ADC08L060CIMT/NOPB - National Semiconductor

February 8, 2008

ADC08L060

8-Bit, 10 MSPS to 60 MSPS, 0.65 mW/MSPS A/D Converter

with Internal Sample-and-Hold

General Description

The ADC08L060 is a low-power, 8-bit, monolithic analog-to-

digital converter with an on-chip track-and-hold circuit. Opti-

mized for low cost, low power, small size and ease of use, this

product operates at conversion rates of 10 MSPS to

60 MSPS while consuming just 0.65 mW per MHz of clock

frequency, or 39 mW at 60 MSPS. Raising the PD pin puts

the ADC08L060 into a Power Down mode where it consumes

about 1 mW.

The unique architecture achieves 7.6 Effective Bits. The AD-

C08L060 is resistant to latch-up and the outputs are short-

circuit proof. The top and bottom of the ADC08L060s

reference ladder are available for connections, enabling a

wide range of input possibilities. The digital outputs are TTL/

CMOS compatible with a separate output power supply pin to

support interfacing with 1.8V to 3V logic. The output coding is

straight binary and the digital inputs (CLK and PD) are TTL/

CMOS compatible.

The ADC08L060 is offered in a 24-lead plastic package

(TSSOP) and is specified over the industrial temperature

range of −40°C to +85°C.

Features

■Single-ended input

■Internal sample-and-hold function

■Low voltage (single +3V) operation

■Small package

■Power-down feature

Key Specifications

■Resolution 8 bits

■Conversion rate 60 MSPS

■DNL ±0.25 LSB (typ)

■INL +0.5/−0.2 LSB (typ)

■SNR (10.1 MHz) 48 dB (typ)

■ENOB (10.1 MHz) 7.6 bits (typ)

■THD (10.1 MHz) −57 dB (typ)

■Latency 5 Clock Cycles

■No missing codes Guaranteed

■Power Consumption

■—Operating 0.65 mW/MSPS (typ)

—Power Down Mode 1.0 mW (typ)

Applications

■Digital Imaging

■Set-top boxes

■Portable Instrumentation

■Communication Systems

■X-ray imaging

■Viterbi decoders

Pin Configuration

20041701

ADC08L060 8-Bit, 10 MSPS to 60 MSPS, 0.65 mW/MSPS A/D Converter with Internal

Sample-and-Hold

Ordering Information

Order Number Temperature Range Package

ADC08L060CIMT −40°C ≤ TA ≤ +85°C TSSOP

ADC08L060CIMTX −40°C ≤ TA ≤ +85°C TSSOP (tape and reel)

Block Diagram

20041702

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ADC08L060

Pin Descriptions and Equivalent Circuits

Pin No. Symbol Equivalent Circuit Description

6VIN Analog signal input. Conversion range is VRB to VRT.

3VRT

Analog Input that is the high (top) side of the reference ladder

of the ADC. Nominal range is 0.5V to VA. Voltage on VRT and

VRB inputs define the VIN conversion range. Bypass well. See

Section 2.0 for more information.

9VRM

Mid-point of the reference ladder. This pin should be

bypassed to a quiet point in the analog ground plane with a

0.1 µF capacitor.

10 VRB

Analog Input that is the low side (bottom) of the reference

ladder of the ADC. Nominal range is 0.0V to (VRT – 0.5V).

Voltage on VRT and VRB inputs define the VIN conversion

range. Bypass well. See Section 2.0 for more information.

23 PD

Power Down input. When this pin is high, the converter is in

the Power Down mode and the data output pins hold the last

conversion result.

24 CLK CMOS/TTL compatible digital clock Input. VIN is sampled on

the rising edge of CLK input.

13 thru 16

and

19 thru 22

D0–D7

Conversion data digital Output pins. D0 is the LSB, D7 is the

MSB. Valid data is output after the rising edge of the CLK

input.

7VIN GND Reference ground for the single-ended analog input, VIN.

1, 4, 12 VA

Positive analog supply pin. Connect to a quiet voltage source

of +3V. VA should be bypassed with a 0.1 µF ceramic chip

capacitor for each pin, plus one 10 µF capacitor. See Section

3.0 for more information.

18 VDR Power supply for the output drivers. If connected to VA,

decouple well from VA.

17 DR GND The ground return for the output driver supply.

2, 5, 8, 11 AGND The ground return for the analog supply.

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ADC08L060

Absolute Maximum Ratings

(Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltage (VA)3.8V

Driver Supply Voltage (VDR) VA +0.3V

Voltage on Any Input or Output Pin −0.3V to VA

Reference Voltage (VRT, VRB) VA to AGND

CLK, PD Voltage Range −0.05V to

(VA + 0.05V)

Input Current at Any Pin (Note 3) ±25 mA

Package Input Current (Note 3) ±50 mA

Power Dissipation at TA = 25°C See (Note 5)

ESD Susceptibility (Note 6)

Human Body Model

Machine Model

2500V

200V

Soldering Temperature, Infrared,

10 seconds (Note 7) 235°C

Storage Temperature −65°C to +150°C

Operating Ratings (Notes 1, 2)

Operating Temperature Range −40°C ≤ TA ≤ +85°C

Supply Voltage, VA+2.4V to +3.6V

Driver Supply Voltage, VDR +2.4V to VA

Output Driver Voltage, VDR 1.8V to VA

Ground Difference |GND − DR GND| 0V to 300 mV

Upper Reference Voltage (VRT) 0.5V to (VA −0.3V)

Lower Reference Voltage (VRB) 0V to (VRT −0.5V)

VIN Voltage Range VRB to VRT

Package Thermal Resistance

Package θJA

24-Lead TSSOP 92°C/W

Converter Electrical Characteristics

The following specifications apply for VA = VDR = +3.0VDC, VRT = +1.9V, VRB = 0.3V, CL = 10 pF, fCLK = 60 MHz at 50% duty cycle.

Boldface limits apply for TJ = TMIN to TMAX: all other limits TJ = 25°C (Notes 4, 8, 9)

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

DC ACCURACY

INL Integral Non-Linearity +0.5

−0.2

+1.9

−1.35

LSB (max)

LSB (min)

DNL Differential Non-Linearity ±0.25 ±0.90 LSB (max)

Missing Codes 0(max)

FSE Full Scale Error 3.0 ±13 mV (max)

VOFF Zero Scale Offset Error 19 27 mV (max)

ANALOG INPUT AND REFERENCE CHARACTERISTICS

VIN Input Voltage 1.6 VRB V (min)

VRT V (max)

CIN VIN Input Capacitance VIN = 0.75V +0.5 Vrms (CLK LOW) 3 pF

(CLK HIGH) 4 pF

RIN RIN Input Resistance >1 MΩ

BW Full Power Bandwidth 270 MHz

VRT Top Reference Voltage 1.9 VAV (max)

0.5 V (min)

VRB Bottom Reference Voltage 0.3 VRT − 0.5 V (max)

0V (min)

VRT - VRB Reference Delta 1.6 2.3 V (max)

1.0 V (min)

RREF Reference Ladder Resistance VRT to VRB 720 590 Ω (min)

1070 Ω (max)

Iref Reference Ladder Current VRT to VRB 2.2 1.5 mA (min)

2.7 mA (max)

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ADC08L060

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

CLK, PD DIGITAL INPUT CHARACTERISTICS

VIH Logical High Input Voltage VDR = VA = 3.6V 2.0 V (min)

VIL Logical Low Input Voltage VDR = VA = 2.7V 0.8 V (max)

IIH Logical High Input Current VIH = VDR = VA = 3.6V 10 nA

IIL Logical Low Input Current VIL = 0V, VDR = VA = 2.7V −50 nA

CIN Logic Input Capacitance 3 pF

DIGITAL OUTPUT CHARACTERISTICS

VOH High Level Output Voltage VA = VDR = 2.7V, IOH = −400 µA 2.6 2.4 V (min)

VOL Low Level Output Voltage VA = VDR = 2.7V, IOL = 1.0 mA 0.4 0.5 V (max)

DYNAMIC PERFORMANCE

ENOB Effective Number of Bits fIN = 10.1 MHz, VIN = FS − 0.25 dB 7.6 6.9 Bits (min)

fIN = 29 MHz, VIN = FS − 0.25 dB 7.4 Bits

SINAD Signal-to-Noise & Distortion fIN = 10.1 MHz, VIN = FS − 0.25 dB 47.4 43.3 dB (min)

fIN = 29 MHz, VIN = FS − 0.25 dB 46.1 dB

SNR Signal-to-Noise Ratio fIN = 10.1 MHz, VIN = FS − 0.25 dB 48 44.5 dB (min)

fIN = 29 MHz, VIN = FS − 0.25 dB 47.2 dB

SFDR Spurious Free Dynamic Range fIN = 10.1 MHz, VIN = FS − 0.25 dB 59.1 dBc

fIN = 29 MHz, VIN = FS − 0.25 dB 54.5 dBc

THD Total Harmonic Distortion fIN = 10.1 MHz, VIN = FS − 0.25 dB −56.9 dBc

fIN = 29 MHz, VIN = FS − 0.25 dB −53.3 dBc

HD2 2nd Harmonic Distortion fIN = 10.1 MHz, VIN = FS − 0.25 dB -61.1 dBc

fIN = 29 MHz, VIN = FS − 0.25 dB −54.9 dBc

HD3 3rd Harmonic Distortion fIN = 10.1 MHz, VIN = FS − 0.25 dB −64.2 dBc

fIN = 29 MHz, VIN = FS − 0.25 dB −63.1 dBc

IMD Intermodulation Distortion f1 = 11 MHz, VIN = FS − 6.25 dB

f2 = 12 MHz, VIN = FS − 6.25 dB −55 dBc

POWER SUPPLY CHARACTERISTICS

IAAnalog Supply Current DC Input 13 15.9 mA (max)

fIN = 10 MHz, VIN = FS − 3 dB 14 mA

DRIDOutput Driver Supply Current DC Input 0.04 0.2 mA (max)

fIN = 10 MHz, VIN = FS − 3 dB (Note 11) 4.2 mA

IA + DRIDTotal Operating Current

DC Input 13 16.1 mA (max)

fIN = 10 MHz, VIN = FS − 3 dB, PD = Low 18.2 mA

CLK Low, PD = Hi 0.33 mA

PC Power Consumption

DC Input 39 48.3 mW (max)

fIN = 10 MHz, VIN = FS − 3 dB, PD = Low 53 mW

CLK Low, PD = Hi 1 mW

PSRR1Power Supply Rejection Ratio FSE change with 2.7V to 3.3V change in

−51 dB

PSRR2Power Supply Rejection Ratio SNR reduction with 200 mV at 1MHz on

supply 45 dB

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ADC08L060

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

AC ELECTRICAL CHARACTERISTICS

fC1 Maximum Conversion Rate 80 60 MHz (min)

fC2 Minimum Conversion Rate 10 MHz

tCL Minimum Clock Low Time 0.62 ns (min)

tCH Minimum Clock High Time 0.62 ns (min)

DC Clock Duty Cycle 5

95 %(min)

%(max)

tOH Output Hold Time CLK to Data Invalid 5.2 ns

tOD Output Delay CLK to Data Transition 7.1 5.0 ns (min)

9.4 ns (max)

Pipeline Delay (Latency) 5 Clock Cycles

tAD Sampling (Aperture) Delay CLK Rise to Acquisition of Data 2.6 ns

tAJ Aperture Jitter 2 ps rms

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability

and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in

the Recommended Operating Conditions is not implied. The recommended Operating Conditions indicate conditions at which the device is functional and the

device should not be operated beyond such conditions..

Note 2: All voltages are measured with respect to GND = AGND = DR GND = 0V, unless otherwise specified.

Note 3: When the input voltage at any pin exceeds the power supplies (that is, less than AGND or DR GND, or greater than VA or VDR), the current at that pin

should be limited to 25 mA. The 50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input

current of 25 mA to two.

Note 4: The Electrical characteristics tables list guaranteed specifications under the listed Recommended Conditions except as otherwise modified or specified

by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations for room temperature only and are not guaranteed.

Note 5: The absolute maximum junction temperature (TJmax) for this device is 150°C. The maximum allowable power dissipation is dictated by TJmax, the

junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA) / θJA. The values

for maximum power dissipation will be reached only when this device is operated in a severe fault condition (e.g., when input or output pins are driven beyond

the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided.

Note 6: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through ZERO Ohms.

Note 7: See AN-450, “Surface Mounting Methods and Their Effect on Product Reliability”.

Note 8: The analog inputs are protected as shown below. Input voltage magnitudes up to VA + 300 mV or to 300 mV below GND will not damage this device.

However, errors in the A/D conversion can occur if the input goes above VDR or below GND by more than 100 mV. For example, if VA is 2.7VDC the full-scale

input voltage must be ≤2.8VDC to ensure accurate conversions.

20041707

Note 9: To guarantee accuracy, it is required that VA and VDR be well bypassed. Each supply pin must be decoupled with separate bypass capacitors.

Note 10: Typical figures are at TJ = 25°C, and represent most likely parametric norms at specific conditions at the time of product characterization and are not

guaranteed. Test limits are guaranteed to National's AOQL (Average Outgoing Quality Level).

Note 11: IDR is the current consumed by the switching of the output drivers and is primarily determined by the load capacitance on the output pins, the supply

voltage, VDR, and the rate at which the outputs are switching (which is signal dependent), IDR = VDR (CO x fO + C1 x f1 + … + C71 x f7) where VDR is the output

driver power supply voltage, Cn is the total capacitance on any given output pin, and fn is the average frequency at which that pin is toggling.

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ADC08L060

Specification Definitions

APERTURE (SAMPLING) DELAY is that time required after

the rise of the clock input for the sampling switch to open. The

Sample/Hold circuit effectively stops capturing the input sig-

nal and goes into the “hold” mode tAD after the clock goes

high.

APERTURE JITTER is the variation in aperture delay from

sample to sample. Aperture jitter shows up as input noise.

CLOCK DUTY CYCLE is the ratio of the time that the clock

wave form is at a logic high to the total time of one clock pe-

riod.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

the maximum deviation from the ideal step size of 1 LSB.

Measured at 60 MSPS with a ramp input.

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE

BITS) is another method of specifying Signal-to-Noise and

Distortion Ratio, or SINAD. ENOB is defined as (SINAD –

1.76) / 6.02 and says that the converter is equivalent to a per-

fect ADC of this (ENOB) number of bits.

FULL POWER BANDWIDTH is a measure of the frequency

at which the reconstructed output fundamental drops 3 dB

below its low frequency value for a full scale input.

FULL-SCALE ERROR is a measure of how far the last code

transition is from the ideal 1½ LSB below VRT and is defined

as:

Vmax + 1.5 LSB – VRT

where Vmax is the voltage at which the transition to the maxi-

mum (full scale) code occurs.

INTEGRAL NON-LINEARITY (INL) is a measure of the de-

viation of each individual code from a line drawn from zero

scale (½ LSB below the first code transition) through positive

full scale (½ LSB above the last code transition). The devia-

tion of any given code from this straight line is measured from

the center of that code value. The end point test method is

used. Measured at 60 MSPS with a ramp input.

INTERMODULATION DISTORTION (IMD) is the creation of

additional spectral components as a result of two sinusoidal

frequencies being applied to the ADC input at the same time.

it is defined as the ratio of the power in the second and third

order intermodulation products to the power in one of the

original frequencies. IMD is usually expressed in dBFS.

LSB (LEAST SIGNIFICANT BIT) is the bit that has the small-

est value or weight of all bits. This value is

(VRT − VRB) / 2n

where “n” is the ADC resolution, which is 8 in the case of the

ADC08L060.

MISSING CODES are those output codes that are skipped

and will never appear at the ADC outputs. These codes can-

not be reached with any input value.

MSB (MOST SIGNIFICANT BIT) is the bit that has the largest

value or weight. Its value is one half of full scale.

OUTPUT DELAY is the time delay after the rising edge of the

input clock before the data update is present at the output

pins.

OUTPUT HOLD TIME is the length of time that the output data

is valid after the rise of the input clock.

PIPELINE DELAY (LATENCY) is the number of clock cycles

between initiation of conversion and when that data is pre-

sented to the output driver stage. New data is available at

every clock cycle, but the data lags the conversion by the

Pipeline Delay plus the Output Delay.

POWER SUPPLY REJECTION RATIO (PSRR) is a measure

of how well the ADC rejects a change in the power supply

voltage. For the ADC08L060, PSRR1 is the ratio of the

change in Full-Scale Error that results from a change in the

d.c. power supply voltage, expressed in dB. PSRR2 is a mea-

sure of how well an a.c. signal riding upon the power supply

is rejected and is here defined as

where SNR0 is the SNR measured with no noise or signal on

the supply lines and SNR1 is the SNR measured with a

1 MHz, 200 mVP-P signal riding upon the supply lines.

SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in

dB, of the rms value of the input signal at the output to the rms

value of the sum of all other spectral components below one-

half the sampling frequency, not including harmonics or d.c.

SIGNAL TO NOISE PLUS DISTORTION (S/(N+D) or

SINAD) is the ratio, expressed in dB, of the rms value of the

input signal at the output to the rms value of all of the other

spectral components below half the clock frequency, includ-

ing harmonics but excluding d.c.

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-

ence, expressed in dB, between the rms values of the input

signal at the output and the peak spurious signal, where a

spurious signal is any signal present in the output spectrum

that is not present at the input.

TOTAL HARMONIC DISTORTION (THD) is the ratio ex-

pressed in dB, of the rms total of the first nine harmonic levels

at the output to the level of the fundamental at the output. THD

is calculated as

where Af1 is the RMS power of the fundamental (output) fre-

quency and Af2 through Af10 are the RMS power of the first 9

harmonic frequencies in the output spectrum.

ZERO SCALE OFFSET ERROR is the error in the input volt-

age required to cause the first code transition. It is defined as

VOFF = VZT − VRB

where VZT is the first code transition input voltage.

2nd HARMONIC DISTORTION (2nd HARM) is the differ-

ence, expressed in dB, between the rms power in the output

fundamental frequency and the power in its 2nd harmonic at

the output.

3rd HARMONIC DISTORTION (3rd HARM) is the difference,

expressed in dB, between the rms power in the output fun-

damental frequency and the power in its 3rd harmonic at the

output.

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ADC08L060

Timing Diagram

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FIGURE 1. ADC08L060 Timing Diagram

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ADC08L060

Typical Performance Characteristics VA = VDR = 3V, fCLK = 60 MHz, fIN = 10 MHz, unless otherwise

stated

INL

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INL vs. Temperature

20041712

INL vs. Supply Voltage, VA

20041713

INL vs. Sample Rate

20041714

INL vs. Clock Duty Cycle

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DNL

20041754

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ADC08L060

DNL vs. Temperature

20041717

DNL vs. Supply Voltage, VA

20041718

DNL vs. Sample Rate

20041719

DNL vs. Clock Duty Cycle

20041720

SNR, SINAD and SFDR vs. Temperature

20041721

SNR, SINAD and SFDR vs. Supply Voltage, VA

20041722

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ADC08L060

SNR, SINAD and SFDR vs. Sample Rate

20041723

SNR, SINAD and SFDR vs. Input Frequency

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SNR, SINAD and SFDR vs. Clock Duty Cycle

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Distortion vs. Temperature

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Distortion vs. Supply Voltage, VA

20041727

Distortion vs. Sample Rate

20041728

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ADC08L060

Distortion vs. Input Frequency

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Distortion vs. Clock Duty Cycle

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Power Consumption (Active) vs.

Sample Rate (fIN = d.c.)

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Power Consumption (Active) vs.

Sample Rate (fIN = d.c.)

20041738

Power Consumption (Active) vs.

Sample Rate (fIN = 1 MHz)

20041739

Power Consumption (Active) vs.

Sample Rate (fIN = 1 MHz)

20041740

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ADC08L060

Spectral Response @ fIN = 10 MHz

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Spectral Response @ fIN = 29 MHz

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Spectral Response @ fIN = 75 MHz

20041756

Spectral Response @ fIN = 98.9 MHz

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ADC08L060

Functional Description

The ADC08L060 uses a unique architecture that achieves

over 7 effective bits at input frequencies up to and beyond

Nyquist.

The analog input signal that is within the voltage range set by

VRT and VRB is digitized to eight bits. Input voltages below

VRB will cause the output word to consist of all zeroes. Input

voltages above VRT will cause the output word to consist of

all ones.

Incorporating a switched capacitor bandgap, the ADC08L060

exhibits a power consumption that is proportional to frequen-

cy, limiting power consumption to what is needed at the clock

rate that is used. This and its excellent performance over a

wide range of clock frequencies makes it an ideal choice as

a single ADC for many 8-bit needs.

Data is acquired at the rising edge of the clock and the digital

equivalent of that data is available at the digital outputs 5 clock

cycles plus tOD later. The ADC08L060 will convert as long as

an adequate clock signal is present at pin 24. The output cod-

ing is straight binary.

The device is in the active state when the Power Down pin

(PD) is low. When the PD pin is high, the device is in the power

down mode, where the output pins hold the last conversion

before the PD pin went high and the device consumes about

1.4 mW. Holding the clock input low will further reduce the

power consumption in the power down mode to about 1 mW

Applications Information

1.0 REFERENCE INPUTS

The reference inputs VRT and VRB are the top and bottom of

the reference ladder, respectively. Input signals between

these two voltages will be digitized to 8 bits. External voltages

applied to the reference input pins should be within the range

specified in the Operating Ratings table. Any device used to

drive the reference pins should be able to source sufficient

current into the VRT pin and sink sufficient current from the

VRB pin to keep these voltages stable.

20041732

FIGURE 2. Simple, low component count reference biasing. Because of the ladder and external resistor tolerances, the

reference voltage of this circuit can vary too much for some applications.

The reference bias circuit of Figure 2 is very simple and the

performance is adequate for many applications. However,

circuit tolerances will lead to a wide reference voltage range.

Better reference stability can be achieved by driving the ref-

erence pins with low impedance sources.

The circuit of Figure 3 will allow a more accurate setting of the

reference voltages. The upper amplifier must be able to

source the reference current as determined by the value of

the reference resistor and the value of (VRT - VRB). The lower

amplifier must be able to sink this reference current. Both

should be stable with a capacitive load. The LM8272 was

chosen because of its rail-to-rail input and output capability,

its high current output and its ability to drive large capacitance

loads. Of course, the divider resistors at the amplifier input

could be changed to suit your reference voltage needs, or the

divider can be replaced with potentiometers for precise set-

tings. The bottom of the ladder (VRB) may simply be returned

to ground if the minimum input signal excursion is 0V. Be sure

that the driving source can source sufficient current into the

VRT pin and sink enough current from the VRB pin to keep

these pins stable.

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ADC08L060

VRT should always be more positive than VRB by the minimum

VRT - VRB difference in the Electrical Characteristics table to

minimize noise. Furthermore, the difference between VRT and

VRB should not exceed the maxumum value specified in the

Electrical Characteristics table to avoid signal distortion.

The VRM pin is the center of the reference ladder and should

be bypassed to a quiet point in the analog ground plane with

a 0.1 µF capacitor. DO NOT allow this pin to float.

20041733

FIGURE 3. Driving the reference to force desired values requires driving with a low impedance source.

2.0 THE ANALOG INPUT

The analog input of the ADC08L060 is a switch followed by

an integrator. The input capacitance changes with the clock

level, appearing as 3 pF when the clock is low, and 4 pF when

the clock is high. The sampling nature of the analog input

causes current spikes that result in voltage spikes at the ana-

log input pin. Any circuit used to drive the analog input must

be able to drive that input and to settle within the clock low

time. The LMH6702 has been found to be a good amplifier to

drive the ADC08L060.

Figure 4 shows an example of an input circuit using the

LMH6702. Any input amplifier should incorporate some gain

as operational amplifiers exhibit better phase margin and

transient response with gains above 2 or 3 than with unity

gain. If an overall gain of less than 3 is required, attenuate the

input and operate the amplifier at a higher gain, as shown in

Figure 4.

The RC at the amplifier output filters the clock rate energy that

comes out of the analog input due to the input sampling circuit.

The optimum time constant for this circuit depends not only

upon the amplifier and ADC, but also on the circuit layout and

board material. A resistor value should be chosen between

10Ω and 47Ω and the capacitor value chose according to the

formula

This will provide optimum SNR performance. Best THD per-

formance is realized when the capacitor and resistor values

are both zero. To optimize SINAD, reduce the capacitor value

until SINAD performance is optimized. That is, until SNR =

−THD. This value will usually be in the range of 20& to 65%

of the value calculated with the above formula. An accurate

calculation is not possible because of the board material and

layout dependence.

The circuit of Figure 4 has both gain and offset adjustments.

If you eliminate these adjustments normal circuit tolerances

may result in signal clipping unless care is exercised in the

worst case analysis of component tolerances and the input

signal excursion is appropriately limited to account for the

worst case conditions.

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ADC08L060

20041734

FIGURE 4. The input amplifier should incorporate some gain for best performance (see text).

3.0 POWER SUPPLY CONSIDERATIONS

A/D converters draw sufficient transient current to corrupt

their own power supplies if not adequately bypassed. A

10 µF tantalum or aluminum electrolytic capacitor should be

placed within an inch (2.5 cm) of the A/D power pins, with a

0.1 µF ceramic chip capacitor placed within one centimeter of

the converter's power supply pins. Leadless chip capacitors

are preferred because they have low lead inductance.

While a single voltage source is recommended for the VA and

VDR supplies of the ADC08L060, these supply pins should be

well isolated from each other to prevent any digital noise from

being coupled into the analog portions of the ADC. A choke

or 27Ω resistor is recommended between these supply lines

with adequate bypass capacitors close to the supply pins.

As is the case with all high speed converters, the ADC08L060

should be assumed to have little power supply rejection. None

of the supplies for the converter should be the supply that is

used for other digital circuitry in any system with a lot of digital

power being consumed. The ADC supplies should be the

same supply used for other analog circuitry.

No pin should ever have a voltage on it that is in excess of the

supply voltage or below ground by more than 300 mV, not

even on a transient basis. This can be a problem upon appli-

cation of power and power shut-down. Be sure that the sup-

plies to circuits driving any of the input pins, analog or digital,

do not come up any faster than does the voltage at the AD-

C08L060 power pins.

4.0 THE DIGITAL INPUT PINS

The ADC08L060 has two digital input pins: The PD pin and

the Clock pin.

4.1 The PD Pin

The Power Down (PD) pin, when high, puts the ADC08L060

into a low power mode where power consumption is reduced

to 1.4 mW with the clock running, or to about 1 mW with the

clock held low. Output data is valid and accurate about 1 mi-

crosecond after the PD pin is brought low.

The digital output pins retain the last conversion output code

when either the clock is stopped or the PD pin is high.

4.2 The ADC08L060 Clock

Although the ADC08L060 is tested and its performance is

guaranteed with a 60 MHz clock, it typically will function well

with clock frequencies from 10 MHz to 80 MHz.

4.2.1 Clock Duty Cycle

The low and high times of the clock signal can affect the per-

formance of any A/D Converter. Because achieving a precise

duty cycle is difficult, the ADC08L060 is designed to maintain

performance over a range of duty cycles. While it is specified

and performance is guaranteed with a 50% clock duty cycle

and 60 Msps, ADC08L060 performance is typically main-

tained with clock high and low times of 0.83 ns, corresponding

to a clock duty cycle range of 5% to 95% with a 60 MHz clock.

Note that minimum low and high times may not be simulta-

neously asserted.

4.2.2 Clock Line Termination

The CLOCK line should be series terminated at the clock

source in the characteristic impedance of that line. If the clock

line is longer than

www.national.com 16

ADC08L060

where tr is the clock rise time and tprop is the propagation rate

of the signal along the trace. The CLOCK pin should be a.c.

terminated with a series RC to ground such that the resistor

value is equal to the characteristic impedance of the clock line

and the capacitor value is

where “L” is the line length in inches and ZO is the character-

istic impedance of the clock line. Typical tPROP is about 150

ps/inch on FR-4 board material. For FR-4 board material, the

value of C becomes

This termination should be located as close as possible to,

but within one centimeter of, the ADC08L060 clock pin.

5.0 LAYOUT AND GROUNDING

Proper grounding and proper routing of all signals are essen-

tial to ensure accurate conversion. A combined analog and

digital ground plane should be used.

Since digital switching transients are composed largely of

high frequency components, total ground plane copper

weight will have little effect upon the logic-generated noise

because of the skin effect. Total surface area is more impor-

tant than is total ground plane volume. Capacitive coupling

between the typically noisy digital circuitry and the sensitive

analog circuitry can lead to poor performance that may seem

impossible to isolate and remedy. The solution is to keep the

analog circuitry well separated from the digital circuitry.

The DR GND connection to the ground plane should not use

the same feedthrough used by other ground connections.

High power digital components should not be located on or

near a straight line between the ADC or any linear component

and the power supply area as the resulting common return

current path could cause fluctuation in the analog input

“ground” return of the ADC.

Keeping analog and digital return (ground) currents separate

from each other will improve system noise performance. Two

methods may be used to do this. Use of traces rather than a

solid plane to route power to all components will accomplish

this because return currents follow the path of the outgoing

currents. However, the advantage of the distributed capaci-

tance of a power plane and a ground plane is lost. Analog and

digital power should be routed as far from each other as is

practical. The analog power trace should also be routed away

from digital areas of the board.

The use of power and ground planes in adjacent layers will

provide distributed capacitance for a low impedance power

distribution system and better system noise performance.

The use of separate analog and digital power planes, both in

the same PC board layer, and the use of a single, non-split

ground plane will keep analog and digital currents separated

from each other. Of course, locate all analog circuitry and

traces over the analog power plane and the digital circuitry

and traces over the digital power plane. To minimize RFI/EMI,

give proper attention to any lines crossing the analog/digital

power plane boundary.

Noise performance is also enhanced by driving a single gate

with each ADC output pin and locating the gate as close as

possible to the ADC output. Inserting a 47Ω resistor in series

with the ADC digital output pins will also help reduce ADC

noise. Be sure to keep the resistors as close to the ADC out-

put pins as possible. Eliminating ground plane copper be-

neath the ADC output lines can also help ADC noise

performance, but could produce unacceptable radiation from

the board.

Analog and digital circuitry should be kept well away from

each other. Especially troublesome is high power digital com-

ponents such as processors and large PLDs. Switch mode

power supplies, including capacitive DC-DC converters, can

cause noise problems with high speed ADCs. Keep such

components well away from ADCs and low level analog signal

areas. Such components should be located as close to the

power supply as possible and should not be in the path of

analog signal or power supply currents.

Digital circuits create substantial supply and ground current

transients. The noise thus generated could have significant

impact upon system noise performance. The best logic family

to use in systems with A/D converters is one that employs

non-saturating transistor designs, or has low noise charac-

teristics, like the 74LS and the 74AC(T)Q families. The worst

noise generators are logic families that draw the largest sup-

ply current transients during clock or signal edges, like the

74HC, 74F and 74AC(T) families.

Since digital switching transients are composed largely of

high frequency components, total ground plane copper

weight will have little effect upon logic-generated noise. This

is because of the skin effect. Total surface area is more im-

portant than is total ground plane volume.

Clock lines should be isolated from ALL other lines, analog

AND digital. Even the generally accepted 90° crossing should

be avoided as even a little coupling can cause problems at

high frequencies. Best performance at high frequencies is

obtained with a straight signal path.

20041736

FIGURE 5. Layout Example

The analog input should be isolated from noisy signal traces

to avoid coupling of spurious signals into the input. Any ex-

ternal component (e.g., a filter capacitor) connected between

the converter's input and ground should be connected to a

very clean point in the ground plane.

17 www.national.com

ADC08L060

Figure 5 gives an example of a suitable layout. All analog cir-

cuitry (input amplifiers, filters, reference components, etc.)

should be placed together away from any digital components.

6.0 DYNAMIC PERFORMANCE

The ADC08L060 is a.c. tested and its dynamic performance

is guaranteed. To meet the published specifications, the clock

source driving the CLK input should exhibit less than 10 ps

(rms) of jitter. For best a.c. performance, isolating the ADC

clock from any digital circuitry should be done with adequate

buffers, as with a clock tree. See Figure 6.

It is good practice to keep the ADC clock line as short as pos-

sible and to keep it well away from any other signals. Other

signals can introduce jitter into the clock signal. The clock

signal can also introduce noise into the analog path.

20041737

FIGURE 6. Isolating the ADC Clock from Digital Circuitry

7.0 COMMON APPLICATION PITFALLS

Driving the inputs (analog or digital) beyond the power

supply rails. For proper operation, all inputs should not go

more than 300 mV below the ground pins or 300 mV above

the supply pins. Exceeding these limits on even a transient

basis may cause faulty or erratic operation. It is not uncom-

mon for high speed digital circuits (e.g., 74F and 74AC de-

vices) to exhibit undershoot that goes more than a volt below

ground. A 51Ω resistor in series with the offending digital input

will usually eliminate the problem.

Care should be taken not to overdrive the inputs of the AD-

C08L060. Such practice may lead to conversion inaccuracies

and even to device damage.

Attempting to drive a high capacitance digital data bus.

The more capacitance the output drivers must charge for

each conversion, the more instantaneous digital current is re-

quired from VDR and DR GND. These large charging current

spikes can couple into the analog section, degrading dynamic

performance. Buffering the digital data outputs (with a

74F541, for example) may be necessary if the data bus ca-

pacitance exceeds 5 pF. Dynamic performance can also be

improved by adding 100Ω series resistors at each digital out-

put, reducing the energy coupled back into the converter input

pins.

Using an inadequate amplifier to drive the analog input.

As explained in Section 2.0, the capacitance seen at the input

alternates between 3 pF and 4 pF with the clock. This dynamic

capacitance is more difficult to drive than is a fixed capaci-

tance, and should be considered when choosing a driving

device.

Driving the VRT pin or the VRB pin with devices that can

not source or sink the current required by the ladder. As

mentioned in Section 1.0, care should be taken to see that

any driving devices can source sufficient current into the

VRT pin and sink sufficient current from the VRB pin. If these

pins are not driven with devices than can handle the required

current, these reference pins will not be stable, resulting in a

reduction of dynamic performance.

Using a clock source with excessive jitter, using an ex-

cessively long clock signal trace, or having other signals

coupled to the clock signal trace. This will cause the sam-

pling interval to vary, causing excessive output noise and a

reduction in SNR performance. The use of simple gates with

RC timing is generally inadequate as a clock source.

www.national.com 18

ADC08L060

Physical Dimensions inches (millimeters) unless otherwise noted

NOTES: UNLESS OTHERWISE SPECIFIED

REFERENCE JEDEC REGISTRATION mo-153, VARIATION AD, DATED 7/93.

24-Lead Package TC

Order Number ADC08L060CIMT

NS Package Number MTC24

19 www.national.com

ADC08L060

Notes

ADC08L060 8-Bit, 10 MSPS to 60 MSPS, 0.65 mW/MSPS A/D Converter with Internal

Sample-and-Hold

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