ADS831E/2K5G4 - Texas Instruments High-Performance Analog

8-Bit, 80MHz Sampling

ANALOG-T O-DIGITAL CONVERTER

FEATURES

●HIGH SNR: 49dB

●INTERNAL OR EXTERNAL REFERENCE

OPTION

●SINGLE-ENDED OR

DIFFERENTIAL ANALOG INPUT

●PROGRAMMABLE INPUT RANGE:

1Vp-p/2Vp-p

●LOW POWER: 275mW

●LOW DNL: 0.35LSB

●SINGLE +5V SUPPLY OPERATION

●SSOP-20 PACKAGE

APPLICATIONS

●MEDICAL IMAGING

●VIDEO DIGITIZING

●COMPUTER SCANNERS

●COMMUNICATIONS

●DISK-DRIVE CONTROL

DESCRIPTION

The ADS831 is a pipeline, CMOS Analog-to-Digital (A/D) con-

verter that operates from a single +5V power supply. This con-

verter provides excellent performance with a single-ended input

and can be operated with a differential input

for added spurious performance. This high performance converter

includes an 8-bit quantizer, high bandwidth

track/hold, and a high accuracy internal reference. It also allows for

the user to disable the internal reference and utilize external

references. This external reference option provides excellent gain

and offset matching when used in multi-channel applications or in

applications where DC full scale range adjustment is required.

The ADS831 employs digital error correction techniques to pro-

vide excellent differential linearity for demanding imaging appli-

cations. Its low distortion and high SNR give the extra margin

needed for medical imaging, communications, video, and test

instrumentation.

The ADS831 is specified at a maximum sampling frequency of

80MHz and a single-ended input range of 1.5V to 3.5V. The

ADS831 is available in an SSOP-20 package and is pin-for-pin

compatible with the 8-bit, 60MHz ADS830.

ADS831

8-Bit

Pipelined

A/D Core

Internal

Reference

Optional External

Reference

Timing

Circuitry

Error

Correction

Logic

3-State

Outputs

T/H

CLK VDRV

ADS831

Int/Ext

•

INV

(Opt)

ADS831

SBAS087A – MAY 2001

www.ti.com

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of Texas Instruments

standard warranty. Production processing does not necessarily include

testing of all parameters.

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

ADS831

2SBAS087A

PACKAGE SPECIFIED

DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT

PRODUCT PACKAGE NUMBER(1) RANGE MARKING NUMBER MEDIA

ADS831E SSOP-20 (QSOP) 349 –40°C to +85°C ADS831E ADS831E Rails

" " " " " ADS831E/1K Tape and Reel

NOTE: (1) Models with a slash ( /) are available only in Tape and Reel in the quantities indicated (e.g., /1K indicates 1000 devices per reel). Ordering 1000 pieces

of “ADS831E/1K” will get a single 1000-piece Tape and Reel.

PACKAGE/ORDERING INFORMATION

ELECTROSTATIC

DISCHARGE SENSITIVITY

This integrated circuit can be damaged by ESD. Texas Instruments

recommends that all integrated circuits be handled with

appropriate precautions. Failure to observe proper handling and

installation procedures can cause damage.

ESD damage can range from subtle performance degradation to

complete device failure. Precision integrated circuits may be more

susceptible to damage because very small parametric changes could

cause the device not to meet its published specifications.

PRODUCT DEMO BOARD

ADS831 DEM-ADS831E

DEMO BOARD ORDERING INFORMATION

+VS.......................................................................................................+6V

Analog Input ............................................................. –0.3V to (+VS + 0.3V)

Logic Input ............................................................... –0.3V to (+VS + 0.3V)

Case Temperature ......................................................................... +100°C

Junction Temperature .................................................................... +150°C

Storage Temperature..................................................................... +150°C

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

At TA = full specified temperature range, single-ended input range = 1.5V to 3.5V, sampling rate = 80MHz, and external reference, unless otherwise noted.

ADS831E

PARAMETER CONDITIONS MIN TYP MAX UNITS

RESOLUTION 8 Guaranteed Bits

SPECIFIED TEMPERATURE RANGE Ambient Air –40 to +85 °C

ANALOG INPUT

Standard Single-Ended Input Range 2Vp-p 1.5 3.5 V

Optional Single-Ended Input Range 1Vp-p 2 3 V

Common-Mode Voltage 2.5 V

Optional Differential Input Range 2Vp-p 2 3 V

Analog Input Bias Current 1µA

Input Impedance 1.25 || 5 MΩ || pF

Track-Mode Input Bandwidth –3dBFS 300 MHz

CONVERSION CHARACTERISTICS

Sample Rate 10k 80M Samples/s

Data Latency 4 Clk Cyc

DYNAMIC CHARACTERISTICS

Differential Linearity Error (largest code error)

f = 1MHz ±0.25 ±1.0 LSB

f = 10MHz ±0.35 LSB

No Missing Codes Guaranteed

Integral Nonlinearity Error, f = 1MHz ±0.5 ±2.0 LSBs

Spurious Free Dynamic Range(1)

f = 1MHz (–1dB input) 67 dBFS(2)

f = 10MHz (–1dB input) 50 65 dBFS

Two-Tone Intermodulation Distortion(3)

f = 9.5MHz and 9.9MHz (–7dB each tone) –57 dBc

Signal-to-Noise Ratio (SNR) Referred to Full Scale

f = 1MHz 49 dB

f = 10MHz 46 49 dB

Signal-to-(Noise + Distortion) (SINAD) Referred to Full Scale

f = 1MHz 48.5 dB

f = 10MHz 44 48.5 dB

Effective Number of Bits(4), f = 1MHz 7.8 Bits

Output Noise Input Tied to Common-Mode 0.2 LSBs rms

Aperture Delay Time 3ns

Aperture Jitter 1.2 ps rms

Overvoltage Recovery Time 2ns

Full-Scale Step Acquisition Time 2.5 ns

ADS831 3

SBAS087A

ELECTRICAL CHARACTERISTICS (Cont.)

At TA = full specified temperature range, single-ended input range = 1.5V to 3.5V, sampling rate = 80MHz, and external reference, unless otherwise noted.

ADS831E

PARAMETER CONDITIONS MIN TYP MAX UNITS

CMOS Compatible

Rising Edge of Convert Clock

CMOS/TTL Compatible

Straight Offset Binary

DIGITAL INPUTS

Logic Family

Convert Command Start Conversion

High Level Input Current(5) (VIN = 5V) 100 µA

Low Level Input Current (VIN = 0V) 10 µA

High Level Input Voltage +3.5 V

Low Level Input Voltage +1.0 V

Input Capacitance 5pF

DIGITAL OUTPUTS

Logic Family

Logic Coding

Low Output Voltage (IOL = 50µA) VDRV = 5V +0.1 V

Low Output Voltage, (IOL = 1.6mA) +0.2 V

High Output Voltage, (IOH = 50µA) +4.9 V

High Output Voltage, (IOH = 0.5mA) +4.8 V

Low Output Voltage, (IOL = 50µA) VDRV = 3V +0.1 V

High Output Voltage, (IOH = 50µA) +2.8 V

Output Capacitance 5pF

ACCURACY

(Internal Reference, 2Vp-p, Unless Otherwise Noted)

fS = 2.5MHz

Zero Error (Referred to –FS) at 25°C–2.5 ±0.5 +2.5 %FS

Zero Error Drift (Referred to –FS) ±53 ppm/°C

Gain Error(6) at 25°C–2.5 ±0.5 +2.5 %FS

Gain Error Drift(6) ±75 ppm/°C

Power Supply Rejection of Gain ∆ VS = ±5% 55 dB

Internal REFT Tolerance Deviation from Ideal 3.0V ±10 ±100 mV

Internal REFB Tolerance Deviation from Ideal 2.0V ±10 ±100 mV

External REFT Voltage Range REFB + 0.8 3.0 VS – 1.25 V

External REFB Voltage Range 1.25 2.0 REFT – 0.8 V

Reference Input Resistance REFT to REFB 800 Ω

POWER SUPPLY REQUIREMENTS

Supply Voltage: +VSOperating +4.75 +5.0 +5.25 V

Supply Current: +ISOperating 58 70 mA

Power Dissipation: VDRV = 5V External Reference 290 350 mW

VDRV = 3V External Reference 275 mW

VDRV = 5V Internal Reference 310 mW

VDRV = 3V Internal Reference 285 mW

Thermal Resistance,

SSOP-20 115 °C/W

NOTES: (1) Spurious Free Dynamic Range refers to the magnitude of the largest harmonic. (2) dBFS means dB relative to Full Scale. (3) Two-tone

intermodulation distortion is referred to the largest fundamental tone. This number will be 6dB higher if it is referred to the magnitude of the two-tone fundamental

envelope. (4) Effective number of bits (ENOB) is defined by (SINAD – 1.76) /6.02. (5) A 50kΩ pull-down resistor is inserted internally. (6) Excludes internal

reference.

ADS831

4SBAS087A

TIMING DIAGRAM

SYMBOL DESCRIPTION MIN TYP MAX UNITS

tCONV Convert Clock Period 12.5 100µsns

tLClock Pulse Low 5.8 6.25 ns

tHClock Pulse High 5.8 6.25 ns

tDAperture Delay 3 ns

t1Data Hold Time, CL = 0pF 3.9 ns

t2New Data Delay Time, CL = 15pF max 5.9 12 ns

4 Clock Cycles

Data Invalid

CONV

N–4N–3N–2N–1 N N+1 N+2 N+3

Data Out

Clock

Analog In N

N+1 N+2 N+3 N+4 N+5 N+6 N+7

GND

Bit 1 (MSB)

Bit 2

Bit 3

Bit 4

Bit 5

Bit 6

Bit 7

Bit 8 (LSB)

CLK

VDRV

+VS

GND

REFT

REFB

INT/EXT

RSEL

ADS831

PIN DESIGNATOR DESCRIPTION

1 GND Ground

2 Bit 1 Data Bit 1 (D7) (MSB)

3 Bit 2 Data Bit 2 (D6)

4 Bit 3 Data Bit 3 (D5)

5 Bit 4 Data Bit 4 (D4)

6 Bit 5 Data Bit 5 (D3)

7 Bit 6 Data Bit 6 (D2)

8 Bit 7 Data Bit 7 (D1)

9 Bit 8 Data Bit 8 (D0) (LSB)

10 CLK Convert Clock

11 RSEL Input Range Select: HI = 2V; LO = 1V

12 INT/EXT Reference Select: HI = External; LO = Internal

13 REFB Bottom Reference

14 REFT Top Reference

15 CM Common-Mode Voltage Output

16 IN Complementary Input

17 IN Analog Input

18 GND Ground

19 +VS+5V Supply

20 VDRV Output Logic Driver Supply Voltage

PIN DESCRIPTIONS

PIN CONFIGURATION

Top View SSOP

ADS831 5

SBAS087A

SPECTRAL PERFORMANCE

Frequency (MHz)

010203040

= 20MHz

SNR = 49dBFS

SFDR = 66dBFS

Magnitude (dB)

–10

–20

–30

–40

–50

–60

–70

–80

–90

SPECTRAL PERFORMANCE

Frequency (MHz)

010203040

Magnitude (dB)

–10

–20

–30

–40

–50

–60

–70

–80

–90

= 10MHz

SNR = 49dBFS

SFDR = 67dBFS

SPECTRAL PERFORMANCE

Frequency (MHz)

Magnitude (dB)

010203040

–10

–20

–30

–40

–50

–60

–70

–80

–90

= 1MHz

SNR = 49dBFS

SFDR = 68dBFS

TWO-TONE INTERMODULATION DISTORTION

Fre

uenc

(

MHz

)

Magnitude (dBFSR)

–10

–20

–30

–40

–50

–60

–70

–80

–90 010203040

f1 = 9.5MHz at –7dBFS

f2 = 9.9MHz at –7dBFS

IMD(3) = –57dBc

SPECTRAL PERFORMANCE

(Single-Ended, 1Vp-p)

Fre

uenc

(

MHz

)

Magnitude (dB)

–10

–20

–30

–40

–50

–60

–70

–80

–90 010203040

= 10MHz

SNR = 49dBFS

SFDR = 66dBFS

TYPICAL CHARACTERISTICS

At TA = full specified temperature range, single-ended input range = 1.5V to 3.5V, sampling rate = 80MHz, and external reference, unless otherwise noted.

DIFFERENTIAL LINEARITY ERROR

Output Code

DLE (LSB)

0.5

0.25

–0.25

–0.5 0 64 128 192 256

fIN = 10MHz

ADS831

6SBAS087A

TYPICAL CHARACTERISTICS (Cont.)

At TA = full specified temperature range, single-ended input range = 1.5V to 3.5V, sampling rate = 80MHz, and external reference, unless otherwise noted.

INTEGRAL LINEARITY ERROR

Output Code

ILE (LSB)

1.0

0.5

–0.5

–1.0 0 64 128 192 256

= 1MHz

DYNAMIC PERFORMANCE vs INPUT FREQUENCY

Input Frequency (MHz)

SFDR, SNR (dBFS)

40 0.1 1 10 100

SFDR

SNR

POWER DISSIPATION vs TEMPERATURE

Temperature (°C)

350

330

310

290

270

250–50 –25 0 25 50 10075

Power Dissipation (mW)

External Reference

VDRV = +5V

Internal Reference

Output Code

DLE (LSB)

1.0

0.5

0.25

–0.25

–0.5

–1.0 0 64 128 192 256

= 20MHz

DIFFERENTIAL LINEARITY ERROR

800k

600k

400k

200k

OUTPUT NOISE HISTOGRAM (DC Input)

Counts

N–2N–1 N N+1 N+2

Output Code

ADS831 7

SBAS087A

APPLICATION INFORMATION

THEORY OF OPERATION

The ADS831 is a high-speed CMOS A/D converter which

employs a pipelined converter architecture consisting of 6

internal stages. Each stage feeds its data into the digital error

correction logic ensuring excellent differential linearity and

no missing codes at the 8-bit level. The output data becomes

valid on the rising clock edge (see Timing Diagram). The

pipeline architecture results in a data latency of 4 clock

cycles.

The analog input of the ADS831 is a differential track and

hold, as shown in Figure 1. The differential topology along

with tightly matched capacitors produce a high level of ac

performance while sampling at very high rates.

The ADS831 allows its analog inputs to be driven either

single-ended or differentially. The typical configuration for

the ADS831 is for the single-ended mode in which the input

track and hold performs a single-ended to differential con-

version of the analog input signal.

Both inputs (IN, IN) require external biasing using a com-

mon-mode voltage that is typically at the mid-supply level

(+VS/2).

The following application discussion focuses on the single-

ended configuration. Typically, its implementation is easier

to achieve and the rated specifications for the ADS831 are

characterized using the single-ended mode of operation.

DRIVING THE ANALOG INPUT

The ADS831 achieves excellent ac performance either in the

single-ended or differential mode of operation. The selection

for the optimum interface configuration will depend on the

individual application requirements and system structure.

For example, communication applications often process a

band of frequencies that does not include DC, whereas in

imaging applications, the previously restored DC level must

be maintained correctly up to the A/D converter. Features on

the ADS831, such as the input range select (RSEL pin) or

the option for an external reference, provide the needed

flexibility to accommodate a wide range of applications. In

any case, the ADS831 should be configured such that the

application objectives are met while observing the headroom

requirements of the driving amplifier in order to yield the

best overall performance.

INPUT CONFIGURATIONS

AC-Coupled, Single-Supply Interface

Figure 2 shows the typical circuit for an ac-coupled analog

input configuration of the ADS831 where all components

are powered from a single +5V supply.

With the RSEL pin connected HIGH, the full scale input

range is set to 2Vp-p. In this configuration, the top and

bottom references (REFT, REFB) provide an output voltage

of +3.0V and +2.0V, respectively. Two resistors ( 2 x 1kΩ)

are used to create a common-mode voltage (VCM) of ap-

proximately +2.5V to bias the inputs of the driving ampli-

fier. Using the OPA681 on a single +5V supply, its ideal

common-mode point is +2.5V. This coincides with the

recommended common-mode input level for the ADS831

thus, obviating the need for a coupling capacitor between the

amplifier and the converter. Even though the OPA681 has an

ac gain of +2, the dc gain is only +1 due to the blocking

capacitor at resistor RG.

The addition of a small series resistor (RS) between the

output of the op amp and the input of the ADS831 will be

beneficial in almost all interface configurations. This will

de-couple the op amp’s output from the capacitive load and

avoid gain peaking, which can result in increased noise. For

best spurious and distortion performance, the resistor value

should be kept below 75Ω. The series resistor in combina-

tion with the 47pF capacitor establishes a passive low-pass

filter limiting the bandwidth for the wideband noise thus

help improving the SNR performance.

AC-Coupled, Dual Supply Interface

The circuit provided in Figure 3 shows typical connections

for the analog input in case the selected amplifier operates

on dual supplies. This might be necessary to take full

advantage of very low distortion operational amplifiers, like

the OPA642. The advantage is that the driving amplifier can

be operated with a ground referenced bipolar signal swing.

This will keep the distortion performance at its lowest since

the signal range stays within the linear region of the op amp

and sufficient headroom to the supply rails can be main-

tained. By capacitively coupling the single-ended signal to

the input of the ADS831, its common-mode requirements

can easily be satisfied with two resistors connected between

the top and bottom reference.

FIGURE 1. Simplified Circuit of Input Track and Hold with

Timing Diagram.

φ1

φ1φ2φ1

φ1φ1

φ1

φ2

φ1φ2φ1

φ2

OUT

Op Amp

Bias V

Op Amp

Bias V

Input Clock (50%)

Internal Non-overlapping Clock

ADS831

8SBAS087A

OPA642

VIN

402Ω

1kΩ

402Ω

ADS831

24.9Ω

1kΩ

47pF

0.1µF

0.1µFIN

REFB

+2.0V INT/EXT GND

REFT

+3.0V RSEL +VS

+5V

–5V

For applications requiring the driving amplifier to provide a

signal amplification with a gain ≥ 5, consider using decom-

pensated voltage feedback op amps, such as the OPA643, or

current feedback op amps OPA681 and OPA658.

DC-Coupled with Level Shift

Several applications may require that the bandwidth of the

signal path includes DC, in which case the signal has to be

DC-coupled to the A/D converter. In order to accomplish

this, the interface circuit has to provide a DC level shift to

the analog input signal. The circuit shown in Figure 4

employs a dual op amp, A1, to drive the input of the

ADS831 and level shift the signal to be compatible with

the selected input range. With the RSEL pin tied to the

supply and the INT/EXT pin to ground, the ADS831 is

configured for a 2Vp-p input range and uses the internal

references. The complementary input (IN) may be appropri-

ately biased using the +2.5V common-mode voltage avail-

able at the CM pin. One-half of the amplifier (OPA2681)

buffers the REFB pin and drives the voltage divider R1, R2.

Because of the op amp’s noise gain of +2V/V, assuming

RF = RIN , the common-mode voltage (VCM) has to be re-

scaled to +1.25V, resulting in the correct DC level of +2.5V

for the signal input (IN). Any DC voltage differences

between the IN and IN inputs of the ADS831 effectively

produce an offset, which can be corrected for by adjusting

the resistor values of the divider, R1 and R2. The selection

criteria for a suitable op amp should include the supply

voltage, input bias current, output voltage swing, distortion,

and noise specification. Note that in this example the overall

signal phase is inverted. To re-establish the original signal

polarity, it is always possible to interchange the IN and IN

connections.

FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS831 for a 2Vp-p Full-Scale Input Range.

–V

OPA681

402Ω

1kΩ

402Ω

ADS831

39Ω

47pF

0.1µF

INT/EXT GND

REFT

+3.0V

1kΩ

= +2.5V

REFB

+2.0V

0.1µF

RSEL +V

+5V

FIGURE 2. AC-Coupled Input Configuration for a 2Vp-p Full-Scale Range and a Common-Mode Voltage, VCM, at +2.5V

Derived From the Internal Top (REFT) and Bottom Reference (REFB). The OPA680 can be used in place of the

OPA681 if a voltage feedback amplifier is preferred.

ADS831 9

SBAS087A

FIGURE 5. Transformer Coupled Input. FIGURE 6. Equivalent Reference Circuit with Recommended

Reference Bypassing.

SINGLE ENDED-TO-DIFFERENTIAL CONFIGURATION

(Transformer Coupled)

If the application requires a signal conversion from a single-

ended source to feed the ADS831 differentially, a RF trans-

former might be a good solution. The selected transformer

must have a center tap in order to apply the common-mode

DC voltage necessary to bias the converter inputs.

AC grounding the center tap will generate the differential

signal swing across the secondary winding. Consider a step-

up transformer to take advantage of a signal amplification

without the introduction of another noise source. Further-

more, the reduced signal swing from the source may lead to

an improved distortion performance.

The differential input configuration may provide a notice-

able advantage of achieving good SFDR performance over

a wide range of input frequencies. In this mode both inputs

of the ADS831 see closely matched impedances, and the

differential signal swing is reduced to half of the swing

required for single-ended drive. Figure 5 shows the sche-

matic for the suggested transformer coupled interface cir-

cuit. The component values of the R-C lowpass may be

optimized depending on the desired roll-off frequency. The

resistor across the secondary side (RT) should be calculated

using the equation RT = n2 x RG to match the source

impedance (RG) for good power transfer and VSWR.

REFERENCE OPERATION

Figure 6 depicts the simplified model of the internal refer-

ence circuit. The internal blocks are the bandgap voltage

reference, the drivers for the top and bottom reference, and

VIN IN

IN CM

22Ω

47pF

+10µF 0.1µF

INT/EXTRSEL

+5V

ADS831

1:n

0.1µF

ADS831

REFT CM REFB

Bypass Capacitors: 0.1µF || 2.2µF each

Bandgap Reference and Logic

VREF

400Ω400Ω

+1+1

+VS

50kΩ50kΩ

INT/EXTRSEL

2Vp-p

NOTE: R

= R

, G = –1

301Ω

499Ω

ADS831

39Ω

47pF

0.1µF

CM (+2.5V)

INT/EXT

499Ω

= +1.25V

REFB

(+2.0V) REFT

(+3.0V)

1/2

OPA2681

1/2

OPA2681

1kΩ

50Ω0.1µF

0.1µF

RSEL +V

+5V

FIGURE 4. DC-Coupled Interface Circuit with Dual Current-Feedback Amplifier OPA2681. The OPA2680 can be used in place

of the OPA2681 if a voltage feedback amplifier is preferred.

ADS831

10 SBAS087A

REFT

+3.0V

ADS831

CMV

+2.5V

REFB

+2.0V

1kΩR

1kΩ

0.1µF0.1µF2.2µF +2.2µF

FIGURE 8. Configuration Example for External Reference Operation.

the resistive reference ladder. The bandgap reference circuit

includes logic functions that allow to set the analog input

swing of the ADS831 to either a 1Vp-p or 2Vp-p full-scale

range simply by tying the RSEL pin to a LOW or HIGH

potential, respectively. While operating the ADS831 in the

external reference mode, the buffer amplifiers for REFT and

REFB are disconnected from the reference ladder.

As shown, the ADS831 has internal 50kΩ pull-up resistors

at the Range Select pin (RSEL) and Reference Select pin

(INT/EXT). Leaving those pins open configures the ADS831

for a 2Vp-p input range and external reference operation.

Setting the ADS831 up for internal reference mode requires

to bring the INT/EXT pin LOW.

The reference buffers can be utilized to supply up to 1mA

(sink and source) to external circuitry. To ensure proper

operation with any reference configurations, it is necessary

to provide solid bypassing at the reference pins in order to

keep the clock feedthrough to a minimum (Figure 6). All

bypassing capacitors should be located as close to their

respective pins as possible.

The common-mode voltage available at the CM pin may be

used as a bias voltage to provide the appropriate offset for

the driving circuitry. However, care must be taken not to

appreciably load this node, which is not buffered and has a

high impedance. An alternative way of generating a com-

mon-mode voltage is given in Figure 7. Here, two external

precision resistors (1% tolerance or better) are located

between the top and bottom reference pins. The common-

mode voltage, CMV, will appear at the midpoint.

EXTERNAL REFERENCE OPERATION

For even more design flexibility, the internal reference can

be disabled and an external reference voltage be used. The

utilization of an external reference may be considered for

applications requiring higher accuracy, improved tempera-

ture performance, or a wide adjustment range of the

converter’s full-scale range. Especially in multichannel

applications, the use of a common external reference has the

benefit of obtaining better matching of the full-scale range

between converters.

The external references can vary as long as the value of the

external top reference REFTEXT stays within the range of

(VS – 1.25V) and (REFB + 0.8V), and the external bottom

reference REFBEXT stays within 1.25V and (REFT – 0.8V),

as shown in Figure 8.

The full-scale input signal range (FSR) of the ADS831 is

determined by the voltage difference across the reference

pins REFT and REFB (FSR = REFT – REFB), while the

common-mode voltage is defined by CMV = (REFT +

REFB)/2. In order to maintain good ac performance, it is

recommended that the typical common-mode voltage be

kept at +2.5V while setting the external reference voltages.

It is possible, however, to deviate from this common-mode

level without significantly impacting the performance. In

particular, DC-coupled applications may benefit from a

lower CMV as it increases the signal headroom of the

ADS831

INT/EXT

REFT GND REFB

External Top Reference

REFT = REFB +0.8V to +3.75V

+VS

RSEL GND

+5V

External Bottom Reference

REFB = REFT –0.8V to +1.25V

A - Short for 1Vp-p Input Range

B - Short for 2Vp-p Input Range (Default)

CMV

FIGURE 7. Alternative Circuit to Generate Common-Mode

Voltage.

ADS831 11

SBAS087A

driving amplifier. The internal reference ladder has a nomi-

nal impedance of 800Ω. Depending on the selected refer-

ence voltages, the required drive current will vary accord-

ingly and the external reference circuitry should be designed

to supply the maximum required current.

DIGITAL INPUTS AND OUTPUTS

Clock Input Requirements

Clock jitter is critical to the SNR performance of high speed,

high resolution Analog to Digital Converters. It leads to

aperture jitter (tA) which adds noise to the signal being

converted. The ADS831 samples the input signal on the

rising edge of the CLK input. Therefore, this edge should

have the lowest possible jitter. The jitter noise contribution

to total SNR is given by the following equation. If this value

is near your system requirements, input clock jitter must be

reduced.

Jitter SNR = 20log 1

rms signal to rms noise

IN A

Where: ƒIN is Input Signal Frequency

tA is rms Clock Jitter

Particularly in udersampling applications, special consider-

ation should be given to clock jitter. The clock input should

be treated as an analog input in order to achieve the highest

level of performance. Any overshoot or undershoot of the

clock signal may cause degradation of the performance.

When digitizing at high sampling rates, the clock should

have a 50% duty cycle (tH = tL), along with fast rise and fall

times of 2ns or less.

Digital Outputs

The output data format of the ADS831 is in positive Straight

Offset Binary code, see Table I. This format can easily

converted into the Two’s Binary Complement code by

inverting the MSB.

Digital Output Driver (VDRV)

The ADS831 features a dedicated supply pin for the output

logic drivers, VDRV, which is not internally connected to

the other supply pins. Setting the voltage at VDRV to +5V

or +3V the ADS831 produces corresponding logic levels

and can directly interface to the selected logic family. The

output stages are designed to supply sufficient current to

drive a variety of logic families. However, it is recom-

mended to use the ADS831 with +3V logic supply. This will

lower the power dissipation in the output stages due to the

lower output swing and reduce current glitches on the supply

line which may affect the ac performance of the converter.

In some applications, it might be advantageous to decouple

the VDRV pin with additional capacitors or a pi-filter.

GROUNDING AND DECOUPLING

Proper grounding and bypassing, short lead length, and the

use of ground planes are particularly important for high

frequency designs. Multilayer PC boards are recommended

for best performance since they offer distinct advantages

like minimizing ground impedance, separation of signal

layers by ground layers, etc. The ADS831 should be treated

as an analog component. Whenever possible, the supply pins

should be powered by the analog supply. This will ensure

the most consistent results since digital supply lines often

carry high levels of noise which otherwise would be coupled

into the converter and degrade the achievable performance.

All ground connections on the ADS831 are internally joined

together, obviating the design of split ground planes. The

ground pins (1, 18) should directly connect to an analog

ground plane which covers the PC board area around the

converter. While designing the layout, it is important to keep

the analog signal traces separated from any digital lines to

prevent noise coupling onto the analog signal path. Because

of its high sampling rate, the ADS831 generates high fre-

quency current transients and noise (clock feedthrough) that

are fed back into the supply and reference lines. This

requires that all supply and reference pins are sufficiently

bypassed. Figure 9 shows the recommended decoupling

scheme for the ADS831. In most cases, 0.1µF ceramic chip

capacitors at each pin are adequate to keep the impedance

low over a wide frequency range. Their effectiveness largely

depends on the proximity to the individual supply pin.

Therefore, they should be located as close to the supply pins

as possible. In addition, a larger bipolar capacitor (1µF to

22µF) should be placed on the PC board in proximity of the

converter circuit.

It is recommended to keep the capacitive loading on the data

lines as low as possible (≤ 15pF). Higher capacitive loading

will cause larger dynamic currents as the digital outputs are

changing. Those high current surges can feed back to the

analog portion of the ADS831 and affect the performance. If

necessary, external buffers or latches close to the converter’s

output pins may be used to minimize the capacitive loading.

They also provide the added benefit of isolating the ADS831

from any digital noise activities on the bus coupling back

high frequency noise.

GND

ADS831

0.1µF

+VS

19 18

GND

10µF

+5V

VDRV

0.1µF

+3/+5V

FIGURE 9. Recommended Bypassing for the Supply Pins.

+FS (IN = +3.5V) 1111 1111

+1/2 FS 1100 0000

+1LSB 1000 0001

Bipolar Zero (IN = 2.5V) 1000 0000

–1LSB 0111 1111

–1/2 FS 0100 0000

–FS (IN = +1.5V) 0000 0000

SINGLE-ENDED INPUT (2Vp-p) STRAIGHT OFFSET BINARY

(IN = CMV) (SOB)

TABLE I. Coding Table for the ADS831.

PACKAGE OPTION ADDENDUM

www.ti.com 16-Aug-2012

Addendum-Page 1

PACKAGING INFORMATION

Orderable Device Status (1) Package Type Package

Drawing Pins Package Qty Eco Plan (2) Lead/

Ball Finish MSL Peak Temp (3) Samples

(Requires Login)

ADS831E ACTIVE SSOP DBQ 20 50 Green (RoHS

& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR

ADS831E/2K5 ACTIVE SSOP DBQ 20 2500 Green (RoHS

& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR

ADS831E/2K5G4 ACTIVE SSOP DBQ 20 2500 Green (RoHS

& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR

ADS831EG4 ACTIVE SSOP DBQ 20 50 Green (RoHS

& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR

(1) The marketing status values are defined as follows:

ACTIVE: Product device recommended for new designs.

LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.

NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.

PREVIEW: Device has been announced but is not in production. Samples may or may not be available.

OBSOLETE: TI has discontinued the production of the device.

(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability

information and additional product content details.

TBD: The Pb-Free/Green conversion plan has not been defined.

Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that

lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.

Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between

the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.

Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight

in homogeneous material)

(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.

Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information

provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and

continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.

TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.

In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.

TAPE AND REEL INFORMATION

*All dimensions are nominal

Device Package

Type Package

Drawing Pins SPQ Reel

Diameter

(mm)

Reel

Width

W1 (mm)

(mm) B0

(mm) K0

(mm) P1

(mm) W

(mm) Pin1

Quadrant

ADS831E/2K5 SSOP DBQ 20 2500 330.0 16.4 6.5 9.0 2.1 8.0 16.0 Q1

PACKAGE MATERIALS INFORMATION

www.ti.com 16-Aug-2012

Pack Materials-Page 1

*All dimensions are nominal

Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)

ADS831E/2K5 SSOP DBQ 20 2500 367.0 367.0 38.0

PACKAGE MATERIALS INFORMATION

www.ti.com 16-Aug-2012

Pack Materials-Page 2

IMPORTANT NOTICE

Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other

changes to its semiconductor products and services per JESD46C and to discontinue any product or service per JESD48B. Buyers should

obtain the latest relevant information before placing orders and should verify that such information is current and complete. All

semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time

of order acknowledgment.

TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms

and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary

to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily

performed.

TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and

applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide

adequate design and operating safeguards.

TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or

other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information

published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or

endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the

third party, or a license from TI under the patents or other intellectual property of TI.

Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration

and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered

documentation. Information of third parties may be subject to additional restrictions.

Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service

voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.

TI is not responsible or liable for any such statements.

Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements

concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support

that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which

anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause

harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use

of any TI components in safety-critical applications.

In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to

help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and

requirements. Nonetheless, such components are subject to these terms.

No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties

have executed a special agreement specifically governing such use.

Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in

military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components

which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and

regulatory requirements in connection with such use.

TI has specifically designated certain components which meet ISO/TS16949 requirements, mainly for automotive use. Components which

have not been so designated are neither designed nor intended for automotive use; and TI will not be responsible for any failure of such

components to meet such requirements.

Products Applications

Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive

Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications

Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers

DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps

DSP dsp.ti.com Energy and Lighting www.ti.com/energy

Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial

Interface interface.ti.com Medical www.ti.com/medical

Logic logic.ti.com Security www.ti.com/security

Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense

Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video

RFID www.ti-rfid.com

OMAP Mobile Processors www.ti.com/omap TI E2E Community e2e.ti.com

Wireless Connectivity www.ti.com/wirelessconnectivity

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265