AD6620AS-REEL - Analog Devices

REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties that

may result from its use. No license is granted by implication or otherwise

under any patent or patent rights of Analog Devices.

AD6620

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700 www.analog.com

Fax: 781/326-8703 © Analog Devices, Inc., 2001

67 MSPS Digital Receive

Signal Processor

FUNCTIONAL BLOCK DIAGRAM

REAL,

DUAL REAL,

OR COMPLEX

INPUTS

SERIAL OR

PARALLEL

OUTPUTS

CIC

FILTERS OUTPUT

FORMAT

COMPLEX

NCO

␮P

OR SERIAL

CONTROL

–SINCOS

EXTERNAL

SYNC

CIRCUITRY

JTAG

PORT

FIR

FILTER

AD6620

FEATURES

High Input Sample Rate

67 MSPS Single Channel Real

33.5 MSPS Diversity Channel Real

33.5 MSPS Single Channel Complex

NCO Frequency Translation

Worst Spur Better than –100 dBc

Tuning Resolution Better than 0.02 Hz

2nd Order Cascaded Integrator Comb FIR Filter

Linear Phase, Fixed Coefficients

Programmable Decimation Rates: 2, 3 . . . 16

5th Order Cascaded Integrator Comb FIR Filter

Linear Phase, Fixed Coefficients

Programmable Decimation Rates: 1, 2, 3 . . . 32

Programmable Decimating RAM Coefficient FIR Filter

Up to 134 Million Taps per Second

256 20-Bit Programmable Coefficients

Programmable Decimation Rates: 1, 2, 3 . . . 32

Bidirectional Synchronization Circuitry

Phase Aligns NCOs

Synchronizes Data Output Clocks

Serial or Parallel Baseband Outputs

Pin Selectable Serial or Parallel

Serial Works with SHARC®, ADSP-21xx, Most Other

DSPs

16-Bit Parallel Port, Interleaved I and Q Outputs

Two Separate Control and Configuration Ports

Generic ␮P Port, Serial Port

3.3 V Optimized CMOS Process

JTAG Boundary Scan

GENERAL DESCRIPTION

The AD6620 is a digital receiver with four cascaded signal-

processing elements: a frequency translator, two fixed-

coefficient decimating filters, and a programmable coefficient

decimating filter. All inputs are 3.3 V LVCMOS compatible.

All outputs are LVCMOS and 5 V TTL compatible.

As ADCs achieve higher sampling rates and dynamic range, it

becomes increasingly attractive to accomplish the final IF stage

of a receiver in the digital domain. Digital IF Processing is less

expensive, easier to manufacture, more accurate, and more

flexible than a comparable highly selective analog stage.

The AD6620 diversity channel decimating receiver is designed

to bridge the gap between high-speed ADCs and general pur-

pose DSPs. The high resolution NCO allows a single carrier to

be selected from a high speed data stream. High dynamic range

decimation filters with a wide range of decimation rates allow

both narrowband and wideband carriers to be extracted. The

RAM-based architecture allows easy reconfiguration for multi-

mode applications.

The decimating filters remove unwanted signals and noise from

the channel of interest. When the channel of interest occupies

less bandwidth than the input signal, this rejection of out-of-

band noise is called “processing gain.” By using large decimation

factors, this “processing gain” can improve the SNR of the

ADC by 36 dB or more. In addition, the programmable RAM

Coefficient filter allows antialiasing, matched filtering, and

static equalization functions to be combined in a single, cost-

effective filter.

The input port accepts a 16-bit Mantissa, a 3-bit Exponent,

and an A/B Select pin. These allow direct interfacing with the

AD6600, AD6640, AD6644, AD9042 and most other high-

speed ADCs. Three input modes are provided: Single Channel

Real, Single Channel Complex, and Diversity Channel Real.

When paired with an interleaved sampler such as the AD6600,

the AD6620 can process two data streams in the Diversity

Channel Real input mode. Each channel is processed with coher-

ent frequency translation and output sample clocks. In addition,

external synchronization pins are provided to facilitate coherent

frequency translation and output sample clocks among several

AD6620s. These features can ease the design of systems with

diversity antennas or antenna arrays.

Units are packaged in an 80-lead PQFP (plastic quad flatpack)

and specified to operate over the industrial temperature range

(–40°C to +85°C).

SHARC is a registered trademark of Analog Devices, Inc.

AD6620

–2– REV. A

I-RAM

256 ⴛ 18

C-RAM

256 ⴛ 20

Q-RAM

256 ⴛ 18

MRCF

RCF

MCICS

CIC5

SCALING

INTERLEAVE DE-

INTERLEAVE

MULTI-

PLEXER

MCICS

CIC2

SCALING

MULTI-

PLEXER

EXP

SCALING

FREQUENCY

TRANSLATOR 3

INPUT

DATA

EXP[2:0]

IN[15:0]

COMPLEX

NCO

fSAMP5

EXPLNV,

EXPOFF

TIMING

SYNC

I/O

CLK

A/B

RESET

SYNC RCF

SYNC CIC

SYNC NCO

PHASE

OFFSET

fSAMP2

fSAMP

MULTIPLEXER

SCALING, SOUT

SERIAL

PARALLEL

DVOUT

I/QOUT

A/BOUT

PARALLEL

OUTPUTS

AND

SERIAL I/O

OUT[15:0]

SCLK

SDI

SDO

SDFS

SDFE

SBM

WL[1:0]

SDIV[3:0]

RCF COEFFICIENTS

NUMBER OF TAPS

DECIMATE FACTOR

ADDRESS OFFSET

CIC2, CIC5

DECIMATE FACTORS

SCALE FACTORS

NCO FREQUENCY

PHASE OFFSET

DITHER

SYNC MASK

INPUT MODE

REAL, DUAL, COMPLEX

FIXED OR WITH EXPONENT

SYNC M/S

OUTPUT

SCALE

FACTOR

JTAG

TRST TCK TMS TDI TDO

MICROPROCESSOR INTERFACE

D[7:0] A[2:0] R/W DTACKCS MODE PAR/SER

CONTROL REGISTERS

MICROPORT AND

SERIAL ACCESS

(W/R)(RDY)

(R/D)

OUTPUT

Figure 1. Block Diagram

TABLE OF CONTENTS

GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . 1

ARCHITECTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

TIMING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . 11

EXPLANATION OF TEST LEVELS . . . . . . . . . . . . . . . . 11

ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

PIN FUNCTION DESCRIPTIONS . . . . . . . . . . . . . . . . . 12

PIN CONFIGURATIONS . . . . . . . . . . . . . . . . . . . . . . . . . 13

INPUT DATA PORT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

OUTPUT DATA PORT . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

FREQUENCY TRANSLATOR . . . . . . . . . . . . . . . . . . . . . 19

SECOND ORDER CASCADED INTEGRATOR

COMB FILTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

FIFTH ORDER CASCADED INTEGRATOR

COMB FILTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

RAM COEFFICIENT FILTER . . . . . . . . . . . . . . . . . . . . . 25

CONTROL REGISTERS AND ON-CHIP RAM . . . . . . . 27

PROGRAMMING THE AD6620 . . . . . . . . . . . . . . . . . . . 30

ACCESS PROTOCOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

MICROPORT CONTROL . . . . . . . . . . . . . . . . . . . . . . . . 32

SERIAL PORT CONTROL . . . . . . . . . . . . . . . . . . . . . . . . 35

JTAG BOUNDARY SCAN . . . . . . . . . . . . . . . . . . . . . . . . 37

APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . 44

ARCHITECTURE

As shown in Figure 1, the AD6620 has four main signal pro-

cessing stages: a Frequency Translator, two Cascaded Integrator

Comb FIR Filters (CIC2, CIC5), and a RAM Coefficient FIR

Filter (RCF). Multiple modes are supported for clocking data

into and out of the chip. Programming and control is accom-

plished via serial and microprocessor interfaces.

Input data to the chip may be real or complex. If the input data

is real, it may be clocked in as a single channel or interleaved

with a second channel. The two-channel input mode, called

Diversity Channel Real, is typically used in diversity receiver

applications. Input data is clocked in 16-bit parallel words,

IN[15:0]. This word may be combined with exponent input bits

EXP[2:0] when the AD6620 is being driven by floating-point or

gain-ranging analog-to-digital converters such as the AD6600.

Frequency translation is accomplished with a 32-bit complex

Numerically Controlled Oscillator (NCO). Real data entering

this stage is separated into in-phase (I) and quadrature (Q)

components. This stage translates the input signal from a digital

intermediate frequency (IF) to baseband. Phase and amplitude

dither may be enabled on-chip to improve spurious performance

of the NCO. A phase offset word is available to create a known

phase relationship between multiple AD6620s.

Following frequency translation is a fixed coefficient, high speed

decimating filter that reduces the sample rate by a program-

mable ratio between 2 and 16. This is a second order, cascaded

integrator comb FIR filter shown as CIC2 in Figure 1. (Note:

Decimation of 1 in CIC2 requires 2× or greater clock into

AD6620). The data rate into this stage equals the input data

rate, f

SAMP

. The data rate out of CIC2, f

SAMP2

, is determined by

the decimation factor, M

CIC2

AD6620

–3–REV. A

Following CIC2 is the second fixed-coefficient decimating filter.

This filter, CIC5, further reduces the sample rate by a program-

mable ratio from 1 to 32. The data rate out of CIC5, f

SAMP5

, is

determined by the decimation factors of M

CIC5

and M

CIC2

Each CIC stage is a FIR filter whose response is defined by the

decimation rate. The purpose of these filters is to reduce the

data rate of the incoming signal so that the final filter stage, a FIR

RAM coefficient sum-of-products filter (RCF), can calculate

more taps per output. As shown in Figure 1, on-chip multiplex-

ers allow both CIC filters to be bypassed if a multirate clock

is used.

The fourth stage is a sum-of-products FIR filter with program-

mable 20-bit coefficients, and decimation rates programmable

from 1 to 32. The RAM Coefficient FIR Filter (RCF in Figure

1) can handle a maximum of 256 taps.

The overall filter response for the AD6620 is the composite of

all three cascaded decimating filters: CIC2, CIC5, and RCF. Each

successive filter stage is capable of narrower transition band-

widths but requires a greater number of CLK cycles to calculate

the output. More decimation in the first filter stage will minimize

overall power consumption. Data comes out via a parallel port

or a serial interface.

Figure 2 illustrates the basic function of the AD6620: to select

and filter a single channel from a wide input spectrum. The

frequency translator “tunes” the desired carrier to baseband.

CIC2 and CIC5 have fixed order responses; the RCF filter

provides the sharp transitions. More detail is provided in later

sections of the data sheet.

–fS/2 –3fS/8 –5fS/16 –fS/4 –3fS/16 –fS/8 –fS

/16 DC fS/16 fS/8 3fS/16 fS/4 5fS/16 fS/23fS/8

SIGNAL OF

INTEREST

SIGNAL OF INTEREST "IMAGE"

WIDEBAND INPUT SPECTRUM (–fsamp/2 TO fsamp/2)

A' AC

Figure 2a. Wideband Input Spectrum (e.g., 30 MHz from High-Speed ADC)

–fS/2 –3fS/8 –5fS/16 –fS/4 –3fS/16 –fS/8 –fS/16 DC fS/16 fS/8 3fS/16 fS/4 5fS/16 fS/23fS/8

AFTER FREQUENCY TRANSLATION

NCO "TUNES" SIGNAL TO BASEBAND

DD'

B' A'

Figure 2b. Frequency Translation (e.g., Single 1 MHz Channel Tuned to Baseband)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

CIC2, CIC5, AND RCF

dBc

FREQUENCY

Figure 2c. Baseband Signal is Decimated and Filtered by CIC2, CIC5, RCF

–4– REV. A

AD6620–SPECIFICATIONS

RECOMMENDED OPERATING CONDITIONS

Test AD6620AS

Parameter Level Min Typ Max Unit

VDD I 3.0 3.3 3.6 V

AMBIENT

IV –40 +25 +85 °C

ELECTRICAL CHARACTERISTICS

Test AD6620AS

Parameter (Conditions) Temp Level Min Typ Max Unit

LOGIC INPUTS

1, 2, 3, 4, 5, 6, 7

(NOT 5 V TOLERANT)

Logic Compatibility Full 3.3 V CMOS

Logic “1” Voltage Full I 2.0 VDD + 0.3 V

Logic “0” Voltage Full I –0.3 0.8 V

Logic “1” Current Full I 1 10 µA

Logic “0” Current Full I 1 10 µA

Input Capacitance 25°CV 4 pF

LOGIC OUTPUTS

2, 4, 7, 8, 9, 10, 11

Logic Compatibility Full 3.3 V CMOS/TTL

Logic “1” Voltage (I

= 0.5 mA) Full I 2.4 VDD – 0.2 V

Logic “0” Voltage (I

= 1.0 mA) Full I 0.2 0.4 V

IDD SUPPLY CURRENT

CLK = 20 MHz

Full V 52 mA

CLK = 65 MHz

Full I 167 227 mA

Reset Mode

Full I 1 mA

POWER DISSIPATION

CLK = 20 MHz

Full V 170 mW

CLK = 65 MHz

Full I 550 750 mW

Reset Mode

Full I 3.3 mW

NOTES

Input-Only Pins: CLK, RESET, IN[15:0], EXP[2:0], A/B, PAR/SEL.

Bidirectional Pins: SYNC_NCO, SYNC_CIC, SYNC_RCF.

Microinterface Input Pins: DS (RD), R/W (WR), CS.

Microinterface Bidirectional Pins: A[2:0], D[7:0].

JTAG Input Pins: TRST, TCK, TMS, TDI.

Serial Mode Input Pins: SDI, SBM, WL[1:0], AD, SDIV[3:0].

Serial Mode Bidirectional Pins: SCLK, SDFS.

Output Pins: OUT[15:0], DV

OUT

, A/B

OUT

, I/Q

OUT

Microinterface Output Pins: DTACK (RDY).

JTAG Output Pins: TDO.

Serial Mode Output Pins: SDO, SDFE.

Conditions for IDD @ 20 MHz. M

CIC2

= 2, M

CIC5

= 2, M

RCF

= 1, 4 RCF taps of alternating positive and negative full scale.

Conditions for IDD @ 65 MHz. M

CIC2

= 2, M

CIC5

= 2, M

RCF

= 1, 4 RCF taps of alternating positive and negative full scale.

Conditions for IDD in Reset (RESET = 0).

Specifications subject to change without notice.

–5–REV. A

AD6620

TIMING CHARACTERISTICS

(CLOAD = 40 pF All Outputs)

Test AD6620AS

Parameter (Conditions) Temp Level Min Typ Max Unit

CLK Timing Requirements:

CLK

CLK Period Full I 14.93

CLK

CLK Period Full I 15.4 ns

CLKL

CLK Width Low Full IV 7.0 0.5 × t

CLK

CLKH

CLK Width High Full IV 7.0 0.5 × t

CLK

Reset Timing Requirements:

RESL

RESET Width Low Full I 30.0 ns

Input Data Timing Requirements:

Input

to CLK Setup Time Full IV –1.0 ns

Input

to CLK Hold Time Full IV 6.5 ns

Parallel Output Switching Characteristics:

DPR

CLK to OUT[15:0] Rise Delay Full IV 8.0 19.5 ns

DPF

CLK to OUT[15:0] Fall Delay Full IV 7.5 19.5 ns

DPR

CLK to DV

OUT

Rise Delay Full IV 6.5 19.0 ns

DPF

CLK to DV

OUT

Fall Delay Full IV 5.5 11.5 ns

DPR

CLK to IQ

OUT

Rise Delay Full IV 7.0 19.5 ns

DPF

CLK to IQ

OUT

Fall Delay Full IV 6.0 13.5 ns

DPR

CLK to AB

OUT

Rise Delay Full IV 7.0 19.5 ns

DPF

CLK to AB

OUT

Fall Delay Full IV 5.5 13.5 ns

SYNC Timing Requirements:

SYNC

to CLK Setup Time Full IV –1.0 ns

SYNC

to CLK Hold Time Full IV 6.5 ns

SYNC Switching Characteristics:

CLK to SYNC

Delay Time Full V 7.0 23.5 ns

Serial Input Timing:

SSI

SDI to SCLKt Setup Time Full IV 1.0 ns

HSI

SDI to SCLKt Hold Time Full IV 2.0 ns

HSRF

SDFS to SCLKu Hold Time Full IV 4.0 ns

SSF

SDFS to SCLKt Setup Time

Full IV 1.0 ns

HSF

SDFS to SCLKt Hold Time

Full IV 2.0 ns

Serial Frame Output Timing:

DSE

SCLKu to SDFE Delay Time Full IV 3.5 11.0 ns

SDFEH

SDFE Width High Full V t

SCLK

DSO

SCLKu to SDO Delay Time Full IV 4.5 11.0 ns

SCLK Switching Characteristics, SBM = “1”:

SCLK

SCLK Period

Full I 2 × t

CLK

SCLKL

SCLK Width Low Full V 0.5 × t

SCLK

SCLKH

SCLK Width High Full V 0.5 × t

SCLK

SCLKD

CLK to SCLK Delay Time Full V 6.5 13.0 ns

Serial Frame Timing, SBM = “1”:

DSF

SCLKu to SDFS Delay Time Full IV 1.0 4.0 ns

SDFSH

SDFS Width High Full V t

SCLK

SCLK Timing Requirements, SBM = “0”:

SCLK

SCLK Period Full I 15.4 ns

SCLKL

SCLK Width Low Full IV 0.4 × t

SCLK

0.5 × t

SCLK

SCLKH

SCLK Width High Full IV 0.4 × t

SCLK

0.5 × t

SCLK

NOTES

This specification valid for VDD >= 3.3 V. t

CLKL

and t

CLKH

still apply.

Specification pertains to: IN[15:0], EXP[2:0], A/B.

Specification pertains to: SYNC_NCO, SYNC_CIC, SYNC_RCF.

SCLK period will be ≥ 2 × t

CLK

when AD6620 is Serial Bus Master (SBM = 1) depending on the SDIV word.

SDFS setup and hold time must be met, even when configured as outputs, since internally the signal is sampled at the pad.

Specifications subject to change without notice.

AD6620

–6– REV. A

TIMING CHARACTERISTICS

(CLOAD = 40 pF All Outputs)

Test AD6620AS

Parameter (Conditions) Temp Level Min Typ Max Unit

MICROPROCESSOR PORT, MODE = 0

MODE0 Input Timing Requirements:

Control

to CLK Setup Time Full IV 3.0 ns

Control

to CLK Hold Time Full IV 5.0 ns

Address

to CLK Hold Time Full IV 3.0 ns

CS to Data Enabled Time Full IV 5.0 ns

CS to Data Disabled Time Full IV 5.0 ns

SAM

CS to Address/Data Setup Time Full IV 0.0 ns

MODE0 Read Switching Characteristics:

CLK to Data Valid Time Full I 10.0 15.0 30.0 ns

RDY

RD to RDY Time Full IV 4.0 19.5 ns

MODE0 Write Timing Requirements:

Control

to CLK Setup Time Full IV 3.0 ns

Control

to CLK Hold Time Full IV 5.0 ns

Micro Data

to CLK Hold Time Full IV 3.0 ns

Address

to CLK Hold Time Full IV 3.0 ns

SAM

Address/Data Setup Time to CS Full IV 0.0 ns

MODE0 Write Switching Characteristics:

RDY

RD to RDY Time Full IV 4.0 19.5 ns

MICROPROCESSOR PORT, MODE = 1

MODE1 Input Timing Requirements:

Control

to CLK Setup Time Full IV 3.0 ns

Control

to CLK Hold Time Full IV 5.0 ns

Address

to CLK Hold Time Full IV 3.0 ns

CS to Data Enabled Time Full IV 5.0 ns

CS to Data Disabled Time Full IV 5.0 ns

SAM

Address/Data Setup Time to CS Full IV 0.0 ns

MODE1 Read Switching Characteristics:

CLK to Data Valid Time Full I 10.0 30.0 ns

DTACK

CLK to DTACK Time Full V 5.5 15.5 ns

MODE1 Write Timing Requirements:

Control

to CLK Setup Time Full IV 0.0 ns

Control

to CLK Hold Time Full IV 5.0 ns

Micro Data

to CLK Hold Time Full IV 6.5 ns

Address

to CLK Hold Time Full IV 3.0 ns

SAM

Address/Data Setup Time to CS Full IV 0.0 ns

MODE1 Write Switching Characteristic:

DTACK

CLK to DTACK Time Full V 5.5 15.5 ns

NOTES

Specification pertains to: R/W (WR), DS (RD), CS.

Specification pertains to: A[2:0].

Specification pertains to: D[7:0].

Specifications subject to change without notice.

AD6620

–7–REV. A

TIMING DIAGRAMS

CLK, INPUTS, PARALLEL OUTPUTS

RESET with PAR/SER = “1” establishes Parallel Outputs active.

CLKH

CLKL

CLK

Figure 3. CLK Timing Requirements

CLK

IN[15:0]

EXP[2:0]

A/B

DATA

Figure 4. Input Data Timing Requirements

CLK

OUT[15:0]

VALID OUTPUT DATA

OUT

I/Q

OUT

tDPR tDPF

IQIQ

tDPF

Figure 5. Parallel Output Switching Characteristics

SYNC PULSES: SLAVE OR MASTER

tSY tHY

CLK

SYNC NCO

SYNC CIC

SYNC RCF

NOTE:

IN THE SLAVE MODE WITH SINGLE CHANNEL OPERATION, THE WIDTH

OF THE SYNC_NCO SHOULD BE ONE SAMPLE CLOCK CYCLE. IN DUAL

CHANNEL MODE, THE PULSEWIDTH SHOULD BE TWO SAMPLE CLOCK

CYCLES. IF A PULSE LONGER THAN SPECIFIED IS USED, THE NCO WILL

BE INHIBITED AND NOT INCREMENT PROPERLY.

Figure 6. SYNC Slave Timing Requirements

CLK

tCHP tCPL

tCS tCH

IN[15:0]

E[2:0]

A/B

N+1

tCLK

Figure 7. SYNC Master Delay

tRESL

RESET

Figure 8. Reset Timing Requirements

AD6620

–8– REV. A

SERIAL PORT: BUS MASTER

RESET with PAR/SER = “0” establishes Serial Port active.

SBM = “1” puts AD6620 in Serial Bus Master mode SCLK is

output; SDFS is output.

SCLKD

SCLKL

SCLKH

CLK

SCLK

Figure 9. SCLK Switching Characteristics

SSI

HSI

DATASDI

SCLK

Figure 10. Serial Input Data Timing Requirements

SCLK

tDSF tDSE

tSDFEH

tSDFSH

SDFS

SDFE

Figure 11. Serial Frame Switching Characteristics

tDSO

SCLK

SDO

Figure 12. Serial Output Data Switching Characteristics

SERIAL PORT: CASCADE MODE

RESET with PAR/SER = “0” establishes Serial Port active.

SBM = “0” puts AD6620 in Serial Port Cascade mode, SCLK

is input; SDFS is input.

tSCLK

tSCLKH

tSCLKL

SCLK

Figure 13. SCLK Timing Requirements

tSSI tHSI

SCLK

SDI DATA

Figure 14. Serial Input Data Timing Requirements

HSRF

I15 I14

SCLK

SDO

HSF

Q1Q0

SDFS

SSF

Figure 15. SDO/SDFS Timing Requirements

DSO

I15 I14

SCLK

SDO

DSE

SDFEH

Q1Q0

SDFE

Figure 16. SDO, SDFE Switching Characteristics

AD6620

–9–REV. A

MICROPORT MODE0, READ

Timing is synchronous to CLK; MODE = 0.

tDD

DATA VALID

tHC

tSC

tHC

tZD

tHA

tRDY

ADDRESS VALID

tSAM

tZR

N N+1 N+2 N+3 N+4 N

CLK

D[7:0]

RDY

A[2:0]

NOTES:

RDY IS DRIVEN LOW ASYNCHRONOUSLY BY RD AND CS GOING LOW AND RETURNS HIGH ON THE RISING EDGE

OF CLK "N+3" FOR INTERNAL ACCESS (A[2:0] = 000), CLK "N+2" OTHERWISE.

THE SIGNAL, WR, MAY REMAIN HIGH AND RD MAY REMAIN LOW TO CONTINUE READ MODE.

CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+4 SHOWN) TO COMPLETE READ.

Figure 17. MODE0 Read Timing Requirements and Switching Characteristics

MICROPORT MODE0, WRITE

Timing is synchronous to CLK; MODE = 0.

DATA VALID

tHC

tSC

tHC

tSC

tHM

ADDRESS VALID

N N+1 N+2 N+3 N*

CLK1

WR2

RD2

CS3

D[7:0]

RDY

A[2:0]

NOTES:

1 RDY IS DRIVEN LOW ASYNCHRONOUSLY BY WR AND CS GOING LOW AND RETURNS HIGH ON THE

RISING EDGE OF CLK "N+2".

2 THESE SIGNALS (R/W AND DS) MAY REMAIN IN LOW STATE TO CONTINUE WRITING DATA.

3 CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+3 SHOWN) TO COMPLETE WRITE.

* THE NEXT WRITE MAY BE INITIATED ON CLK, N*.

tHA

tRDY

tSAM

Figure 18. MODE0 Write Timing Requirements and Switching Characteristics

AD6620

–10– REV. A

MICROPORT MODE1, READ

Timing is synchronous to CLK; MODE = 1.

DATA VALID

tSC

ADDRESS VALID

NN+1 N+2 N+3

CLK1

R/W2

DS2

CS3

D[7:0]

DTACK

A[2:0]

tSAM

N+4 N

tHC

tDD

tHC

tZD

tHA

tDTACK

NOTES:

1 DTACK IS DRIVEN LOW ON THE RISING EDGE OF CLK "N+3" FOR INTERNAL ACCESS (A[2:0] = 000),

CLK "N=2" OTHERWISE.

2 THE SIGNAL, R/W MAY REMAIN HIGH AND DS MAY REMAIN LOW TO CONTINUE READ MODE.

3 CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+4 SHOWN) TO COMPLETE ACCESS

AND FORCE DTACK HIGH.

tSC

tZR

Figure 19. MODE1 Read Timing Requirements and Switching Characteristics

MICROPORT MODE1, WRITE

Timing is synchronous to CLK; MODE = 1.

NN+1 N+2 N+3

SAM

DTACK

CLK

R/W

D[7:0]

DTACK

A[2:0]

DTACK

SAM

NOTES:

ON RISING EDGE OF "N+3" CLK, DTACK IS DRIVEN LOW.

THESE SIGNALS (R/W AND DS) MAY REMAIN IN LOW STATE TO CONTINUE WRITING DATA.

CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+3 SHOWN) TO COMPLETE WRITE

AND FORCE DTACK HIGH.

* THE NEXT WRITE MAY BE INITIATED ON CLK, N*.

DATA VALID

ADDRESS VALID

Figure 20. MODE1 Write Timing Requirements and Switching Characteristics

AD6620

–11–REV. A

ABSOLUTE MAXIMUM RATINGS*

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +4.5 V

Input Voltage . . . –0.3 V to VDD + 0.3 V (Not 5 V Tolerant)

Output Voltage Swing . . . . . . . . . . . . –0.3 V to VDD + 0.3 V

Load Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 pF

Junction Temperature Under Bias . . . . . . . . . . . . . . . . 130°C

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

Lead Temperature (5 sec) . . . . . . . . . . . . . . . . . . . . . . 280°C

*Stresses greater than those listed above may cause permanent damage to the

device. These are stress ratings only; functional operation of the device at these or

any other conditions greater than those indicated in the operational sections of this

specification is not implied. Exposure to absolute maximum rating conditions for

extended periods may affect device reliability.

Thermal Characteristics

80-Lead Plastic Quad Flatpack:

= 44°C/W

= 11°C/W

EXPLANATION OF TEST LEVELS

I. 100% Production Tested.

II. 100% Production Tested at 25°C, and Sampled Tested at

Specified Temperatures.

III. Sample Tested Only.

IV. Parameter Guaranteed by Design and Analysis.

V. Parameter is Typical Value Only.

VI. 100% Production Tested at 25°C, and Sampled Tested at

Temperature Extremes.

ORDERING GUIDE

Package

Model Temperature Range Package Description Option

AD6620AS –40°C to +85°C (Ambient) 80-Lead PQFP (Plastic Quad Flatpack) S-80A

AD6620S/PCB Evaluation Board with AD6620AS and Software

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily

accumulate on the human body and test equipment and can discharge without detection.

Although the AD6620 features proprietary ESD protection circuitry, permanent damage may

occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD

precautions are recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

AD6620

–12– REV. A

Name Type Description

VDD P 3.3 V Supply

VSS G Ground

CLK I Input Clock

RESET I Active Low Reset Pin

IN[15:0] I Input Data (Mantissa)

EXP[2:0] I Input Data (Exponent)

A/B I Channel (A/B) Select

SYNC_NCO I/O Sync Signal for NCO

SYNC_CIC I/O Sync Signal for CIC Stages

SYNC_RCF I/O Sync Signal for RCF

MODE I Sets Microport Mode: Mode 1, (MODE = 1), Mode 0, (MODE = 0)

A[2:0] I Microprocessor Interface Address

D[7.0] I/O/T Microprocessor Interface Data

DS or RD I Mode 1: Data Strobe Line, Mode 0: Read Signal

R/W or WR I Read/Write Line (Write Signal)

CS I Chip Select, Enables the Chip for µP Access

DTACK or RDY O Acknowledgment of a Completed Transaction (Signals when µP Port Is Ready for an Access)

PAR/SER I Parallel/Serial Control Select (PAR = 1, SER = 0)

OUT

O Data Valid Pin for the Parallel Output Data

A/B

OUT

O Signals to Which Channel the Output Belongs to (A = 1, B = 0)

I/Q

OUT

O Signals Whether I or Q Data Is Present (I = 1, Q = 0)

TRST I Test Reset Pin

TCK I Test Clock Input

TMS I Test Mode Select Input

TDI I Test Data Input

TDO I Test Data Output

Pin Types: I = Input, O = Output, P = Power Supply, G = Ground, T = Three-state.

SHARED PINS

Parallel Outputs (PAR/SER = 1 at RESET) Serial Port (PAR/SER = 0 at RESET)

Name Type Description Name Type Description

OUT15 O Parallel Output Data SCLK I/O Serial Clock Input (SBM =0)

Serial Clock Output (SBM = 1)

OUT14 O Parallel Output Data SDI I Serial Data Input

OUT13 O Parallel Output Data SDO O/T Serial Data Output

OUT12 O Parallel Output Data SDFS I/O Serial Data Frame Sync Input (SBM = 0)

Serial Data Frame Sync Output (SBM = 1)

OUT11 O Parallel Output Data SDFE O Serial Data Frame End

OUT10 O Parallel Output Data SBM I Serial Bus Master (Master = 1, Cascade = 0)

OUT9 O Parallel Output Data WL1 I Serial Port Word Length, Bit 1

OUT8 O Parallel Output Data WL0 I Serial Port Word Length, Bit 0

OUT7 O Parallel Output Data AD I Append Data

OUT[6:4] O Parallel Output Data NC NC Unused, Do Not Connect

OUT3 O Parallel Output Data SDIV3 I SCLK Divide Value, Bit 3

OUT2 O Parallel Output Data SDIV2 I SCLK Divide Value, Bit 2

OUT1 O Parallel Output Data SDIV1 I SCLK Divide Value, Bit 1

OUT0 O Parallel Output Data (LSB) SDIV0 I SCLK Divide Value, Bit 0

Pin Types: I = Input, O = Output, P = Power Supply, G = Ground, T = Three-state.

PIN FUNCTION DESCRIPTIONS

AD6620

–13–REV. A

PIN CONFIGURATIONS

Parallel Output Data

80 79 78 77 76 71 70 69 68 67 66 6575 74 73 72 64 63 62 61

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

PIN 1

IDENTIFIER

TOP VIEW

(Not to Scale)

(MSB) OUT15

OUT14

VDD

OUT13

OUT12

OUT11

VSS

OUT10

OUT9

OUT8

OUT7

VDD

OUT6

OUT5

OUT4

VSS

VDD

DTACK

R/W

VSS

MODE

OUT0 (LSB)

A/BOUT

I/QOUT

VDD

DVOUT

PAR/SER

RESET

TRST

TCK

TMS

TDO

TDI

VDD

SYNC NCO

SYNC CIC

SYNC RCF

VSS

EXP2

IN15 (MSB)

IN14

VSS

IN13

IN12

IN11

VDD

IN10

IN9

IN7

VSS

IN6

IN5

IN4

IN8

AD6620

VSS

OUT3

OUT2

OUT1

CLK

A/B

IN0 (LSB)

VDD

IN3

IN2

IN1

EXP0

EXP1

Serial Port

80 79 78 77 76 71 70 69 68 67 66 6575 74 73 72 64 63 62 61

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

PIN 1

IDENTIFIER

TOP VIEW

(Not to Scale)

SCLK

SDI

VDD

SDO

SDFS

SDFE

VSS

SBM

WL1

WL0

VDD

VSS

VDD

DTACK

R/W

VSS

MODE

SDIV0

A/BOUT

I/QOUT

VDD

DVOUT

PAR/SER

RESET

TRST

TCK

TMS

TDO

TDI

VDD

SYNC NCO

SYNC CIC

SYNC RCF

VSS

EXP2

IN15

IN14

VSS

IN13

IN12

IN11

VDD

IN10

IN9

IN7

VSS

IN6

IN5

IN4

IN8

AD6620

VSS

SDIV3

SDIV2

SDIV1

CLK

A/B

IN0

VDD

IN3

IN2

IN1

EXP0

EXP1

THE HIGHEST NUMBERED BIT IS THE MSB FOR ALL PORTS

NC = NO CONNECT

AD6620

–14– REV. A

CIC2 DECIMATION

LOG

(M)

225 51

POWER – mW

234

250

275

300

325

350

375

400

CIC5 DECIMATION

RCF DECIMATION

TPC 1. Typical Power vs. Decimation Rates

–132

–120

–108

–96

–84

–72

–60

–48

–36

–24

–12

fSAMP

SPUR = –104dB

PHASE DITHER OFF

TPC 2. Typical NCO Spur Without Dither

–132

–120

–108

–96

–84

–72

–60

–48

–36

–24

–12

SAMP

SPUR = –118dB

PHASE DITHER ON

TPC 3. Typical NCO Spur with Dither

–Typical Performance Characteristics

–140

–120

–100

–80

–60

–40

–20

REJECTION – dB

COMPOSITE FREQUENCY RESPONSE – MHz

TPC 4. High Decimation GSM Filter

Input sample rate 65 MSPS, decimation is 240, FIR taps is 240.

Unshown spectrum is below that shown. Decimation distribu-

tion is 3, 10, 8, respectively.

–140

–120

–100

–80

–60

–40

–20

REJECTION – dB

COMPOSITE FREQUENCY RESPONSE – MHz

486

TPC 5. High Decimation AMPS Filter

Input sample rate 58.32 MSPS, decimation is 300, FIR taps is

128. Unshown spectrum is below that shown. Decimation distri-

bution among CIC2, CIC5, and RCF is 10, 30 and 1, respectively.

AD6620

–15–REV. A

INPUT DATA PORT

The input data port accepts a clock (CLK), a 16-bit mantissa

IN[15:0], a 3-bit exponent EXP[2:0], and channel select Pin A/B.

These pins allow direct interfacing to both standard fixed-point

ADCs such as the AD9225 and AD6640, as well as to gain-

ranging ADCs such as the AD6600. These inputs are not 5 V

tolerant and the ADC I/O should be set to 3.3 V.

The input data port accepts data in one of three input modes:

Single Channel Real, Diversity Channel Real, or Single Channel

Complex. The input mode is selected by programming the Input

Mode Control Register located at internal address space 300h.

Single Channel Real mode is used when a single channel ADC

drives the input to the AD6620. Diversity Channel Real mode is

the two channel mode used primarily for diversity receiver appli-

cations. Single Channel Complex mode accepts complex data in

conjunction with the A/B input which identifies in-phase and

quadrature samples (primarily for cascaded 6620s).

The input data port is sampled on the rising edge of CLK at a

maximum rate of 67 MSPS. The 16-bit mantissa, IN[15:0] is

interpreted as a twos complement integer. For most applications

with ADCs having fewer than 16 bits, the active bits should be

MSB justified and the unused LSBs should be tied low.

The 3-bit exponent, EXP[2:0] is interpreted as an unsigned

integer. The exponent can be modified by the 3-bit exponent

offset ExpOff (Control Register 0x305, Bits (7–5)) and an expo-

nent invert ExpInv (Control Register 0x305, Bit 4).

ExpOff sets the offset of the input exponent, EXP[2:0]. ExpInv

determines the direction of this offset. Equations below show

how the exponent is handled.

scaled input IN ExpInv

Exp ExpOff

–mod( , )

=× =

scaled input IN ExpInv

Exp ExpOff

–mod( –,)

=× =

where: IN is the value of IN[15:0], Exp is the value of EXP[2:0],

and ExpOff is the value of ExpOff.

Input Scaling

In general there are two reasons for scaling digital data. The

first is to avoid “clipping” or, in the case of the AD6620 regis-

ter, “wrap-around” in subsequent stages. Wrap-around is not a

concern for the input data since the NCO is designed to accept

the largest possible input at the AD6620 data port.

The second use of scaling is to preserve maximum dynamic

range through the chip. As data flows from one stage to the next

it is important to keep the math functions performed in the

MSBs. This will keep the desired signal as far above the noise

floor as possible, thus maximizing signal-to-noise ratio.

Scaling with Fixed-Point ADCs

For fixed-point ADCs, the AD6620 exponent inputs EXP[2:0]

are typically not used and should be tied low. The ADC outputs

are tied directly to the AD6620 Inputs, MSB-justified. The

exponent offset (ExpOff) and exponent invert (ExpInv) should

both be programmed to 0. Thus the input equation,

scaled input IN ExpInv

Exp ExpOff

–mod( , )

=× =

where: IN is the value of IN[15:0], Exp is the value of EXP[0:2],

and ExpOff is the value of ExpOff, simplifies to,

scaled input IN_–mod( , )

=×208

Thus for fixed-point ADCs, the exponents are typically static

and no input scaling is used in the AD6620.

IN4

IN3

IN2

IN1

IN0

EXP2

EXP1

EXP0

IN15

D11 (MSB)

D0 (LSB)

AD6640

AD6620

A/B

+3.3V

Figure 21. Typical Interconnection of the AD6640 Fixed

Point ADC and the AD6620

Scaling with Floating-Point ADCs

An example of the exponent control feature combines the AD6600

and the AD6620. The AD6600 is an 11-bit ADC with three bits

of gain ranging. In effect, the 11-bit ADC provides the mantissa,

and the three bits of relative signal strength indicator (RSSI) are

the exponent. Only five of the eight available steps are used by

the AD6600. See the AD6600 data sheet for additional details.

For gain-ranging ADCs such as the AD6600,

scaled input IN ExpInv

Exp ExpOff

–mod( –,)

=× =

where: IN is the value of IN[15:0], Exp is the value of EXP[2:0],

and ExpOff is the value of ExpOff.

The RSSI output of the AD6600 numerically grows with increas-

ing signal strength of the analog input (RSSI = 5 for a large

signal, RSSI = 0 for a small signal). With the Exponent Offset

equal to zero and the Exponent Invert Bit equal to zero, the

AD6620 would consider the smallest signal at the parallel input

(EXP = 0) the largest and, as the signal and EXP word increase,

it shifts the data down internally (EXP = 5, will shift the 11-bit

data right by 5 bits internally before going into the CIC2). The

AD6620 regards the largest signal possible on the AD6600 as

the smallest signal. Thus the Exponent Invert Bit is used to make

the AD6620 exponent agree with the AD6600 RSSI. When it

is set high, it forces the AD6620 to shift the data up for growing

EXP instead of down. The exponent invert bit should always be

set high for use with the AD6600.

Table I. AD6600 Transfer Function with AD6620 ExpInv = 1,

and No ExpOff

ADC Input AD6600 AD6620 Signal

Level RSSI[2.0] Data Reduction

Largest 101 (5) ⫼ 4 (>> 2) –12 dB

100 (4) ⫼ 8 (>> 3) –18 dB

011 (3) ⫼ 16 (>> 4) –24 dB

010 (2) ⫼ 32 (>> 5) –30 dB

001 (1) ⫼ 64 (>> 6) –36 dB

Smallest 000 (0) ⫼ 128 (>> 7) –42 dB

(ExpInv = 1, ExpOff = 0)

AD6620

–16– REV. A

The Exponent Offset is used to shift the data right. For example,

Table I shows that with no ExpOff shift, 12 dB of range is

lost when the ADC input is at the largest level. This is undesired

because it lowers the Dynamic Range and SNR of the system

by reducing the signal of interest relative to the quantization

noise floor.

To avoid this automatic attenuation of the full-scale ADC sig-

nal, the Exponent Offset is used to move the largest signal (RSSI =

5) up to the point where there is no downshift. In other words,

once the Exponent Invert bit has been set, the Exponent Offset

should be adjusted so that mod(7–5 + ExpOff,8) = 0. This is

the case when Exponent Offset is set to 6 since mod(8, 8) = 0.

Table II illustrates the use of ExpInv and ExpOff when used

with the AD6600 ADC.

Table II. AD6600 Transfer Function with AD6620 ExpInv = 1,

and ExpOff = 6

ADC Input AD6600 AD6620 Signal

Level RSSI[2.0] Data Reduction

Largest 101 (5) ⫼ 1 (>> 0) –0 dB

100 (4) ⫼ 2 (>> 1) –6 dB

011 (3) ⫼ 4 (>> 2) –12 dB

010 (2) ⫼ 8 (>> 3) –18 dB

001 (1) ⫼ 16 (>> 4) –24 dB

Smallest 000 (0) ⫼ 32 (>> 5) –30 dB

(ExpInv = 1, ExpOff = 6)

This flexibility in handling the exponent allows the AD6620 to

interface with other gain ranging ADCs besides the AD6600.

The Exponent Offset can be adjusted to allow up to seven

RSSI(EXP) ranges to be used as opposed to the AD6600s five.

It also allows the AD6620 to be tailored in a system that employs

the AD6600, but does not utilize all of its signal range. For

example, if only the first four RSSI ranges are expected to occur

then the Exponent Offset could be adjusted to five, which would

then make RSSI = 4 correspond to the 0 dB point of the AD6620.

IN4

IN3

IN2

IN1

IN0

EXP2

EXP1

EXP0

IN15D10 (MSB)

D0 (LSB)

AD6600

AD6620

A/B

RSS12

RSS11

RSS10

A/B OUT

Figure 22. Typical Interconnection of the AD6600 Gain-

Ranging ADC and the AD6620 in a Diversity Application

Input Timing

The CLK signal is used to sample the input port and clock the

synchronous signal processing stages that follow. The CLK signal

can operate up to 67 MHz and have a duty cycle of 45% to

55%. In applications using high speed ADCs, the ADC sample

clock is typically used to clock the AD6620. Applications that

require a faster signal processing clock than the ADC sample

clock, may employ fractional rate input timing as shown in the

following sections. The input timing requirements vary according

to the mode of operation. Fractional rate input timing creates a

longer “don’t care” time for the input data so that slower ADCs

need only meet the setup-and-hold conditions for their data

with respect to their own sample clock cycle, rather than the

faster signal processing clock. The ADC sample clock may be

any integer fraction of CLK up to and including 1, as long as

the clock and data rate are less than or equal to 67 MSPS.

Single Channel Real Mode

In the Single Channel Real mode the A/B input pin functions as

an active high input enable. If the A/D sample clock is fast enough

to perform the necessary filter functions, full rate input timing

can be used and A/B should be tied high as shown in Figure 23.

N N+1 N+2 N+3 N+4

CLK

IN[15:0]

EXP[2:0]

A/B

Figure 23. Full Rate Input Timing, Single Channel

Real Mode

When a faster processing clock is used to achieve better filter

performance, the A/D data must be synchronized with the faster

AD6620 CLK signal. This is achieved by having the ADC clock

rate an integer fraction of the AD6620 clock rate. AD6620 input

data is sampled at the slower ADC clock rate. In the Single

Channel Real Mode this is achieved by dynamically controlling

the A/B input and bringing it high before each rising CLK edge

that data is to be sampled on. A/B must be returned low before

the next high speed clock pulse and the duty cycle of the A/B

signal will therefore be equal to the data-to-clock ratio.

N N+1

tSI tHI

CLK

IN[15:0]

EXP[2:0]

A/B

Figure 24. Fractional Rate Input Timing (4

CLK), Single

Channel Real Mode

Diversity Channel Real Mode

In the Diversity Channel Real mode the A/B pin serves not only

as an input enable but also to determine which channel is being

sampled on a given CLK edge. A high on the A/B pin marks

channel A data and a low on A/B marks channel B data. The

AD6620 only accepts the first sample after an A/B transition.

All subsequent samples are disregarded until A/B changes again.

When full rate input timing is employed in the Diversity Chan-

nel Real mode, A/B must toggle on every rising edge of CLK for

new data to be clocked into the AD6620.

AD6620

–17–REV. A

CLK

IN[15:0]

EXP[2:0]

A/B

BNBN+1

AN+1 AN+2 BN+2

CLK

IF CLK 2x IS USED TO CLOCK THE AD6620, THE FIRST RISING EDGE AFTER

THE A/B TRANSITION WILL LATCH THE DATA.

Figure 25. Full Rate Input Timing, Diversity Channel Real

Mode

If fractional rate input timing is necessary in the Diversity Chan-

nel Real Mode, the A/B pin must toggle at half the rate of the

A/D sample clock. The timing diagram below shows a 3× pro-

cessing clock. In this situation there will be one ADC encode

pulse for every three AD6620 CLK pulses and data must be

taken on every third CLK pulse. The CLK edges that corre-

spond to the latching of A and B channel data are shown in

Figure 26.

CLK

IN[15:0]

EXP[2:0]

A/B

Figure 26. Fractional Rate Input Timing (3

CLK), Diversity

Channel Real Mode

Single Channel Complex Mode

In the Single Channel Complex input mode, A/B high identi-

fies the in-phase samples and A/B low identifies quadrature

samples. The quadrature samples are paired with the previous

in-phase samples. The timing for this mode is the same as that

of the Diversity Channel Real Mode. This mode is useful for

accepting complex output data from another AD6620 or another

source to increase filtering and or decimation rates.

In the Single Channel Complex Mode the CIC2 decimation

must be set to two (M

CIC2

= 2). This is necessary in order to

allow enough CLK cycles to process the complex input data as

described below.

First clock cycle: (A/B high).

–I data loaded from the input port.

–The I data-path gets I × cosine.

–The Q data-path gets I × sine.

–The first integrator of the CIC2 adds these values to its

previous sums.

–The rest of the CIC2 is idle.

Second clock cycle: (A/B low).

–Q data loaded from the input port.

–The I data-path gets Q × sine.

–The Q data-path gets Q × cosine.

–The first integrator of the I path of the CIC2 completes the

sum (I × cosine - Q × sine) and the first integrator of the Q

path of the CIC2 completes the sum j(I × sine + Q × cosine).

–The rest of the CIC2 operates on these sums, which is the

complete complex multiply. The data is then multiplexed

through the rest of the chip as if it were single channel real data.

Simplified Input Data Port Schematic

Figure 27 details a simplified schematic for the input data port.

The first thing to note is that IN[15:0], EXP[2:0] and A/B are

all synchronously latched with CLK. Note also that upon soft

reset, a seven pipeline delay (sample clock delay) exists in the

data path. This delay is synchronous with CLK, but is in fact

seven valid sample data delays. For instance, in single channel

CLK

LOGIC "1"

SOFT RESET

CLR

ENB

IN[15:0]

EXP[2:0]

A/B

CLK

MULTIPLEXER

DUAL CHANNEL REAL

SINGLE CHANNEL COMPLEX

INT IN[15:0]

INT EXP[2:0]

INT DATA STROBE

CLR

DELAY 7

ENB

DSET Q

CLR Q

Figure 27. Simplified Input Data Port Schematic for the AD6620

AD6620

–18– REV. A

real mode with full rate timing the delay is seven CLKs. If

instead the data rate is one-fourth CLK, then 28 CLKs (i.e.,

seven sample data delays, gated via A/B) occur before valid data

is passed to the NCO stage.

Interfacing AD6620 Inputs to 5 V Logic Gates

None of the inputs to the AD6620 are tolerant of 5 V logic

signals. When interfacing 5 V devices to this product, an interface

gate such as the 74LCX2244 is recommended. If latching must

be performed, 74LCX574 latches may be used. This gate runs

from the 3.3 V supply and is tolerant of 5 V inputs.

OUTPUT DATA PORT

Parallel Output Data Port

The AD6620 provides a choice of two output ports: a 16-bit

parallel port and a synchronous serial port. Output operation

using the serial port is discussed in the next section. The parallel

port is limited to 16 bits. Because pins are shared between the

parallel and serial output ports, only one output mode can be

used. The output mode must be set with a hard reset generated

by at least a 30 ns low time on the RESET pin. If the PAR/SER

line is high (Logic “1”), then parallel output data is activated.

The PAR/SER pin should remain static after the output mode

has been set (i.e., PAR/SER should only change when RESET is

low). Data out of the AD6620 is two’s complement.

A scale factor is associated with the output port, which allows

the signal level to be adjusted. This scale factor is mapped to

location 309h, Bits 2–0 in the AD6620 internal address space.

This scalar controls the weight of the 16-bit data going to the

parallel port. The scale factor is discussed in the RAM Coeffi-

cient Filter (RCF) section.

The Parallel Mode provides a 16-bit output port, which consti-

tutes the I and Q data for either one or both channels. This port

can run at a maximum of 67 MHz (33.5 MHz I, 33.5 MHz Q).

This rate assumes that there is a minimum decimation of 2 in

the first filter stage (CIC2) or a 2× or greater CLK is used. This

decimation is required because for every input word there is

both an I and a Q output. When the data rate and clock rate are

the same (Full Rate Input Timing), the minimum decimation of

2 must occur in CIC2. Refer to CIC2 for more detail.

OUT

is provided to signal that valid data is present. If this pin

is high, there is a valid data word on the bus. DV

OUT

remains high

for two high-speed clock cycles in Single Channel Real and Single

Channel Complex Mode and for four high-speed clock cycles in

Diversity Channel Real mode. After DV

OUT

returns low the Q data

will remain until the next data sample.

I/Q

OUT

When this pin is high the data word represents I data; when

I/Q

OUT

is low Q data is present. This signal will also be low when

OUT

is low since the last word of every data phase is Q data.

A/B

OUT

If DV

OUT

is low, A/B

OUT

is always low. When A/B

OUT

is high, A

Channel data is available on the output. If DV

OUT

remains high

while A/B

OUT

is low, then B Channel data is on the output pins

of the chip OUT[15:0].

CLK

OUT[15:0]

VALID DATA

A DATA

tDPR tDPF tDPF

OUT

I/Q

OUT

A/B

OUT

Figure 28. Parallel Output Data Timing (Single-Channel

Mode)

DPR

DPF

IQ IQ

IAQAIBQB

VALID DATA

A DATA B DATA

CLK

OUT[15:0]

DVOUT

I/QOUT

A/BOUT

Figure 29. Parallel Output Data Timing (Diversity Channel

Mode)

Serial Output Data Port

The AD6620 provides a choice of two output ports: a 16-bit

parallel port and a synchronous serial port. The advantage of

using the serial port is that all 23 bits of available data can be

output in the 24-bit or 32-bit mode. The serial output port

shares some of the same pins used by the parallel output port.

As a result, one or the other mode of output may be utilized,

but not both. The output mode must be set with a hard reset

generated by at least a 30 ns low time on the RESET pin. If the

PAR/SER line is low (Logic “0”) upon reset, then serial output

data is activated. The PAR/SER pin should remain static after

the output mode has been set (i.e., PAR/SER should only change

when RESET is low).

Note that the AD6620 cannot be booted through the serial port.

The microport must be used to initialize the device, then serial

operation is supported.

Figure 30 shows the typical interconnections between an AD6620

in serial master mode and a DSP. Refer to the Serial Control

Port section for a detailed description of pin functions and pro-

cedures for writing and reading with relation to the serial port.

Note the 10 kΩ resistors connected to SDI and SDO. These

prevent the lines from toggling when the AD6620 or DSP

three-states these pins.

AD6620

–19–REV. A

SCLK

AD6620 DSP

+3.3V

SBM

SCLK

SDI DT

SDO DR

SDFS RFS

SDFE

10k⍀10k⍀

SDIV

2 4

ADWL

Figure 30. Typical Serial Data Output Interface to DSP

(Serial Master Mode, SBM = 1)

Figure 31 shows two AD6620s illustrating the cascade capability

for the chip. The first is connected as a serial master and the

second is configured in serial cascade mode. The SDFE signal

of the master is connected to the SDFS of the slave. This allows

the master AD6620 data to be obtained first by the DSP, fol-

lowed by the cascaded AD6620 data.

SCLK

AD6620 DSP

+3.3V

SBM

SCLK

SDI DT

SDO DR

SDFS RFS

SDFE

10k⍀

SDIV

2 4

ADWL

10k⍀

SCLK

AD6620

CASCADE

SBM

SDI

SDO

SDFS

SDFE

SDIV

2 4

ADWL

Figure 31. Typical Serial Data Output Interface to DSP

(Serial Cascade Mode, SBM = 0)

The AD6620 also supports a serial slave mode, where the serial

clock and interface is provided by a DSP or ASIC that is set to

operate in the master mode. Note that the AD6620 cannot be

booted through the serial port. The microport must be used to

initialize the device, then serial operation is supported.

In the serial slave mode, DV

OUT

is valid and indicates the pres-

ence of a new word in the output buffers of the shift register.

This pin may thus be used by the DSP to generate an interrupt

to service the serial port. The DSP then generates an SFDS

pulse to drive the AD6620. The first serial clock rising edge

after SDFS makes the first bit available at SDO. The falling

edge of serial clock can be used to sample the data. The total

number of bits are then read from the AD6620 (determined by

the serial port word length). If the DSP has the ability to count

bits, the DSP will know when the complete frame is read. If not,

the DSP can monitor the SDFE pin to determine that the com-

plete frame is read. The serial clock provided by the DSP can be

asynchronous with the AD6620 clock and input data.

SCLK

AD6620

DSP

SBM

SCLK

SDI DT

SDO DR

SDFS RFS

SDFE

10k⍀10k⍀

SDIV

2 4

ADWL

DVOUT IRQ

Figure 32. Typical Serial Data Output Interface to DSP

(Serial Slave Mode, SBM = 0)

In either the serial master or slave mode, there are two con-

straints that must be observed. The first is that the clock must

be fast enough to read the serial frame prior to the next frame

becoming available. Since the AD6620 output is synchronous

with its input sample rate, the output update rate can be deter-

mined by the user-programmed decimation rate. The timing

diagram in Figure 33 details how serial slave mode is imple-

mented. The second constraint is that the time between serial

frames may be either zero SCLK periods (the end of one frame

adjoins the beginning of the next) or two or more SCLK peri-

ods. One SCLK period between frames is not allowed.

DSO

OUT

SCLK

SDFS

SDO

DSP USES FALLING EDGE OF

OUT

TO GENERATE SDFS

FIRST DATA IS AVAILABLE THE FIRST

RISING SCLK AFTER SDFS GOES HIGH

MSB

MSB – 1

OUT

PULSEWIDTH IS 2 CLKIN

SINGLE CHANNEL AND 4 CLKIN

DUAL CHANNEL

Figure 33. Timing for Serial Slave Mode (SBM = 0)

FREQUENCY TRANSLATOR

The first signal processing stage is a frequency translator con-

sisting of two multipliers and a 32-bit complex numerically

controlled oscillator (NCO). The NCO serves as a quadrature

local oscillator capable of producing any analytic frequency

between –f

SAMP

/2 and +f

SAMP

/2 with a resolution of f

SAMP

. In

the Single Channel Real input mode, f

SAMP

is equal to f

CLK

multi-

plied by the fraction of CLK cycles that A/B is high. In the

Diversity Channel Real and Single Channel Complex input

AD6620

–20– REV. A

modes, f

SAMP

is equal to f

CLK

multiplied by the fraction of CLK

cycles on which A/B has been toggled. The NCO worst case

discrete spur is better than –100 dBc for all output frequencies.

The control word, NCO_FREQ is interpreted as a 32-bit unsigned

integer. To translate a channel centered at f

to dc, calculate

NCO_FREQ using the equation below. The mod function is

used here to allow for Super Nyquist sampling where the IF

carrier (fCH) is larger than the sample rate (fSAMP). The mod

removes the integer portion of the number and forces it into the

32-bit NCO Frequency Register. If the fraction remaining is

larger than 0.5, the NCO will be tuning above the Nyquist rate.

The corresponding signal is then aliased back into the first Nyquist

Zone as a negative frequency.

NCO FREQ f

SAMP

_,=× 









mod

In both Single and Diversity Channel Real Input modes, the out-

put of the translation stage is the complex product of the real

input samples and the complex samples from the NCO. It is

necessary for the subsequent decimating filters to reject the

unwanted image of the channel of interest, as well as any unwanted

neighboring signals (and their images) not rejected by previ-

ous analog filters.

In the Diversity Channel Real Input mode, the same NCO output

words are used for both channel A and B streams, resulting in

identical phase shifts. In Single Channel Complex mode both I

and Q inputs are multiplied by the quadrature outputs of the

NCO. The I and Q products of the multiply are then processed

in the AD6620 filter stages.

In single channel real or dual channel real operation, the frequency

translation and filtering processes provide a gain of –6 dB. This

can be visualized since the input data is usually a real sampled

signal consisting of both positive and negative frequency compo-

nents (Figure 2a). After being mixed with the complex NCO,

the normal filtering of the AD6620 will remove one component

or the other resulting in an analytic signal (Figure 2b). This

filtering thus removes one-half or 6 dB of the signal keeping

consistent with the mathematics involved. If however, the filter-

ing of the device allows both the positive and negative frequency

components to pass (i.e., the original signal is near dc), the gain

of the frequency translation is 0 dB. Finally, if the NCO is

bypassed, the gain of the frequency translation block is –12 dB.

Phase Dither

The AD6620 provides a phase dither option for improving the

spurious performance of the NCO. This is controlled via the

NCO Control Register at address 301 hex. When phase dither is

enabled by setting Bit 1 of this register high, spurs due to phase

truncation in the NCO are randomized. The energy from these

spurs is spread into the noise floor and Spurious Free Dynamic

Range is increased at the expense of very slight decreases in the

SNR. Phase dither should be experimented with for each desired

NCO frequency and if it is seen to reduce spurs, it should be

considered. The choice of whether Phase Dither is used in a

system will ultimately be decided by the system goals. If lower

spurs are desired at the expense of a slightly raised noise floor, it

should be employed. If a low noise floor is desired and the higher

spurs can be tolerated or filtered by subsequent stages, then

Phase Dither is not needed.

Amplitude Dither

The second dither option is Amplitude Dither or “Complex

Dither.” Amplitude Dither is enabled by setting Bit 2 of the

NCO Control Register at address 0x301 high. Amplitude Dither

improves performance by randomizing the amplitude quantiza-

tion errors within the angular to Cartesian conversion of the

NCO. This dither will be particularly useful when the NCO

frequency is close to an integer submultiple of the Input Data

Rate. However, this option may reduce spurs at the expense of a

slightly raised noise floor. Amplitude Dither and Phase Dither

can be used together, separately or not at all.

Phase Offset

The phase offset register adds an offset to the phase accumula-

tor of the NCO. This is a 16-bit register and is interpreted as a

16-bit unsigned integer. A 0 in this register corresponds to a 0

Radian offset and an FFFF hex corresponds to an offset of 2 π

(1 – 1/(2^16)) Radians. This register can be used to allow mul-

tiple AD6620s whose NCOs are synchronized to produce sine

waves with a known and steady phase difference.

NCO Synchronization

In order to achieve phase coherence between several AD6620s,

a SYNC_NCO pin is provided. When the internal register

bit, SYNC_M/S (Bit 3 of internal register 0x300), is set high,

SYNC_NCO provides a synchronization pulse on the rising

edge of CLK. When the SYNC_M/S bit is low, SYNC_NCO

accepts an external synchronization signal sampled on the rising

edge of CLK. When the AD6620 is a slave, the SYNC_NCO

signal need not be a short pulse. It may be taken high and held

for more than a CLK cycle in which case the NCO will be held

inactive until this pin is again lowered. If the device is run as a

sync slave in Single Channel Mode, the SYNC_NCO pin must

be held low for one sample period, usually one clock cycle. If the

device is run in Diversity Channel Real mode, the SYNC_NCO

must be high for two sample periods (clock cycles). In a system

with an array of AD6620s it is not necessary to use one as a

master. It may be desirable to generate a synchronization signal

elsewhere in the system and use that to control the AD6620. An

example of this may be in systems that receive packets of data.

In this case, the NCO may be resynchronized prior to the begin-

ning of the packet, thus giving a consistent phase relationship on

each burst. This allows for ease of use in a large system where

many AD6620s need be synchronized accurately across a large

backplane or installation.

CLK

SYNC NCO

SYNC CIC

SYNC RCF

NOTE:

IN THE SLAVE MODE WITH SINGLE CHANNEL OPERATION, THE WIDTH

OF THE SYNC_NCO SHOULD BE ONE SAMPLE CLOCK CYCLE. IN DUAL

CHANNEL MODE, THE PULSEWIDTH SHOULD BE TWO SAMPLE CLOCK

CYCLES. IF A PULSE LONGER THAN SPECIFIED IS USED, THE NCO WILL

BE INHIBITED AND NOT INCREMENT PROPERLY.

Figure 34. SYNC_NCO Pin

AD6620

–21–REV. A

The frequency of the SYNC_NCO pulses, and therefore the

accuracy of the synchronization, is determined by the value of

the NCO Sync Control Register at address 302 hex. The value

in this register is the SYNC_MASK and is interpreted as a

32-bit unsigned integer. This value controls the window around

the zero crossing of the NCO output sine wave in which the

NCO will output a SYNC_NCO pulse as a master. As a slave,

the value in this register will determine the number of MSBs

of the output sine wave that are synchronized with the master.

The Master and all slaves should use the same SYNC_MASK

word. This value should almost always be written as all 1s

(FFFFFFFF hex).

Effects of A/B Input on the NCO

If the AD6620 is run in Single Channel Real mode using frac-

tional rate input timing, the A/B input is used to enable the

NCO advancement. If the A/B line is held high longer than one

clock period, the NCO will advance for each rising edge of the

CLK while A/B is high. This is not normally the desired result

and thus A/B must be taken low after the first CLK period to

prevent anomalous NCO results. See additional details under

Fractional Rate Timing.

Phase Continuous Tuning with the AD6620

For synchronization purposes, the AD6620 NCO phase is reset

each time the NCO frequency register is either written to or

read from. This is accomplished by forcing an NCO Sync to

occur. Normally, phase-continuous tuning is required on the

transmit path to control spectral leakage. On the receive path

this in not usually a constraint. However, if phase-continuous

tuning is required with the AD6620, it can be accomplished by

configuring the AD6620 as a Sync Slave. In this manner, no

internal NCO sync is generated when the NCO frequency regis-

ter is written to. If multiple AD6620s are synchronized together,

a common external sync pulse can be used to lock each of the

receivers together at the appropriate point in time. It is also

possible to reconfigure the AD6620 after the NCO frequency

Master. The next time the NCO phase cycles through 0 degrees,

the NCO sync is exerted and the chip is again synchronized.

2ND ORDER CASCADED INTEGRATOR COMB FILTER

The CIC2 filter is a fixed-coefficient, decimating filter. It is

constructed as a second order CIC filter whose characteristics

are defined only by the decimation rate chosen. This filter can

process signals at the full rate of the input port (67 MHz) in all

input modes. The output rate of this stage is given by the equa-

tion below.

SAMP

CIC

The decimation ratio, MCIC2, is an unsigned integer that may

be between 1 and 16. This stage may be bypassed under certain

conditions by setting, M

CIC2

equal to 1. For this to happen the

processing clock rate, f

CLK

must be two or more times the input

data rate, f

SAMP

. This is because the I and Q data is processed in

parallel within the CIC2 filter, and the I and Q output data is

then multiplexed through the same data pipe before it enters the

CIC5 filter.

The frequency response of the CIC2 filter is given by the follow-

ing equations.

Hz z

CIC

() –

–

=×











CIC

SAMP

CIC

()

sin

=×

































The scale factor, S

CIC2

is a programmable unsigned integer

between 0 and 6. This serves as an attenuator that can reduce

the gain of the CIC2 in 6 dB increments. For the best dynamic

range, S

CIC2

should be set to the smallest value possible (i.e.,

lowest attenuation) without creating an overflow condition.

This can be safely accomplished using the equation below, where

input_level is the largest fraction of full scale possible at the

input to this AD6620 (normally 1). The CIC2 scale factor is

not ignored when the CIC2 is bypassed.

MASKED

COUNT = 0?

SYNC

MASK

SYNC_NCO

32 32

NCO FREQ

AMPLITUDE

DITHER

COS

SIN

PHASE

DITHER

PHASE

OFFSET

PHASE

ACCUMULATOR

Figure 35. NCO Block Diagram

AD6620

–22– REV. A

S ceil M input level

OL input level

CIC CIC

CIC S

CIC

222

=×







=×

log ( _ )

The equations for calculating CIC2 output level is correct when

stage is not bypassed (normal operation). However, when by-

passed, the following equations should be used instead.

CIC2

= Input Level

The gain and pass band droop of the CIC2 should be calculated

by the equations above, as well as the filter transfer equations

that follow. If these are unacceptable, they can be compensated

for in subsequent stages.

CIC2 Rejection

The table below illustrates the amount of bandwidth in percent

of the data rate into the CIC2 stage. The data in this table may

be scaled to any allowable sample rate up to 67 MHz in Single

Channel Mode or 33.5 MHz in Diversity Channel Mode. The

table can be used as a tool to decide how to distribute the deci-

mation between CIC2, CIC5 and the RCF.

The data in this table may be scaled to any allowable sample

rate up to 67 MHz in Single Channel Mode or 33.5 MHz in

Diversity Channel Mode.

Table III. SSB CIC2 Alias Rejection Table (f

SAMP

= 1)

Bandwidth Shown in Percentage of f

SAMP

CIC2

–50 dB –60 dB –70 dB –80 dB –90 dB –100 dB

21.79 1.007 0.566 0.318 0.179 0.101

31.508 0.858 0.486 0.274 0.155 0.087

41.217 0.696 0.395 0.223 0.126 0.071

51.006 0.577 0.328 0.186 0.105 0.059

60.853 0.49 0.279 0.158 0.089 0.05

70.739 0.425 0.242 0.137 0.077 0.044

80.651 0.374 0.213 0.121 0.068 0.038

90.581 0.334 0.19 0.108 0.061 0.034

10 0.525 0.302 0.172 0.097 0.055 0.031

11 0.478 0.275 0.157 0.089 0.05 0.028

12 0.439 0.253 0.144 0.082 0.046 0.026

13 0.406 0.234 0.133 0.075 0.043 0.024

14 0.378 0.217 0.124 0.07 0.04 0.022

15 0.353 0.203 0.116 0.066 0.037 0.021

16 0.331 0.19 0.109 0.061 0.035 0.02

Example Calculations

Goal: Implement a filter with an Input Sample Rate of 10 MHz

requiring 100 dB of Alias Rejection for a ±7 kHz pass band.

Solution: First determine the percentage of the sample rate that

is represented by the pass band.

BW kHz

MHz

FRACTION

=× =100 7

007.%

Find the –100 dB column on the right of the table and look

down this column for a value greater than or equal to your

pass band percentage of the clock rate. Then look across to the

extreme left column and find the corresponding decimation

rate. Referring to the table, notice that for a decimation of 4, the

frequency having –100 dB of alias rejection is 0.071 percent

which is slightly greater than the 0.07 percent calculated. There-

fore, the maximum bound on CIC2 decimation for this condi-

tion is four. Additional decimation means less alias rejection

than the 100 dB required.

Note that although an M

CIC2

less then four would still yield the

required rejection, overall power consumption is reduced by

decimating as much as possible in this stage. Decimation in

CIC2 lowers the data rate and thus reduces power consumed in

subsequent stages.

The plot below shows the CIC2 transfer function using a deci-

mation of four. The first plot is referenced to the input sample

rate, the complex spectrum from –f

SAMP

/2 to f

SAMP

/2. The sec-

ond plot is referenced to the CIC2 output rate, the complex

spectrum from –f

SAMP2

/2 to f

SAMP2

/2. The aliases of the CIC2

can be seen to be “folding back” in toward the edge of the

desired filter pass band. It is the level of these aliases as they

move into the desired pass band that are important.

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/fSAMP

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/fSAMP2

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

Figure 36. CIC2 Alias Rejection, M

CIC2

= 4

The set of plots below show a decimation of 16 in the CIC2

filter. The lobes of the filter drop as the decimation rate

increases, but the amplitudes of the aliased frequencies increase

because the output rate has been reduced.

AD6620

–23–REV. A

The frequency response of the filter is given by the following

equations. The gain and pass band droop of CIC5 should be

calculated by these equations. Both parameters may be compen-

sated for in the RCF stage.

Hz z

CIC

() –

–

=×











CIC

SAMP

CIC

()

sin

=×

































The scale factor, S

CIC5

is a programmable unsigned integer

between 0 and 20. It serves to control the attenuation of the

data into the CIC5 stage in 6 dB increments. For the best dynamic

range, S

CIC5

should be set to the smallest value possible (lowest

attenuation) without creating an overflow condition. This can

be safely accomplished using the equation below, where OL

CIC2

is the largest fraction of full scale possible at the input to this

filter stage. This value is output from the CIC2 stage then pipe-

lined into the CIC5. S

CIC5

is ignored when this filter is bypassed

by setting M

CIC5

= 1.

S ceil M OL

CIC CIC CIC

CIC

SCIC

CIC

525

=×

()

log ( ) –

when CIC5 is bypassed;

OL OL

CIC CIC52

The output rate of this stage is given by the equation below.

SAMP

CIC

≤

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/fSAMP

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/f

SAMP2

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

Figure 37. CIC2 Alias Rejection, M

CIC2

= 16

5TH ORDER CASCADED INTEGRATOR COMB FILTER

The third signal processing stage, CIC5, implements a sharper

fixed-coefficient, decimating filter than CIC2. The input rate to

this filter is f

SAMP2

. The maximum input rate is given by the

equation below. N

equals two for Diversity Channel Real

input mode; otherwise N

equals one. In order to satisfy this

equation, M

CIC2

can be increased, N

can be reduced, or f

CLK

can be increased (reference fractional rate input timing described

in the Input Timing section).

SAMP

CLK

≤×

The decimation ratio, M

CIC5

, may be programmed from 1 to 32

(all integer values). When M

CIC5

= 1, this stage is bypassed and

the CIC5 scale factor is ignored.

AD6620

–24– REV. A

This table helps to calculate an upper bound on decimation,

CIC5

, given the desired filter characteristics.

The plots following (Figure 38) represent the CIC5 transfer

function with respect to the CIC5 output rate for a decimation

of 4. The first plot is referenced to the input sample rate and

shows the complex spectrum from –f

SAMP/

2 to +f

SAMP

/2. The

second plot is referenced to the CIC5 output rate; the complex

spectrum ranges from –f

SAMP5

/2 to +f

SAMP5

/2. Aliased images in

CIC5 “fold back” toward the edge of the desired filter pass

band. It is the level of these aliases as they move into the desired

pass band that are of concern. The improved roll-off of CIC5

over CIC2 can be seen when these plots are compared to those

previously shown for CIC2.

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/f

SAMP

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/f

SAMP5

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

Figure 38. CIC5 Alias Rejection, M

CIC5

= 4

CIC5 Rejection

The table below illustrates the amount of bandwidth in percent-

age of the clock rate that can be protected with various decimation

rates and alias rejection specifications. The maximum input rate

into the CIC5 is 32.5 MHz. As in the previous table, these

are the 1/2 bandwidth characteristics of the CIC5. Note that

the CIC5 stage can protect a much wider band than the CIC2 for

any given rejection.

Table IV. SSB CIC5 Alias Rejection Table (f

SAMP2

= 1)

Bandwidth Shown in Percentage of f

SAMP2

CIC5

–50 dB –60 dB –70 dB –80 dB –90 dB –100 dB

210.227 8.078 6.393 5.066 4.008 3.183

37.924 6.367 5.11 4.107 3.297 2.642

46.213 5.022 4.057 3.271 2.636 2.121

55.068 4.107 3.326 2.687 2.17 1.748

64.267 3.463 2.808 2.27 1.836 1.48

73.68 2.989 2.425 1.962 1.588 1.281

83.233 2.627 2.133 1.726 1.397 1.128

92.881 2.342 1.902 1.54 1.247 1.007

10 2.598 2.113 1.716 1.39 1.125 0.909

11 2.365 1.924 1.563 1.266 1.025 0.828

12 2.17 1.765 1.435 1.162 0.941 0.76

13 2.005 1.631 1.326 1.074 0.87 0.703

14 1.863 1.516 1.232 0.998 0.809 0.653

15 1.74 1.416 1.151 0.932 0.755 0.61

16 1.632 1.328 1.079 0.874 0.708 0.572

17 1.536 1.25 1.016 0.823 0.667 0.539

18 1.451 1.181 0.96 0.778 0.63 0.509

19 1.375 1.119 0.91 0.737 0.597 0.483

20 1.307 1.064 0.865 0.701 0.568 0.459

21 1.245 1.013 0.824 0.667 0.541 0.437

22 1.188 0.967 0.786 0.637 0.516 0.417

23 1.137 0.925 0.752 0.61 0.494 0.399

24 1.09 0.887 0.721 0.584 0.474 0.383

25 1.046 0.852 0.692 0.561 0.455 0.367

26 1.006 0.819 0.666 0.54 0.437 0.353

27 0.969 0.789 0.641 0.52 0.421 0.34

28 0.934 0.761 0.618 0.501 0.406 0.328

29 0.902 0.734 0.597 0.484 0.392 0.317

30 0.872 0.71 0.577 0.468 0.379 0.306

31 0.844 0.687 0.559 0.453 0.367 0.297

32 0.818 0.666 0.541 0.439 0.355 0.287

AD6620

–25–REV. A

The maximum number of taps this filter can calculate, N

TAPS

, is

given by the equation below. The value N

TAPS

minus 1 is writ-

ten to the AD6620 internal address space at address 30C hex.

The decimation ratio of this filter, M

RCF

, may be programmed

from 1 to 32. The input rate into the RCF is f

SAMP5

. N

is equal

to two for Diversity Channel Real Input mode; otherwise N

= 1.

TAPS

CLK RCF

SAMP

≤











min ,

256

The RCF coefficients are located in addresses 0x000 to 0x0FF

and are interpreted as 20-bit twos complement numbers. When

writing the coefficient RAM, the lower addresses will be multi-

plied by relatively older data from the CIC5 and the higher

coefficient addresses will be multiplied by relatively newer data

from the CIC5. The coefficients need not be symmetric and the

coefficient length, N

TAPS

, may be even or odd. If the coefficients

are symmetric, then both sides of the impulse response must be

written into the coefficient RAM.

The RCF stores the data from the CIC5 into a 256 × 36 RAM.

256 × 18 is assigned to I data and 256 × 18 is assigned to Q data.

The RCF uses the RAM as a circular buffer, so that it is difficult

to know in which address a particular data element is stored. To

avoid start-up transients due to undefined data RAM values, the

data RAM should be cleared upon initialization. The RCF

utilizes the number of data RAM locations equal to N

TAPS

× N

rounded up to the nearest even number, starting from address

0x100, so these are the only values that need be cleared.

When the RCF is triggered to calculate a filter output, it starts

by multiplying the oldest value in the data RAM by the first

coefficient (located by the RCF

OFF

This value is accumulated with the products of newer data words

multiplied by the subsequent locations in the coefficient RAM

until the coefficient address RCF

OFF

+ N

TAPS

–1 is reached.

Table V. Three-Tap Filter

Coefficient Address Impulse Response Data

0 h(0) n(0) Newest

1 h(1) n(1)

2 (N

TAPS

– 1) h(2) n(2) Oldest

The output rate of this filter is determined by the output rate of

the CIC5 stage and MRCF.

SAMPR

SAMP

RCF

RCF Coefficient Address Offset

This register at address 30b hex allows the AD6620 to hold

multiple filters in the RAM. However, the sum of the taps

required may not exceed 256 divided by the number of chan-

nels. The RCF will compute the filter from RCF_OFFSET to

(RCF_OFFSET + N

TAPS

). A single access can then be used to

select which of the filters is used without requiring coefficients

be rewritten.

The set of plots below (Figure 39) represents a decimation of 32

in the CIC5 filter. It can be seen that the lobes of the filter drop

as the decimation rate increases, but the aliased frequencies

increase due to the reduction of the output rate.

–120

–100

–80

–60

–40

–20

dBFS

f/f

SAMP

–0.3 –0.2 –0.1 0.1 0.2 0.30

–0.5

–120

–100

–80

–60

–40

–20

dBFS

f/f

SAMP5

–0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.50

Figure 39. CIC5 Alias Rejection, M

CIC5

= 32

RAM COEFFICIENT FILTER

The final signal processing stage is a sum-of-products decimat-

ing filter with programmable coefficients. Figure 40 shows a

simplified block diagram. The data memories I-RAM and Q-RAM

store the 256 most recent complex samples from the previous filter

stage with 18-bit resolution. The coefficient memory, C-RAM,

stores up to 256 coefficients with 20-bit resolution. On each

CLK cycle one tap for I and one tap for Q is calculated using

the same coefficients. The I and Q accumulators provide 3 bits

of headroom. This headroom allows the output of the RCF filter

to contain 23 significant bits.

IIN

QIN

IOUT

QOUT

256ⴛ18b

I-RAM

256ⴛ20b

C-RAM

256ⴛ18b

Q-RAM

Figure 40. RAM Coefficient Filter Block Diagram

AD6620

–26– REV. A

RCF Output Scale Factor

The scale factor associated with the RCF, S

OUT

, behaves differ-

ently than the scale factors in the CIC stages. This scalar, at the

RCF output, controls the weight of the 16-bit output data going

to the parallel port or to the serial port when using 16-bit words.

OUT

determines which of the 23 RCF output bits are used

based on the equation below. OL

RCF

is the 23-bit RCF output

data; POL represents the output port data. POL is rounded to

the 16 bits desired. The weight of the rounding is adjusted by

OUT

. When the serial port is used with 24-bit or 32-bit words,

OUT

is ignored.

POL round OL

bits RCF S

OUT

=×

16 4

2()

(–)

Another way to consider the effects of the RCF Output Scale

factor is discussed here. If both CIC scalars follow the previous

recommendations, the following chart can be used to determine

what value to use for the RCF scale factor. In order to determine

this, the “gain” of the impulse response must first be determined.

This can be done by integrating the coefficients used for the

RCF filter remembering to normalize the values against the full-

scale input range of 2

There are several possibilities when setting the “gain” of the

RCF coefficients. Following these guidelines will preserve at

least three bits in the sum of products registers.

(

)

∑

; 0 dB dc gain in RCF filter. Numeric wraparound

very unlikely. The RCF Scale factor should be nominally

set to 4.

(

)

≤

∑

; slight loss in RCF filter. Numeric wraparound

is impossible. The RCF Scale factor should be nominally

set to 4.

hn m

(

)

∑

; where the absolute value of m is a number less

than 1 and is scaled to account for losses elsewhere in the

system, such as conversion gain errors, attenuator losses or

CIC scaling errors. The gain should be scaled down to avoid

wraparound in the RCF process, however, then the RCF

Scale Factor can be adjusted up to increase the signal level.

The value of m can also be negative to account for an inver-

sion through an amplifier. The RCF Scale factor should be

set where needed to produce the desired full-scale results

with a fully loaded receiver input signal.

The RCF Scale factor has the effect as shown in the following

table. Each successive gain step doubles or halves the overall

gain of the stage. Overall gain through the RCF stage is the

cascaded gain of the RCF Scale factor shown below and the

RCF coefficient gain discussed previously.

Table VI.

RCF Gain RCF Scale Factor (Address 309h)

1/8 7

1/4 6

1/2 5

16 0

Gain through the RCF of the AD6620 is thus:

Gain Gain

coefficients RCF

Unique B Operation

Unique B works in conjunction with dual channel mode. In this

mode, both the A and B channels can have different FIR coeffi-

cients. This can prove useful in many applications where each

signal path has known differences. Another option is that FIR

gain for one path could be different than the other. During

diversity selection, one path could be tailored for weak signals

and the other for strong signals, providing extra dynamic range.

To use the Unique B mode, set Bit 3 high in register 309h. This

will cause the internal state machine to use a different set of

coefficients for the B channel than the A. With Bit 3 set low for

normal operation, the FIR coefficient index is incremented only

after both the A and B channels are computed. However when

this bit is set high, the index is incremented after each A channel

and B channel computation. Therefore, filters are computed nor-

mally. When downloaded to the AD6620, they should be inter-

leaved with the A channel terms occupying the even RCF

Coefficient locations and the B channel terms occupying the

odd locations. Both filters must be the same length and fit in the

allocated memory space.

With Unique B set to ‘0,’ the following table illustrates how the

coefficients are distributed.

Table VII.

Coefficient Address

W(0) 0

W(1) 1

W(2) 2

W(3) 3

……

With Unique B set to ‘1,’ the following table illustrates how the

coefficients are distributed.

Table VIII.

Coefficient Address

Wa(0) 0

Wb(0) 1

Wa(1) 2

Wb(1) 3

……

AD6620

–27–REV. A

If the AD6620s to be synchronized have identical decimation,

then latency through the filter stages will be matched and output

data rates for the Sync master’s filter stages will match the cor-

responding filter stages of the slave.

SYNC M/S

MASTER

SLAVE

SYNC CIC

SYNC RCF

MASTER

SLAVE

Figure 42. SYNC_CIC, SYNC_RCF Pins

The three SYNC inputs to the control block originate from the

same three bidirectional pads from which the three SYNC out-

puts are driven. When the AD6620 is a SYNC MASTER, the

internal circuitry that generates the SYNC pulse outputs is

enabled to the pads. When the AD6620 is a SYNC SLAVE, the

internally produced SYNC pulses are three-stated, and the pads

are driven from an external input. The capacitance on these pins

must be closely monitored since the master responds to the

same SYNC pulse as the slave (its own pulse). There is no input

requirement to the relative phases of these SYNC pulses. In the

absence of SYNC pulses each state machine will free run so the

latter decimation filters can be reliably synchronized by the

SYNC pulses of an earlier stage. However, when sync pulses are

provided externally, setup-and-hold times must be met for each

respective input.

CONTROL REGISTERS AND ON-CHIP RAM

The AD6620 provides a choice of two control ports. It has an

8-bit generic microprocessor port that is used for configuring

the device at boot up and dynamically reconfiguring the AD6620

in the system. It also has a synchronous serial port that can also

dynamically reconfigure the AD6620 for the desired system

operation. All control registers are available from both the serial

port and the microprocessor port. These control methods are

nonexclusive and the two ports can be used simultaneously. If

simultaneous access occurs, the serial port is given precedence

over the microprocessor port unless a micro cycle is already

under way. The microprocessor port deasserts the RDY signal

and waits until the serial access is completed for Mode 0. The

microprocessor port does not assert DTACK for Mode 1 until

the serial access is completed.

Filter Phase Synchronization

Like the NCO, the AD6620 filter stages have phase synchroni-

zation circuitry enabling multiple AD6620s to be used in appli-

cations such as diversity antennas and phased array systems.

For any f

SAMP

, there are M

CIC2

possible phases of f

SAMP2

at the

output of the CIC2 stage. Similarly, at the output of the CIC5

stage, there are M

CIC5

possible phases of f

SAMP5

. This means that

at the output of the CIC stages there is already M

CIC2

× M

CIC5

possible phases of the filtered data. Additional phase uncertainty

is introduced by decimation done in the RCF. At the output of

the AD6620 there are a total of M

CIC2

× M

CIC5

× M

RCF

possible

output phases of the data.

In diversity systems using multiple AD6620s, it is necessary to

ensure that the output of each AD6620 in the system is in phase.

A variety of system issues (e.g., not bringing the AD6620s on

line at the same time, excessive digital noise) could cause the

AD6620s to start out-of-phase or to drift out-of-phase as the

system runs. To achieve output phase coherence in such systems

the SYNC_CIC and SYNC_RCF pins are provided.

The function of these pins is controlled by the SYNC_M/S bit

in the Mode Control Register at address 300 hex of internal

address space. When the SYNC_M/S bit is high, SYNC_CIC

and SYNC_RCF provide synchronization pulses on the rising

edge of CLK. When the SYNC_M/S bit is low, SYNC_CIC and

SYNC_RCF accept external synchronization pulses sampled on

the rising edge of clock. This pulse edge synchronizes the CIC2,

CIC5 and RCF filter stages of all AD6620 in the chain.

Below is an example of the output SYNC pulse waveforms.

The SYNC_NCO pulse is not shown and is described in the

preceding NCO Synchronization section. Each SYNC_RCF

output pulse is concurrent with a SYNC_CIC pulse. The

SYNC_RCF output pulse can be connected to the SYNC_CIC,

and SYNC_RCF inputs of another AD6620 to achieve full

decimation synchronization.

CLK

SYNC CIC

SYNC RCF

Figure 41. SYNC Output Pulses

In the example above, M

CIC2

= 3, and M

CIC5

= 1 as evidenced

by the SYNC_CIC pulses that occur every 3 CLK cycles

CIC2

× M

CIC5

). M

RCF

= 3, resulting in SYNC_RCF pulses

that are one third as frequent as the SYNC_CIC pulses. In this

example full rate input timing is employed such that the input

data rate equals the clock rate.

AD6620

–28– REV. A

Table IX. Control Register and RAM Addresses

Address Bit Width Name Notation Description

000–0FF 20 RCF Coefficient RAM RCF Coefficient RAM

100–1FF 36 RCF Data RAM RCF Data RAM

200–27F 0 Reserved Reserved

300 8 MODE CONTROL REGISTER 0: SOFT_RESET

1: Diversity Channel Real Input Mode

2: Single Channel Complex Input Mode

3: Sync Master/Slave

(Master = 1,

Slave = 0)

7–4: Reserved

301 3 NCO CONTROL REGISTER 0: NCO Bypass (Bypass = 1, Active = 0)

1: Enable Phase Dither

2: Enable Amplitude Dither

7–3: Reserved

302 32 NCO SYNC CONTROL REGISTER SYNC_MASK Write: Sync Mask Shadow

Read: Sync Mask

303 32 NCO_FREQ NCO_FREQ Channel Frequency for NCO Tuning

304 16 NCO PHASE_OFFSET NCO Phase Offset

305 8 INPUT/CIC2 SCALE REGISTER 2–0: S

CIC2

3: Reserved

4: ExpInv

7–5: : ExpOff

306 8 M

CIC2

– 1M

CIC2

– 1 CIC2 Decimation Minus One

307 5 CIC5 SCALE REGISTER S

CIC5

4–0: S

CIC5

7–5: Reserved

308 8 M

CIC5

– 1M

CIC5

– 1 CIC5 Decimation Minus One

309 4 OUTPUT/RCF CONTROL REGISTER S

OUT

2–0: Output Scale Factor

3: Unique B Flag (Normal Mode = 0,

Unique B Mode = 1)

7–4: Reserved

30A 8 M

RCF

– 1M

RCF

– 1 RCF Decimation Minus One

30B 8 RCF ADDRESS OFFSET REGISTER RCF

OFF

Filter Coefficient Address Offset

30C 8 N

TAPS

– 1N

TAPS

– 1 Number of Taps Minus One

30D 8 Reserved (Should Be Written 0) Reserved (Should Be Written 0)

NOTES

This bit is set high on RESET. The chip is held into SOFT_RESET until it is written low.

This bit is set low on RESET. This keeps multiple AD6620 SYNC Masters from driving each other.

outputs while acting as a sync master, or as inputs while acting

as a sync slave. This is the only register with a defined power-up

state: on power-up, Bit 0 will be at a Logic “1.” This places the

chip in SOFT_RESET and defines the chip as a sync slave.

Powering up as a sync slave avoids contention problems when

connecting multiple AD6620s.

If Bit 0 is written low and Bits 2 and 1 are low, the AD6620 is in

Single Channel Real Mode. If Bit 1 is high and Bits 0 and 2 are

low, the device is in the Diversity Channel Real Mode. If Bit 2

is high and Bits 0 and 1 are low, the chip is in the Single Chan-

nel Complex Mode. Setting Bit 3 high configures the AD6620

as a SYNC master; the SYNC pins are then used as outputs. If

Bit 3 is low, it is a SYNC slave and the SYNC pins function

as inputs. Bits 7–4 are reserved and should be written low.

(0x301) NCO Control Register

This register allows control of special features of the NCO. If

Bit 0 of this register is high the NCO of the AD6620 is by-passed.

Both the I data and the Q data that are passed through the chip

will be the same and the Spectrum will not be translated. In

bypass the input data is attenuated by 12 dB.

(0x000–0xFF) RCF Coefficient RAM

Memory that stores user-programmable coefficients for the RCF

filter. The RAM will hold 256 20-bit twos complement words

for a maximum filter length of 256 taps. In Diversity Channel

Real Mode the filter length is limited to 128 taps per channel.

The number of taps used is controlled by N

TAPS

–1 (30C) regard-

less of the number of coefficient locations programmed. If filter

size allows, more than one filter can be resident in the memory

at a time. This makes it possible to switch filters without reloading

all of the coefficients.

(0x100–0x1FF) RCF Data RAM

These locations store I and Q data exiting the CIC5 filter stage

while the RCF performs multiply accumulates. The lower 18

bits of the 36-bit location is I data; the upper 18 bits are Q data.

These locations are addressed in memory and are available via

the control ports so that the data RAM can be flushed for test-

ing and simulation purposes. They are not cleared on reset.

(0x300) Mode Control Register

This location brings the chip out of reset and sets the operating

mode. It also specifies how the chip will use its SYNC pins: as

AD6620

–29–REV. A

(0x309) Output/RCF Control Register

Bits 2-0 of this register hold the Output Scale Factor, S

OUT

These bits are interpreted as a 3-bit unsigned integer, the value

of which controls which of the 23 output bits of the RCF are

passed to the output port being used. The data output corre-

sponds to the following equation where OL

RCF

is the 23-bit

output of the RCF and POL is the 16-bit data available at the

parallel output port or the serial port when 16-bit serial words

are used. The truncation function rounds the scaled 23-bit

number to 16 bits. S

OUT

is ignored when WL is 24 or 32 bits. In

most applications, this register should be set to 4 as an initial

starting value.

POL round OL

bits RCF

OUT

=×

2()

(–)

For additional details on determining RCF gain, see the RCF

Output Scale Factor section.

Bit 3 of this register is used to control the Unique B feature of

the chip. When written low, the normal mode, the chip uses the

same FIR coefficients for both the A and B channels. However,

when the bit is set high, different coefficients are used for the A

and B channels. When Unique B mode is selected, the filter

coefficients should be interleaved with the A channel terms

occupying the even RCF Coefficient locations and the B chan-

nel terms occupying the odd locations.

Bits 7–4 of this register are reserved and must be written 0.

(0x30A) (M

RCF

– 1)

This register controls the amount of decimation in the RCF

filter stage. The value contained in this register is the RCF

decimation rate minus one. This is interpreted as an unsigned

8-bit integer, but due to limited number of taps and, therefore,

filtering power in the RCF filter accumulators this value should

be limited to 31 (decimation = 32).

(0x30B) RCF Address Offset Register

This register controls the address offset used by the RCF to

calculate a given filter and is interpreted as an 8-bit unsigned

integer. It allows more than one filter to be placed in the Coeffi-

cient RAM. This makes it possible to switch filters without

reloading all of the coefficients. The RCF filter will compute

taps for all coefficients between RCF

OFF

and (RCF

OFF

+ N

TAPS

)

provided that the decimation, CLK rate and input data rate

provide sufficient time for this.

(0x30C) (N

TAPS

– 1)

This register controls the number of taps calculated by the RCF.

The value in this register is interpreted as an unsigned integer

and is equal to the number of taps desired minus one. This filter

is not inherently symmetric and the number of coefficients placed

in the Coefficient RAM will be equal to the number of taps,

provided that only one filter at a time is loaded. No symmetry is

assumed and preaddition is not used. The total number of taps

for all filters must be less than 256 taps for Single Channel

Real mode, or less than 128 taps/channel for Diversity Channel

Real mode.

(0x30D) Reserved

Reserved, but must be written 0 for correct operation.

The NCO has two features to improve the performance of some

systems: Phase Dither and Amplitude Dither. These can be used

together or alone. If Bit 1 of the register is high, Phase Dither is

activated. If Bit 2 is high, Amplitude Dither is activated. For

more information on dither refer to the NCO section.

(0x302) NCO SYNC Control Register

This holds the SYNC_MASK, which controls the frequency of

the SYNC_NCO pulses and therefore the phase accuracy of the

synchronization. See the NCO section for details.

(0x303) NCO_FREQ

This register holds the NCO frequency control word as described

in the NCO section. This is a 32-bit unsigned integer that sets

the frequency of the AD6620 NCO.

(0x304) NCO PHASE_OFFSET

This register controls the phase offset of the NCO. It is also

described in detail in the NCO section and can be used to allow

for phase differences between multiple antennas receiving the

same carrier.

(0x305) INPUT/CIC2 Scale Register

This register holds the scale factor, S

CIC2

, for CIC2. S

CIC2

scales

down the data before it is accumulated in CIC2. This avoids

eliminates the resulting spectral errors. S

CIC2

is contained in Bits

2–0 of this register. It is treated as an unsigned integer between

0 and 6. Increasing S

CIC2

shifts data down. For more details

refer to the section on the CIC2 filter.

The second function of this register is to scale the input data

from the Parallel Data Input port. This allows the AD6620 to

treat the floating point input data with considerable flexibility.

There are two parts of this function. The first is Bit 4, which

tells the AD6620 how to handle the exponent, EXP[2:0]. If this

bit is low, data is shifted down as the exponent increases. If this

bit is high, then for increasing EXP[2:0] the input data is shifted

up. The second part of the input data shifting is the Exponent

Offset(ExpOff[7 . . 5]) held in Bits 7–5 of this register. This pro-

vides gain to the input data as described in the Input Port section.

(0x306) (M

CIC2

– 1)

This register controls the amount of decimation in the CIC2 filter

stage. The value contained in this register is the CIC2 decima-

tion rate minus one. This is interpreted as an unsigned 8-bit

integer but due to limited growth in the CIC2 filter accumula-

tors this value should be limited to 15 (decimation = 16).

(0x307) S

CIC5

This register holds the scale factor, S

CIC5

, for CIC5. S

CIC5

scales

down the data before it is accumulated in CIC5. This avoids

nates the resulting spectral errors. S

CIC5

is contained in Bits 4–0

of this register. It is treated as an unsigned integer between 0

and 20. Increasing S

CIC5

shifts data down. For more details refer

to the section on the CIC5 filter.

(0x308) (M

CIC5

– 1)

This register controls the amount of decimation in the CIC5

filter stage. The value contained in this register is the CIC5

decimation rate minus one. This is interpreted as an unsigned

8-bit integer, but due to limited growth in the CIC5 filter accu-

mulators this value should be limited to 31 (decimation = 32).

AD6620

–30– REV. A

PROGRAMMING THE AD6620

Initializing the AD6620

Before the AD6620 can be used to down convert and filter the

channel of interest it must be configured for the job. First the

RESET pin should be pulsed low for a minimum of 30 ns and

should then be returned high. This HARD_RESET of the

AD6620 clears the CIC Accumulators as well as the NCO

Phase Accumulator. When RESET is brought high the AD6620 is

removed from the HARD_RESET condition. The AD6620 is

now in SOFT_RESET. In this state the Mode Control Register

at address 0x300 contains a “1” (Bit 0 is high). When the AD6620

is in SOFT_RESET, no data is accepted by the input data port

and no processing occurs. The serial port and parallel output

port is held inactive and the chip is defined as a SYNC slave to

avoid possible contentions on these pins. While the AD6620 is

in this condition it should be programmed by the process below.

It should be noted that this initialization must be performed via

the microprocessor port since the serial port is inactive.

1. If the AD6620 is being reinitialized without performing a

HARD_RESET, then address 0x300 should be written 1 to

place the AD6620 in SOFT_RESET. This allows the non-

dynamic registers to be programmed.

2. Program the Coefficient RAM of the AD6620 with the

desired FIR Filter. The address auto-increment feature can

be used to decrease the amount of time required to program

the Coefficients. This feature is described in detail in the

Microport Control section that follows.

3. (Optional) The first piece of data out of the AD6620 is always

zero due to an output pipeline delay. There will also be a

start-up glitch on the output of the AD6620 due to possible

nonzero data in the I and Q data RAMS of the RCF filter.

These RAMS are not initialized by the HARD_RESET. If

this is a concern then the data RAMS should now be written

to zero. For efficiency the auto-increment feature can be

used as with the programming of the coefficient RAMs.

4. The Configuration Registers of the AD6620 are now pro-

grammed. First, address 0x300 should be written to set the

Operating Mode if Diversity Channel Real or Single Channel

Complex Modes are used. Bit 0 of this register should remain

high at this time. This will hold the SOFT_RESET condition.

The remaining configuration registers can now be programmed.

This should start at address 0x301 and continue to address

0x30D. This defines the operation of the NCO and filter stages.

5. The AD6620 is now ready to be removed from SOFT_RESET

and allowed to process data. This is done by writing address

0x300 to again set the desired mode of operation. This loca-

tion should be set for SYNC MASTER or SYNC SLAVE

operation at this time. Bit 0 of this register is written low at

this time to remove the SOFT_RESET condition.

Dynamic Programming of the AD6620

Many attributes of the AD6620 may be altered dynamically as

the AD6620 processes the received data. This allows the receiver

to be adjusted during operation in order to achieve the maxi-

mum performance. The typical dynamic registers of the AD6620

are listed in the following table. To program the other registers

follow the steps described in the section titled Initializing the

AD6620. Technically all registers can be programmed dynami-

cally, but adverse results may occur if registers other those listed

are written dynamically.

These addresses may be programmed via either the Micropro-

cessor or the Serial Control Ports.

Table X. Dynamic AD6620 Registers

Address Bit Width Name

302 32 NCO SYNC CONTROL REGISTER

303 32 NCO_FREQ

304 16 NCO PHASE_OFFSET

305 8 INPUT/CIC2 SCALE REGISTER

307 5 CIC5 SCALE REGISTER

309 4 OUTPUT/RCF CONTROL REGISTER

30B 8 RCF ADDRESS OFFSET REGISTER

Registers 0x302, 0x303 and 0x304 allow the NCO of the AD6620

to be adjusted. The tuning frequency can be dynamically changed

for frequency hopping. The phase of the carrier can be adjusted

with address 0x304. The phase accuracy of the synchronization

can be changed with 0x302. Registers 0x305, 0x307, and 0x309

allow the user to dynamically control the gain of the AD6620 in

6 dB increments. This can be used to maximize the AD6620s

dynamic range for the signal being tuned at a particular instant.

spectral estimation.

In addition to dynamically writing to these registers, they may

also be read to verify program content. Care should be taken,

however, because reading some registers may affect normal chip

operation. In particular, reading from 303h the NCO frequency

will cause the phase accumulator to be reset via the SYNC_NCO

pulse if the AD6620 is running as a Sync master. If the device is

run as a Sync slave, then the phase accumulator is not reset.

Addresses 000h through 1FFh should not be read dynamically

as doing so will disrupt the internal state machine computing

the FIR taps. These locations may be read statically if needed.

AD6620

–31–REV. A

ACCESS PROTOCOLS

The AD6620 external accesses may be performed through either

the Microprocessor Port or the Serial Port. The Microport and

the serial port both use a three-bit address and eight-bit data to

access these registers. The three-bit address provides access to

seven register locations (External Interface Registers). These

the AD6620 shown in the Control Register section. The seven

registers are the LAR (Low Address Register), the AMR (Address

Mode Register), and the five data registers (DR4–DR0).

Table XI. External Interface Registers

A[2:0] Name Comment

000 Data Register 0 (DR0) D[7:0]

001 Data Register 1 (DR1) D[15:8]

010 Data Register 2 (DR2) D[23:16]

011 Data Register 3 (DR3) D[31:24]

100 Data Register 4 (DR4) D[35:32]

101 Reserved Reserved

110 Low Address Register (LAR) A[7:0]

111 Address Mode Register (AMR) 1-0: A[9:8]

5-2: Reserved

6: Read Increment

7: Write Increment

The internal address space is accessed using a 10-bit internal

address. Many of these address locations are more than a byte

wide and require multiple accesses to the seven External Inter-

face Registers, which are each only 8 bits wide (only 4 bits of

DR4 are used). Accesses to these registers are accomplished

using the 3-bit address and 8-bit data lines the manner described

below. The source of these values depends on the control port

method used.

All internal accesses are accomplished by first writing the inter-

nal address of the register or memory location to be accessed.

The lower eight address bits are written to the LAR register

and the upper two address bits to the LSBs of the AMR. This

defines the internal address of the location to be accessed as

shown in the memory map shown in the Control Registers and

On-Chip RAM section.

Internal Write Access

Up to 36 bits of data (as needed) can be written by the process

described below. Any high order bytes that are needed are writ-

ten to the corresponding data registers defined in the external

3-bit address space. The least significant byte is then written to

DR0 at address (000). When a write to DR0 is detected, the

internal microprocessor port state machine then moves the data

in DR4–DR0 to the internal address pointed to by the address

in the LAR and AMR.

Write Pseudocode

void write_micro(ext_address, int data);

main();

{

/* This code shows the programming of the NCO frequency

variable address is the External Address A[2:0] and data is the

value to be placed in the external interface register. The NCO

// holding registers for NCO byte wide access data

int d3, d2, d1, d0;

// NCO frequency word (32-bits wide)

NCO_FREQ = 0xCBEFEFFF;

// write AMR

write_micro(7, 0x03);

// write LAR

write_micro(6, 0x03);

// DR4 is not needed because NCO_FREQ is only 32-bits, not

// write DR3 with high byte of 32 bit word (D[31:24]

d3 = (NCO_FREQ & 0xFF000000) >> 24;

write_micro(3, d3);

// write DR2 with high byte of 32 bit word (D[23:16]

d2 = (NCO_FREQ & 0xFF0000) >> 16;

write_micro(2, d2);

// write DR1 with D[15:8]

d1 = (NCO_FREQ & 0xFF00) >> 8;

write_micro(1, d1);

// write DR0 with D[7:0]

// Writing to DR0 causes all data to be transferred to the

internal address.

//Therefore, DR1, DR2 and DR3 should already be written

d0 = NCO_FREQ & 0xFF;

write_micro(0, d0);

} // end of main

Internal Read Access

A read is performed by first writing the LAR and AMR as with a

write. The data registers (DR4–DR0) are then read in the reverse

order that they were written. First, the least significant byte of

the data (D[7:0]) is read from DR0. On this transaction the

high bytes of the data are moved from the internal address

pointed to by the LAR and AMR into the remaining data regis-

ters (DR4–DR1). This data can then be read from the data

registers using the appropriate 3-bit addresses. The number of

data registers used depends solely on the amount of data to be

read or written. Any unused bit in a data register should be

masked out for a read.

AD6620

–32– REV. A

Read Pseudocode

int read_micro(ext_address);

main();

{

/* This code shows the reading of the NCO frequency register

using the read_micro function as defined above. The variable

address is the External Address A[2..0] and data is the value to

be placed in the external interface register. The NCO register is

located at Internal Address = 0x303.

// holding registers for NCO byte wide access data

int d3, d2, d1, d0;

// NCO frequency word (32-bits wide)

// write AMR

write_micro(7, 0x03 );

// write LAR

write_micro(6, 0x03);

/* read D[7:0] from DR0, All data is moved from the Internal

Registers to the interface registers on this access. Reading

should be initiated with a read from DR0. Therefore, DR1,

DR2 and DR3 can be read after DR0 */

d0 = read_micro(0) & 0xFF;

// read D[15:8] from DR1

d1 = read_micro(1) & 0xFF;

// read D[23:16] from DR2

d2 = read_micro(2) & 0xFF;

// read D[31:24] from DR3

d3 = read_micro(3) & 0xFF;

// DR4 is not needed because NCO_FREQ is only 32-bits

// Assemble 32-bit NCO_FREQ word from the 4 byte

components

NCO_FREQ = d0 + (d1 << 8) + (d2 << 16) + (d3 << 24);

} // end of main

Auto Increment Feature

To increase throughput, an auto increment feature is provided.

This feature is controlled by Bits 6 and 7 of the AMR. If these

bits are set to 00, the address remains the same after an internal

access. If set to 01, the address is incremented after a read access

has been performed. If set to 10, the address is incremented

after a write access is performed. If set to 11, the address is incre-

mented after each access, read or write. This allows the AD6620

to be initialized in a much shorter time since the access to the

LAR and AMR must occur only once to initialize or read-back

the entire device.

MICROPORT CONTROL

External reads and writes are accomplished in one of two modes

via the Microprocessor Port. The CS, RD (DS), RDY (DTACK),

WR (R/W) and MODE pins are used to control the access. The

specific function of these pins depends on whether the access is

MODE 0 or MODE 1. The Mode 1 signal names are those

listed on the pinout. The access mode is controlled by the

MODE input as described in the following sections.

Table XII. Microprocessor Control Signals

MODE 0 MODE 1

A[2:0] (Address Lines) A[2:0] (Address Lines)

D[7:0] (Data Lines) D[7:0] (Data Lines)

CS (Chip Select) CS (Chip Select)

RD (Read Strobe) DS (Data Strobe)

WR (Write Strobe) R/W (Read/Write Select)

RDY (Ready Signal) DTACK (Data Acknowledge)

MODE (Mode Select) MODE (Mode Select)

The Microport is synchronous with the master clock (CLK) of

the AD6620, but the interface is not required to be. If the speed

of the interface is significantly slower than CLK, synchronicity

should not be an issue. If the interface is relatively fast com-

pared to CLK, the user may need to synchronize the Microport

to CLK or add wait states to the controlling processor. The

timing diagrams show the relationship of the control signals to

clock and the user should use these as a guide to implement a

Microport interface.

AD6620

–33–REV. A

Mode = 0

If MODE is low during the access, the interface is in Mode 0.

In Mode 0 the CS, RD and the WR lines control the access

type. While an access is being performed, or if the serial port

DATA VALID

tHC

tSC

tHC

tSC

tHM

ADDRESS VALID

N N+1 N+2 N+3 N*

CLK1

WR2

RD2

CS3

D[7:0]

RDY

A[2:0]

NOTES:

1 RDY IS DRIVEN LOW ASYNCHRONOUSLY BY WR AND CS GOING LOW AND RETURNS HIGH ON THE

2 THESE SIGNALS (R/W AND DS) MAY REMAIN IN LOW STATE TO CONTINUE WRITING DATA.

3 CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+3 SHOWN) TO COMPLETE WRITE.

*THE NEXT WRITE MAY BE INITIATED ON CLK, N.

tHA

tRDYHtRDYL

tSAM

RISING EDGE OF CLK "N+2".

Figure 44. Mode 0 Write (MODE = GND)

is accessing the chip, the RDY line goes low at the start of

the access. When the internal cycle is complete the RDY line

is released.

DATA VALID

RDY

ADDRESS VALID

SAM

N N+1 N+2 N+3 N+4 N

CLK

D[7:0]

RDY

A[2:0]

NOTES:

RDY IS DRIVEN LOW ASYNCHRONOUSLY BY RD AND CS GOING LOW AND RETURNS HIGH ON THE RISING EDGE

OF CLK "N+3" FOR INTERNAL ACCESS (A[2:0] = 000), CLK "N+2" OTHERWISE.

THE SIGNAL, WR, MAY REMAIN HIGH AND RD MAY REMAIN LOW TO CONTINUE READ MODE.

CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+4 SHOWN) TO COMPLETE READ.

Figure 43. Mode 0 Read (MODE = GND)

AD6620

–34– REV. A

DATA VALID

ADDRESS VALID

NN+1 N+2 N+3

CLK

R/W

D[7:0]

DTACK

A[2:0]

SAM

N+4 N

DTACK

NOTES:

DTACK IS DRIVEN LOW ON THE RISING EDGE OF CLK "N+3" FOR INTERNAL ACCESS (A[2:0] = 000),

CLK "N=2" OTHERWISE.

THE SIGNAL, R/W MAY REMAIN HIGH AND DS MAY REMAIN LOW TO CONTINUE READ MODE.

CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+4 SHOWN) TO COMPLETE ACCESS

AND FORCE DTACK HIGH.

Figure 45. Mode 1 Read (MODE = VDD)

Mode = 1

If the MODE input is held high the interface is in Mode 1. In

Mode 1 the RD signal becomes the data strobe (DS) and the

WR signal becomes a read/write (R/W) select signal. In this

tSC

NN+1 N+2 N+3

tSAM

tDTACK

tSC

CLK

R/W

D[7:0]

DTACK

A[2:0]

tHC

tDTACK

tSAM

tHM

tHA

NOTES:

ON RISING EDGE OF "N+3" CLK, DTACK IS DRIVEN LOW.

THESE SIGNALS (R/W AND DS) MAY REMAIN IN LOW STATE TO CONTINUE WRITING DATA.

CS MUST RETURN TO HIGH STATE AND BE SAMPLED BY CLK (N+3 SHOWN) TO COMPLETE WRITE

AND FORCE DTACK HIGH.

*THE NEXT WRITE MAY BE INITIATED ON CLK

DATA VALID

ADDRESS VALID

Figure 46. Mode 1 Write (MODE = VDD)

mode the DTACK signal goes low when data is available during

a read or when data has been latched during a write. The DTACK

signal stays low until the DS signal is released.

AD6620

–35–REV. A

The data is contained in the low byte of the 16 significant bits.

This data will be placed into the external interface register

pointed to by A[2 . . 0] for a write and will be ignored for a read.

Serial Port Writes

If the WRITE bit is high and the READ bit is low then a write

access is performed to the external interface register pointed to

by A[2 . . 0]. A write to an internal register takes place by first

writing the AMR and LAR. The data registers DR4–DR1 are

then written as needed. A final write to DR0 then moves the

data to the internal register.

Serial Port Reads

If the READ bit is high, then a read to the register indicated is

performed and the data will appear in the RDATA word appended

to the serial frame. The internal data read is loaded into the

serial data word in FIFO fashion. The first byte read is loaded

into the first eight bits, the second read during the frame is

loaded into the second byte, etc. Since the serial data is shifted

MSB first, the first byte will actually be loaded into the most

significant byte of the serial data word.

During a frame (the period between SDFS rising edges) up to

four reads may occur. When a read is requested through the

serial port, a data word is appended to the end of the serial string.

Even if AD is not asserted (see below for AD description) a word is

added to the end of the IQ data stream. Therefore, if the chip is

in single channel mode, the I and Q data are sent followed by a

read word. If the chip is in diversity channel mode, the IQ pairs

are followed by a read word. Thus the serial port responds with

either three or five serial words in a frame, respectively. If AD is

asserted, the read word is sent each frame regardless of a request. If

no requests are made, the appended word is all zeros.

The number of reads accomplished in a frame is limited by the

serial word length. If the serial word length is 16 bits, only two

reads can be performed during a frame. If the serial word length

is 24 bits, three reads can occur in a frame. If the serial word

length is 32 bits, then up to four reads can occur in a frame.

The RDATA word format is shown below. Rows three and four

will not be present when 16-bit words are used, and row four

will not be present when 24-bit words are used.

Table XV. RDATA Word Definition

DA7 DA6 DA5 DA4 DA3 DA2 DA1 DA0

DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

DC7 DC6 DC5 DC4 DC3 DC2 DC1 DC0

DD7 DD6 DD5 DD4 DD3 DD3 DD1 DD0

The number of words in the serial frame depends on the operat-

ing mode of the chip (one or two I/Q pairs) and whether or not

a read access occurs. It also depends on the Append Data pin,

AD. When this signal is asserted, then the RDATA word is

appended to the Serial Frame regardless of whether or not a

read was performed in the frame. This allows time-slotted sys-

tems where multiple AD6620s or other devices share a serial

port of a DSP without hardware handshakes. When AD is

high and there has not been a read during the active frame,

the RDATA word is driven low and SDFE is held off for another

serial word length.

At all times, the serial interface must have time to shift all bits.

The section below Serial Port Guidelines should be consulted to

determine if sufficient time exists.

SERIAL PORT CONTROL

In addition to providing access to the complex output data stream

of the AD6620, the Serial Port can also be used for Dynamic

Control of the device. The dynamic registers of the AD6620

that are typically programmed while the chip is processing are

listed in the table below. In order to use the serial port control,

the chip must first be booted using the microprocessor interface.

Table XIII. Dynamic Registers

Bit

Address Width Name

300 8 MODE CONTROL REGISTER

302 32 NCO SYNC CONTROL REGISTER

303 32 NCO_FREQ

304 16 NCO PHASE_OFFSET

305 8 INPUT/CIC2 SCALE REGISTER

307 5 CIC5 SCALE REGISTER

309 4 OUTPUT/RCF CONTROL REGISTER

30B 8 RCF ADDRESS OFFSET REGISTER

The internal address and data structure are shared between

the microprocessor port and the serial port. When accessing

the internal RAM or registers, the serial port is given priority

over a microprocessor request. If a Mode 0 access occurs on

the microport while the serial port is accessing the internal address

space, the RDY line will go low and stay low until the serial

access has been completed. If a Mode 1 access occurs on the

microport during a serial access, the DTACK signal will not go

low until the serial access has been completed. The microport is

used for booting the AD6620 and either the microport or the

serial port can be used to dynamically change the system param-

eters. Both ports may be used in the same design provided that

the handshaking rules described above are observed.

For each word shifted out of the serial port there is a word

shifted in. Each input word can provide one internal access. Each

access can be a read or a write. All reads and writes are per-

formed via the same 8-bit registers used by the microprocessor

port. Each bit in the SDI words has a predefined meaning and

are used to decode which of these registers are being accessed

and whether the access is a read or a write. The bits are defined

according to the table below.

Table XIV. SDI Input Word Definition

READ WRITE xxxA2A1A0

D7 D6 D5 D4 D3 D2 D1 D0

x x xxxxxx

Only the first 16 bits of the SDI word contain significant data

regardless of the serial word length. The first two bits shifted in

are the READ and WRITE indicator signals. These bits control

the access type as described below and should not be asserted

simultaneously. If the Serial Port is not used for control then the

SDI pin should be tied low to disable register reads and writes.

The three address bits are the three least significant bits of the

upper byte in the 16-bit word. These three bits A[2:0] define

which of the seven external registers are accessed by the serial

port according to Table XI.

AD6620

–36– REV. A

Example of Serial Port W/R Operation

The example shown below demonstrates writing and reading

from the AD6620. For this example, the chip is set up in diver-

sity channel real mode. Therefore, there four data words (two Is

and two Qs) are generated as receiver data. Thus four commands

can be shifted into the SDI port. These are shown below. Addi-

tionally, the chip is configured with a word length of 16 bits.

The AD6620 response with five words per frame (two Is, two

Qs and the appended read word).

Table XVI. SDI Data Format

A-I A-Q B-I B-Q Append

SDO XXXX XXXX XXXX XXXX 0AXX

SDI 4703 4600 80XX 4603 XXXX

The table above shows the serial output bits for this configura-

tion. As the I and Q data are being shifted out, the SDI pin is

telling the chip what data to return during the appended data

field. During the A-I portion of the frame, the hex word 4703 is

shifted into the chip. Breaking this word down, the command

instructs the AD6620 to write an ‘03’ into the AMR register.

The next word, 4600, writes a ‘00’ into the LAR. Therefore, the

chip is so configured that the next command will either read

from or write to internal memory space ‘300’ hex, the Mode

Control Register. The next word on the SDI pin is 80XX. This

indicates a read from DR0. Note that the second half of the read

word is ignored. During the B-Q word, another read or write

can be set up. In this case, 4603 changes the internal memory to

point to ‘303,’ the NCO frequency, thus setting up subsequent

access of this register. Now during the append data frame, the

AD6620 sends any read words that are pending due to read

requests. In this case, the contents of register ‘300.’ Since the

chip is in single channel complex mode and running, the chip

responds with ‘0AXX.’ ‘0A’ indicates that the chip is in diver-

sity channel real mode and running as a Sync master. The ‘XX’

is indeterminate and would have been the results of a second

read if one had been requested.

PAR/SER

The Serial Port shares pins with a Parallel Output Port. These

pins are arbitrated by the PAR/SER pin. In order to operate the

chip with the Parallel Output Data Port PAR/SER must be high

while RESET is brought high. For Serial Port operation, PAR/

SER must be held low while RESET is brought high. PAR/SER

should remain valid while the AD6620 is processing (should

only be changed in RESET). PAR/SER should be hardwired on

a given design.

SBM

Serial Bus Master. When SBM is high, the AD6620 generates

SCLK and SDFS. When SBM is low, the AD6620 accepts

external SCLK and SDFS signals. When configured as a bus

master the SCLK signal can be used to strobe data into the DSP

interface. When used with another AD6620 in Serial Cascade

Mode, SCLK can be taken from the master AD6620 and used

to shift data out from the cascaded device. In this situation SDFS

of the Cascaded AD6620 is connected to the SDFE pin of the

master AD6620. When an AD6620 is in Serial Cascade Mode,

all of the serial port activities are controlled by the external

signals SCLK and SDFS.

Regardless of whether the chip is a Serial Bus Master or is in

Serial Cascade Mode, the AD6620 Serial Port functions are

identical except for the source of the SCLK and SDFS pins.

SCLK

SCLK is an output when SBM is high; SCLK is an input when

SBM is low. In either case the SDI input is sampled on the

falling edge of SCLK, and all outputs are switched on the rising

edge of SCLK. The SDFS pin is sampled on the falling edge of

SCLK. This allows the AD6620 to recognize the SDFS in time

to initiate a frame on the very next SCLK rising edge. The maxi-

mum speed of this port is 33.5 MHz or half of the master CLK

signal, whichever is lower. Care should be taken with this signal.

Even when the AD6620 is selected as a serial bus master, reflec-

tions on this line will cause the output shifters to ‘double shift’

output data causing corrupt serial data. If this signal is going to

a back plane of more than several inches, the line should either

be buffered or be matched to the impedance of the back plane.

See the Applications section of this data sheet for information

on driving the transmission lines.

SDI

Serial Data Input. Serial Data is sampled on the falling edge of

SCLK. This pin is used to write the internal control registers of

the AD6620 or to write the address of an internal location to be

read. These activities are described later in the Serial Frame

Structure section. If this pin is not used to write data into the

control port it should be tied low.

SDO

Serial Data Output. Serial output data is switched on the rising

edge of SCLK. On the very next SCLK cycle after an SDFS,

the MSB of A channel: I data is shifted. On every subsequent

SCLK edge a new piece of data is shifted out on the SDO pin

until the last bit of data is shifted out. The last bit of data

shifted is A channel: Q data in either of the Single Channel

Modes or the B channel: Q data in the Diversity Channel Real

Data. SDO is three-stated when the serial port is outside its

time-slot. This allows the AD6620 to share the SDI of a DSP,

with other AD6620s. In order to ensure that the three-state

condition of this pin does not cause a problem there should

either be a bus holder on this signal or there should be a weak

pull-down resistor placed on it. This will ensure that the SDO

pin is always in a valid logic state.

SDFS

SDFS is the Serial Data Frame Sync signal. SDFS is an output

when SBM is high; SDFS is an input when SBM is low. SDFS

is sampled on the falling edge of SCLK. When SDFS is sampled

high, the AD6620 serial port will become active on the next

rising edge of SCLK for a complete serial time-slot. When SBM

is high SDFS will pulse high for one SCLK cycle before an

active serial time-slot is to be initiated and a transfer will begin

immediately on the next rising edge of SCLK. When used as a

serial slave, the SDFS pin must not receive more than one SDFS

per frame. As with SCLK, care should be taken with this signal.

Even when the AD6620 is selected as a serial bus master, reflec-

tions on this line can cause erratic framing results. If this signal

is going to a back plane of more than several inches, the line

should either be buffered or be matched to the impedance of the

back plane. See the applications section of this data sheet for

information on driving the transmission lines.

SDFE

Serial Data Frame End output. SDFE will go high during the

last SCLK cycle of an active time-slot. The SDFE output of a

master AD6620 can be tied to the input SDFS of an AD6620 in

Serial Cascade Mode in order to provide a hardwired time-slot

scenario. When the Last Bit of SDO data is shifted out of the

AD6620

–37–REV. A

Master AD6620, the SDFE signal will be driven high by the

same SCLK rising edge that this bit is clocked out on. On the

falling edge of this SCLK cycle, the Cascaded AD6620 will

sample its SDFS signal, which is hardwired to the SDFE of the

Master. On the very next SCLK edge, A channel: I data of the

Cascaded AD6620 will start shifting out of the port. There will

be no rest between the time-slots of the master and slave.

WL[1:0]

WL defines the Word Length of the serial data stream. The

possible options are 00–16 bit words, 01–24 bit words, 10–32 bit

words and 11–Undefined. This setting controls the width of all

serial words. All words are shifted MSB first and are left justified,

i.e., the first n-bits are valid and any padding that is needed

to fill the word length is added at the end. When the serial word

length is 24 or 32 bits, the I and Q output data is presented with

23-bit resolution.

Table XVII. Setting Serial Word Length

Serial Word Length WL1 WL0

16-Bit 0 0

24-Bit 0 1

32-Bit 1 0

Disallowed 1 1

Append Data signal. In Single Channel Real Mode, when AD is

low the serial data stream consists only of A channel: I and Q

data. If the AD6620 is in Diversity Channel Real Mode, the

serial frame is four words long and consists of both A and B

channel complex data. When the AD signal is high, an extra

serial word is appended to the Serial Frame. This word consists

of any data that is read from the AD6620 internal registers via

the Serial Port. If a Read has not occurred, the data in this word

is zero. The addition of this word allows a Serial System to be

designed so that any AD6620 can have data read at any time

without changing the fixed timing of the serial port.

If the serial transfer includes a register read, the register data is

appended to the serial frame regardless of the state of the AD pin.

SDIV[3:0]

When the AD6620 is used as a Serial Bus Master the chip gen-

erates a serial clock by dividing down the CLK signal. The

divider ratio is set by the serial division word, SDIV. SDIV is

interpreted as a 4-bit unsigned integer and determines the fre-

quency of the serial clock when the SBM pin is pulled high.

When the AD6620 is in Serial Cascade Mode these bits are

ignored. The following equations express the Serial Clock Fre-

quency as a function of the CLK signal and the SDIV nibble.

ffSDIV

SDIV SDIV

SCLK

CLK

SCLK

CLK

=×≠

Serial Port Guidelines

The serial clock, SCLK, must be run at a rate sufficient to clock

all of the serial data out of the port before new data is latched

into the internal I and Q data registers. See the Serial Output

Data Port section for more details. If the serial port is to be used

as a means of programming the part, some extra serial bandwidth

may also be required to shift data from the internal registers of

the AD6620. There must be two or more or zero high speed

clocks between serial frames. When used as a serial bus master

SCLK can run at a maximum rate of half the processing CLK.

In serial slave mode, the serial clock can be run up to 67 MHz.

The equations below help determine what the minimum serial

clock rate must be in order to insure that data is not lost.

ffWL NR

MMMM

SCLK

SAMP CH D

TOT

TOT CIC CIC RCF

≥××× +

=××

()2

= 1 if AD is asserted or if read operations are used from the

serial port: otherwise R

= 0. This term accounts for the band-

width consumed when data is read from the internal control

registers or memory.

JTAG BOUNDARY SCAN

The AD6620 supports a subset of IEEE Standard 1149.1

specifications. For additional details of the standard, please see

“IEEE Standard Test Access Port and Boundary-Scan

Architecture,” IEEE-1149 publication from IEEE.

The AD6620 has five pins associated with the JTAG interface.

These pins are used to access the on-chip Test Access Port

(TAP) and are listed in the table below.

Table XVIII.

Pin Name Description

TRST TAP Reset

TCLK Test Clock

TMS TAP Mode Select

TDI Test Data Input

TDO Test Data Output

The AD6620 supports four op codes as shown below. These

instructions set the mode of the JTAG interface.

Table XIX.

Instruction Op Code

IDCODE 01

BYPASS 11

SAMPLE/PRELOAD 10

EXTEST 00

The Vendor Identification Code can be accessed through the

IDCODE instruction and has the following format.

Table XX.

MSB LSB

Version Part Number Manufacturing ID # Mandatory

0000 0010 0111 0111 1110 000 1110 0101 1

A BSDL file for this device is available from Analog Devices,

Inc. Contact Analog Devices, Inc. for more information.

AD6620

–38– REV. A

APPLICATIONS

EVALUATION BOARD

An evaluation board is available for the AD6620. This evalua-

tion board comes complete with an AD6620 and interfaces to a

PC through the printer port. The evaluation board comes com-

plete with software to drive the evaluation board and to design

optimized filters for use with the AD6620. The evaluation board

includes a high speed data interface that mates directly with

evaluation boards for high performance converters such as the

AD6600 and AD6640, allowing digital receivers to be bread-

boarded with only an external RF/IF converter and an interface

to the DSP.

The control software allows access to all of the internal registers

to provide complete programming of the device in a lab setting.

The software can process high speed data as well as digitally

filtered data from the AD6620 allowing analysis of both pre and

post filter channel characteristics. The controlling software can

also be used to verify the filter performance by sweeping the NCO,

greatly simplifying verification of any given filter design.

AD6620

HI SPEED

DATA HEADER LATCH FIFO

PC PRINTER PORT

LATCH

TRANSCEIVER

Figure 47. Evaluation Board Block Diagram

As shown in the block diagram below, the high speed data into

the evaluation board is sent to both the AD6620 and the by-pass

latches. On the output of the AD6620, data is available in either

serial or parallel mode. In serial mode, data may be sent directly

to a DSP for system bread-boarding. In parallel mode, the data

may be sent to the on-board FIFO for spectral analysis by the

included software. For additional information, refer to the evalua-

tion board manual.

FILTER DESIGN

The AD6620 implements a pair of cascaded CIC filters with a

sum of products FIR filter. The frequency characteristics of the

CIC filters have already been documented. Additional reading

on this class of filters can be found in “An Economical Class of

Digital Filters for Decimation and Interpolation,” by Eugene B.

Hogenauer, IEEE Transactions on Acoustics, Speech, and Signal

Processing, Volume ASSP-29, Number 2, April 1981.

The characteristics of the FIR filter are fully programmable.

The coefficients of this filter may be generated in any number

of ways, using standard procedures such as Parks-McClellan.

Available software from Analog Devices that assists in the design

of filters for this product. This software allows comparison

between different distributions of decimation. The software

works independently of the evaluation board, but easily allows

transfer of design data directly to the evaluation board for

immediate verification of the designed filter.

The normal procedure for designing a filter for the chip is as

shown in the flow chart. First, the desired characteristics must

be determined based on the receive channel requirements. The

decimation rates for the CIC filters must then be selected such

that their performance is near that of the desired channel require-

ments. Finally, an algorithm such as the Parks-McClellan or

Remez exchange is used to compute the final spectral require-

ments, including droop correction for passband loss of the CIC

filters. If the designed filter meets the requirements, then the

filter is acceptable. If not, another combination of CIC filter

decimation must be examined. Tables III and IV greatly simplify

distribution and selection of CIC requirements. The filter

software available from Analog Devices helps to automate

this procedure.

SELECT FILTER

REJECTION

REQUIREMENTS

DOES CIC2 FILTER

PROTECT ENOUGH

BANDWIDTH?

SELECT

DECIMATION

RATE FOR CIC5

DESIGN RCF WITH

REMEZ EXCHANGE

YES

SELECT

DECIMATION

RATE FOR CIC2

DOES CIC5 FILTER

PROTECT ENOUGH

BANDWIDTH?

DOES COMPOSITE

FILTER PROVIDE

THE DESIRED

RESULTS?

YES

Figure 48. Diagram of Filter Design Software

SERIAL BUFFERING

The AD6620 serial outputs are designed to operate at very high

speed. As such, care must be taken when driving the serial output

lines. These high speed lines must be treated as transmission

lines. Critical lines include the SCLK, SDFS, SDFE, SDI and

SDO. It is recommended that these lines be series source termi-

nated with the characteristic impedance of the driven line. If the

lines are longer than a few inches, digital line buffers should be

used as shown below. Buffering in this manner will prevent

reflections on the serial lines from disrupting operation of the

AD6620. A good reference on transmission lines is found in the

“MECL System Design Handbook” by Motorola Inc., Stock

code HB205R1/D.

AD6620

SCLK

SDO

SDFS

SCLK

SDO

SDFS

Figure 49. Serial Line Buffering and Series Source

Termination

DSP/SHARC INTERFACING

With little effort, the AD6620 will interface to nearly all indus-

try standard DSPs, as shown in the figure below. The figures

below show operation in TDM applications as well as in serial

slave mode.

In TDM mode the first AD6620 is configured to be the master.

This chip is the first to access the serial data bus. When the

master has data available in its output shifters, it generates an

SDFS telling the DSP that serial data will follow. At this point,

AD6620

–39–REV. A

the SDO of the master AD6620 takes control of the SDO line

and begins shifting data out of the device. When all data has

been shifted, the master raises the SDFE on the last shifted.

This signals the next chip (slave) that on the next cycle of the

clock it should take control of the SDO line and begin shifting

data to the DSP. When the second AD6620 completes its shift,

it raises its SDFE to signal the next chip in the chain, if present.

If additional devices are connected to the chain, this would be

used to indicate they should take control on the next clock cycle.

This application does not have a third device and therefore, the

frame would end.

Normally in an application with a single AD6620, the AD6620

would be configured as the serial bus master. However, there

are applications where the DSP or other device may be the serial

bus master. In this case, the diagram below illustrates how to

configure the AD6620 so that it may be used in this mode. In

order to use this in a meaningful application, the DSP must

know when the AD6620 has new data available on its output. If

the DSP polls the AD6620 too early, either old data will be present

or the data could be in an indeterminate state. To prevent this,

the AD6620 has an output pin DV

OUT

that signals the DSP

when new data is available. This should be tied to an interrupt

line of the DSP that is edge-sensitive, as the DV

OUT

line is only

valid for two or four high speed clock cycles depending on the

mode of the chip. The DSP may then invoke an interrupt service

routine to handle the data, see text below. In this application,

the DSP is responsible for generating the framing and clocking

signals to the AD6620 as shown in Figure 51.

SCLK

AD6620 DSP

+3.3V

SBM

SCLK

SDI DT

SDO DR

SDFS RFS

SDFE

10k⍀

SDIV

2 4

ADWL

10k⍀

SCLK

SBM

SDI

SDO

SDFS

SDFE

SDIV

2 4

ADWL

AD6620

CASCADE

Figure 50. Dual AD6620s Using the Serial Bus in a TDM

Application

SCLK

AD6620

DSP

SCLK

SDI DT

SDO DR

SDFS RFS

SDFE 10k⍀

SDIV

2 4

ADWL

10k⍀

SBM DV

OUT

IRQ

Figure 51. AD6620 Configured as a Serial Slave

Software for Single Channel Real Operation

When interfacing Analog Device’s SHARC DSP, the following

code fragments can be used to configure the SHARC. The first

example shows how to configure the registers for use with a single

channel application. The first segment of code defines the memory

for use with the multichannel serial port data. The second segment

of code sets up the serial port for receiving data only. It could

have just as easily been set up for bidirectional data by properly

setting the MTCSI register. The final two code segments are used

when a serial port interrupt occurs. When the SHARC detects

completion of the serial port frame, an interrupt is generated

and the final code segment is executed. The comments in that

section show where user code should be inserted. The SHARC

takes care of moving the serial port buffers data directly to data

memory as shown.

/* —————————————————————————————*/

/* multi-channel register setup */

.SEGMENT/DM dm_data;

.VAR fm_demod_data[2]; /* Array for receiving 1 real and imag

sample */

.VAR fm_demod_tcb[8] = 0, 0, 0, 0, fm_demod_data+7, 2, 1,

fm_demod_data; /* Transfer Control Block for reception of fm data */

/* —————————————————————————————*/

/* Subroutine to setup sport1 for use with the AD6620 */

setup_sport1:

r0 = 0; /* multi-channel enable setup */

dm(MTCS1) = r0; /* do not transmit on any channels */

r0 = 0; /* Compand Setup */

dm(MTCCS1) = r0; /* no companding on transmit */

dm(MRCCS1) = r0; /* no companding on receive */

r0 = 0x00100000; /* Setup sport 1 transmit control register */

dm(STCTL1) = r0; /* mfd = 1 */

r0 = 0x038c20f2; /* Setup sport 1 receive control register */

dm(SRCTL1) = r0; /* slen = 15, sden & schen enabled */

/* sign extend, external SCLK+RFS */

r0 = fm_demod_tcb + 7; /* TCB address */

dm(CP1) = r0; /* Kickoff DMA chain */

rts (db); /* RETURN */

bit set imask SPR1I; /* enable sport1 receive interrupt */

nop;

/* —————————————————————————————*/

spr1_svc: jump spr1_asserted;

RTI;

/* —————————————————————————————*/

/* Process received data here. Data samples located in fm_demod_data

and fm_demod_data+1

spr1_asserted:

push sts; /* Push the status stack */

/* Use secondary set of DAGs and Register file */

AD6620

–40– REV. A

bit set mode1 SRD1H | SRD2L | SRRFH | SRRFL;

nop;

/* Insert code here to process I and Q data. The DSP serial port handler

has placed the samples in fm_demod_data and fm_demod_data+1 */

pop sts; /* Pop the status stack */

rti (db);

/* Switch back to primary set of DAGs and Register file */

bit clr mode1 SRD1H | SRD2L | SRRFH | SRRFL;

nop;

.ENDSEG;

/*—————————————————————————————*/

Software for Diversity Channel Real Operation

The code for interfacing to Diversity Channel Real mode is very

similar to that of single channel. The only difference being the

number of channels allocated on the TDM chain. This process

can easily be extended for any number of TDM channels as

long as there is sufficient time in the frame to completely trans-

mit the data. This procedure works with the appended data as

well as serially cascaded devices. The code below demonstrates

setup and operation in diversity channel mode.

/*—————————————————————————————*/

.SEGMENT/DM dm_data;

/* multi-channel register setup */

.VAR fm_demod_data[4]; /* Array for receiving 2 real and imag

sample from each channel */

.VAR fm_demod_tcb[8] = 0, 0, 0, 0, 0, 4, 1, fm_demod_data;

/* Transfer Control Block for reception of fm data */

/* —————————————————————————————*/

/*—————————————————————————————*/

setup_sport1:

r0 = 0; /* multi-channel enable setup */

dm(MTCS1) = r0; /* do not transmit on any channels */

r0 = 0; /* Compand Setup */

dm(MTCCS1) = r0; /* no companding on transmit */

dm(MRCCS1) = r0; /* no companding on receive */

r0 = 0x00100000; /* Setup sport 1 transmit control register */

dm(STCTL1) = r0; /* mfd = 1 */

r0 = 0x038c00f2; /* Setup sport 1 receive control register */

dm(SRCTL1) = r0; /* slen = 15, sden & schen enabled */

/* sign extend, external SCLK+RFS */

r0 = fm_demod_tcb + 7; /* TCB address */

dm(fm_demod_tcb + 4) = r0; /* TCB point back to itself */

dm(CP1) = r0; /* Kickoff DMA chain */

rts (db) /* RETURN */

bit set imask SPR1I; /* enable sport1 receive interrupt */

bit set imask CB15I; /* Enable circular buffer 15 wrap

interrupt for buffers full */

/*—————————————————————————————*/

spr1_svc: jump spr1_asserted;

RTI;

/*—————————————————————————————*/

spr1_asserted: /* SPORT1 Receive interrupt - do the fm demod and

increment the counter */

push sts; /* Push the status stack */

/* Use secondary set of DAGs and Register file */

SRRFL;

nop;

/* Insert code here for processing I and Q data pairs. The DSP serial

port handler has placed the samples in fm_demod_data through

fm_demod_data+3 */

pop sts; /* Pop the status stack */

rti (db);

/* Switch back to primary set of DAGs and Register file */

SRRFL;

nop;

.ENDSEG;

/*—————————————————————————————*/

TYPICAL LATENCY EXPECTATIONS

In the AD6620 latency can be divided into three components.

For difficult filters, the largest component of latency is Algorith-

mic Latency. This type of latency is tied inseparably to the desired

filter response. For smaller or minimal filters, Fixed Latency begins

to dominate. This is the undesirable fixed delay associated with

the calculation of the output samples. Finally, Variable Latency,

is the smallest component. This is the delay that can be influenced

by the relative phase of internal decimated clocks with respect to

the SYNC_CIC.

Algorithmic Latency is a necessary component of any filtering

process be it analog or digital. Since frequency is a variation

with respect to time, it must take time to discriminate between

analog frequencies. Assuming the AD6620 is used to generate

linear phase, low-pass filters, the algorithmic latency is a direct

function of the number of RCF taps and the CIC decimation

ratios. In general, the largest part of the impulse response of

these filters is the center of the impulse response length, so that

the delay is represented by one-half the composite impulse

response length.

The impulse response length of the RCF is the number of taps

times the RCF input sample period. Therefore relative to the

input sample clock the impulse response length of the RCF is

given by;

NMM

TAPS CIC CIC

ADC

−

(

)

××+11

The impulse response length of the CIC5 is given by;

55 1

×−

(

)

×+MM

CIC CIC

ADC

AD6620

–41–REV. A

The impulse response length of the CIC2 is given by

×−

(

)

CIC

ADC

The composite impulse response length of all three stages is

NMM MM M

TAPS CIC CIC CIC CIC CIC

ADC

××+××−×+

52 52 2

431

The Algorithmic Latency is

NMM MM M

TAPS CIC CIC CIC CIC CIC

ADC

××+××−×+

52 52 2

431

Fixed Latency is the delay due to each register between the input

and the output of the AD6620. The latency is the count of each

fixed latency of the AD6620 can be approximated by the follow-

ing expression:

10 7 7 5

tt M M M N t

CLK SAMP CIC CIC RCF TAPS CLK

++ + +

[]

+×

where:

CLK

is the high speed clock to the AD6620.

SAMP

is the data rate delivered to the AD6620.

Normally t

CLK

and t

SAMP

are the same unless a clock multiplier is

used such as with the AD6600’s 2× clock output.

Variable Latency is due to any differences between the asyn-

chronous edge of the SYNC pulses and the data rate. This

includes use of the internal synchronization options.

Based on the information on latency, the plots shown below

provide typical latency for a variety of different applications.

They were obtained by inserting a –F

dc step into the Input Data

Port of the AD6620. These are I channel step responses for the

input transient. The latency is defined as the output period times

number of output samples until the output reached approxi-

mately 50% of the step value.

OUTPUT SAMPLES

0.20

–0.40

–1.001213

FRACTION OF FS

5791113151719

0.00

–0.20

–0.60

–0.80

23 25 27 29

61.44MHz

SAMPLE RATE

MCIC2 = 16

MCIC5 = 8

MRCF = 8

NTAPS = 256

AT 19 OUTPUT SAMPLES,

THE LATENCY WOULD

BE 0.32ms

EXPECTED LATENCY = 0.303ms

Figure 52. AMPS Example

OUTPUT SAMPLES

0.20

–0.40

1112

FRACTION OF F

345678910

0.00

–0.20

–0.60

–0.80 12 13 14 15

58.9824MHz

SAMPLE RATE

CIC2

= 2

CIC5

= 4

RCF

= 6

TAPS

= 48

AT 8 OUTPUT SAMPLES,

THE LATENCY WOULD

BE 6.51␮s

EXPECTED LATENCY = 6.31␮s

Figure 53. CDMA Example

OUTPUT SAMPLES

0.20

–0.40

1213

FRACTION OF FS

5791113151719

0.00

–0.20

–0.60

–0.80

23 25 27 29

64.512MHz

SAMPLE RATE

MCIC2 = 2

MCIC5 = 14

MRCF = 3

NTAPS = 84

AT 19 OUTPUT SAMPLES,

THE LATENCY WOULD

BE 24.7␮s

EXPECTED LATENCY = 24.31␮s

Figure 54. PHS Example

OUTPUT SAMPLES

0.20

–0.40

1112

FRACTION OF FS

345678910

0.00

–0.20

–0.60

–0.80

12 13 14 15

–1.00

65.0MHz

SAMPLE RATE

MCIC2 = 2

MCIC5 = 6

MRCF = 20

NTAPS = 240

AT 9 OUTPUT SAMPLES,

THE LATENCY WOULD

BE 33.23␮s

EXPECTED LATENCY = 31.2␮s

Figure 55. WB-GSM Example

AD6620

–42– REV. A

PARALLEL PROCESSING USING AD6620

If a single AD6620 does not have enough time to compute an

adequate filter, multiple AD6620s can be operated in parallel as

shown in Figure 56. In this example, the processing is distrib-

uted between four chips so that each chip can process more

taps. The outputs are then combined such that the desired data

rate is achieved.

CLK

SYNC RCF

OUT

AD6620 #1

AIN

ENCODE

CLOCK

CLK

SYNC RCF

OUT

AD6620 #2

CLK

SYNC RCF

OUT

AD6620 #4

CLK

SYNC RCF

OUT

AD6620 #3

LATCH

OUTPUT

SELECTOR

RCF TIMING

CONTROL

AD6640

Figure 56. Parallel Processing with the AD6620

In this application, one high speed ADC can feed parallel

AD6620s. Although not shown in this diagram, the SYNC_NCO

and SYNC_CICs are tied together and synchronized from an

external source with all chips run as SYNC_Slaves.

This architecture allows for each AD6620 to process four times

as many taps as would otherwise be possible. Consider the

example of an ADC clocked at 58.9824 MHz and a desired

output data rate of 4.9152 MHz. If a single AD6620 were used,

the decimation rate would be 12 (58.9824/4.9152) allowing for

only 12 taps in the FIR filter. Not nearly enough for a usable

digital filter. Now consider the case where each AD6620 only

provides an output for one in four samples. In this case, the

decimation rate per chip would be four times larger, 48 in this

example. With a decimation of 48, more taps for the filter can

be generated and produce a much better filter.

COUNTER

0 TO 47

CLOCK IN COUNT = 0

COUNT = 11

COUNT = 23

COUNT = 35

Figure 57. RCF Timing Generator for Parallel Processing

Implementation of such a procedure is quite simple and basi-

cally shown in Figure 57. The filter design would proceed by

designing the filter to have the desired spectral characteristics

at its output rate. For our example here, each AD6620 would

have an output rate of 1.2288 MHz. The filter should be designed

such that the required rejection is attained directly at this rate.

This one filter is loaded into each chip. Upsampling is achieved

on the output by multiplexing between the different AD6620

outputs which are staggered, in this case by 90 degrees of the

output data rate. Therefore, since the decimation rate is 48

and four AD6620s are used, every 12 high speed clock cycles

a new AD6620 output should be selected. The most direct

method is to use these pulses to trigger the SYNC_RCF signals.

This staggering is required to properly phase the AD6620’s inter-

nal computations. Once the chips have been synchronized in this

manner, they will begin producing DV

OUT

signals that can be

used to instruct the Output Selector which output is valid.

The RCF Timing Control is responsible for proper phasing of

the AD6620s in the system. The example shown here is for the

example of four devices in parallel. It can easily be expanded to

any number of devices with this methodology. Since the AD6620s

are decimating by 48, the complete cycle time is 48 system

clocks. Thus the timing control must run modulo 48. When the

count is 0, the first RCF should be reset with a pulse that is one

clock cycle wide. Likewise, when the count is 11, 23 and 35,

RCF2, RCF3 and RCF4 should be reset respectively. This will

properly phase the AD6620s to run 90 degrees out of phase. If

this example consisted of six AD6620s, then they should be

reset on count 0, 7, 15, 23, 31 and 39. Following this method,

any number of AD6620s can be paralleled for higher data rates.

Once the AD6620 RCFs are properly phased, the DV

OUT

signals

will then enable the output selector to know which outputs should

be connected at the correct point in time. In review, the DV

OUT

signal pulses high when the RCF data is being placed on the out-

puts. Since the devices are operated in Single Channel Real

mode, this signal will be high for two clock cycles while two

pieces of data are written to the output. The output pairs consist

of I followed by Q. As each chip’s DV

OUT

cycles high, its data

should be connected to the output bus as shown below. This

effectively forms a MUX that sequentially cycles the output of

each of the AD6620s in the system to the output port. The only

remaining issue is retiming the data. Since each AD6620 clocks its

data out in two clock cycles, there will be 10 cycles where the

data is idle. During this period, the last Q out will remain valid

until the next chip in the sequence generates its DV

OUT

signal. This

normally should pose no problem, but if it does, the output data

could easily go to a FIFO and be retimed so that output data

streams at a regular rate.

In order to meet conventional logic requirements, OE for each

of the input latches should be active low. The DV

OUT

of the

AD6620 is active high, therefore, an inverter must be typically

inserted between the DV

OUT

lines and the OE of the latches as

shown in the updated Figure 58.

AD6620

–43–REV. A

CLOCK

DVOUT1

DVOUT2

DVOUT3

DVOUT4

AD6620–1

AD6620–2

AD6620–3

AD6620–4

SELECTOR

OUTPUT

QIQI

Figure 59. Timing for Parallel Processing

OEINPUT LATCHING

OUT1

CLOCK

OUT1

OUT2

CLOCK

OUT2

OUT3

CLOCK

OUT3

OUT4

CLOCK

OUT4

OEINPUT LATCHING

OUTPUT

LATCHING

Figure 58. Parallel Processing Output Selector

In the Output Selector above each of the DV

OUT

lines is ANDed

with main clock. This allows the data out of each of the AD6620s

to be properly latched into the input latches. The DV

OUT

line is

also responsible for placing the latched outputs on the internal

bus at the proper time. This data is then latched in the output

latch using the internal ORed clocking signals.

The timing for these events is shown in Figure 59. As shown,

the system clock is run at the specified rate. Then the RCF

timing control state machine is responsible for generating the

appropriate sync pulses. When each AD6620 completes its SOP

computation, it generates the DV

OUT

pulses shown below. Concur-

rently, each chip places its IQ data on the output pins of that

device. With this data, the output selector state machine com-

bines all of the data and places the data on the output bus.

Using the AD6620 in a Narrow Band System

A typical interconnection between the AD6600, AD6620 and a

General Purpose DSP is shown in Figure 65. This is an example

of an IF sampling narrow-band system and offers many techni-

cal and cost advantages over traditional solutions. In this example,

the AD6620 is in Diversity Channel Real Mode, with the AD6600

sampling a diversity antenna on its B channel. The AD6620

performs floating-point to fixed-point conversion, digital tuning,

digital filtering and decimation of the A/D output data.

MAIN

INPUT

DIVERSITY

INPUT

2ⴛCLK

A/B OUT

3 RSSI BITS

11 DATA BITS

ENCODE

SCLK

SDI

SDO

SDFS

CLK

A/B

E[2...0]

IN[15...5]

AD6620

AD6600

SCLK

SDO

SDI

SDFS

DSP

Figure 60. Implementation of a Narrow Band Receiver

The 2× CLK on the AD6600 is used as the processing CLK of

the AD6620. The use of this faster clock allows the RCF filter

to process up to twice as many taps per sample. The increased

number of taps available helps to improve the filter characteris-

tics. In some applications an even faster processing clock may be

necessary to allow for improved digital filter performance. In

this case the A/B pin of the AD6620 must be toggled when each

channel input is to be sampled.

For most narrow-band uses of the AD6600/AD6620 combina-

tion, a high oversampling ratio is desired. This spreads the

quantization noise of the A/D over a wider spectrum and allows

the digital filtering of the AD6620 to remove much of this noise.

This effectively increases the SNR of the AD6600. This process

of oversampling and digital filtering is called “process gain”

and its contribution to SNR can be calculated from the equa-

tion below.

PG Sample Rate of Channel

Signal Bandwidth

=









10 log ___

The process of oversampling can also provide the benefit of

lowering the noise floor of the A/D. This can increase the effec-

tive dynamic range of a receiver if the sampling rate is chosen

such that the signal harmonics and/or intermodular distortion

(IMD) products fall out of the band of interest. In this case

these spurs could be filtered by the AD6620 and the quantiza-

tion noise would be the dominant dynamic range limitation of

the AD6600/AD6620 receiver solution.

–44–

C00967–0–6/01(A)

PRINTED IN U.S.A.

AD6620

REV. A

A DSP is then used to perform the demodulation of the digital

channel. This has the advantage of allowing for in-system con-

figuration options and can even allow for improved modulation

techniques to be applied in the future. This assumes that the

AD6600 and the circuitry on its front end are compatible with

the modulation standard to be used.

For more information on using the AD6600 and AD6620 in a

Single Carrier application, refer to Analog Devices’ Application

note AN-502.

Using the AD6620 in a Wideband System

The AD6620 is fully capable of being utilized in a wide-band

architecture system where A/Ds such as the AD6640 or the

AD9042 usually run at higher sample rates than those typically

found in a narrow-band system. A correspondingly wider band

can then be digitized. The digitization of this wide bandwidth

allows many more channels to be digitized using the same A/D

and IF circuitry. The core configuration of such a system is

shown in Figure 61.

The AD6640 and the AD6620 are both designed to run as fast

as 67 MHz. In these applications the AD6620 will be used to

process only one channel and will process the data at the A/D

sample rate. Additional channels can be processed by taking the

AD6640 high speed data stream to additional AD6620s. Each

AD6620 can then be tuned to a different channel.

The AD6620 provides a great deal of selectivity by mixing down

a channel of interest as in the narrow-band case and filtering the

out-of-band noise and adjacent channels. Unlike the narrow-

band solutions it is much more difficult to place the spurious

content out of the band of interest because more of this band-

width is used due to the larger number of carrier channels. The

aliased spurs of one channel are likely to fold back on another

80-Lead Terminal Plastic Quad Flatpack (PQFP)

(S-80A)

SEATING

PLANE

0.134 (3.40)

MAX

0.041 (1.03)

0.029 (0.73)

0.004 (0.10)

MAX

0.120 (3.05)

0.100 (2.55)

0.010 (0.25)

MIN 0.015 (0.38)

0.009 (0.22)

0.690 (17.45)

0.667 (16.95)

0.555 (14.10)

0.547 (13.90)

0.555 (14.10)

0.547 (13.90)

0.690 (17.45)

0.667 (16.95)

2021 41

0.486 (12.35) BSC

TOP VIEW

(PINS DOWN)

0.026 (0.65)

BSC

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

CLK

AD6620 #1

SERIAL I/O

AIN

ENCODE

CLOCK

CLK

AD6620 #2

SERIAL I/O

CLK

AD6620 #3

SERIAL I/O

CLK

AD6620 #4

SERIAL I/O

DSP

ARRAY

LATCH

AD6640

AD6644

Figure 61. Implementation of a Multicarrier Receiver

channel. This places a greater requirement on the Spurious Free

Dynamic Range (SFDR) of the A/D than in the narrow-band

case. The SFDR is then usually the limiting factor of the wide-

band system.

Provided that the A/D has sufficient SFDR for the air interface

requirements, the AD6620 can use process gain, as in the narrow-

band case, by filtering the out-of-band noise and adjacent channel

power. This increases the SNR of the digital data stream.

As in the Narrow-band System a DSP is then used to demodu-

late the digital data. The same advantages of flexibility exist in

the wide-band case as they did in the narrow-band case. Future

improvements in demodulation algorithms can be implemented

in the receiver, provided that the front end hardware is compat-

ible with the desired modulation standard.