ADC12020CIVY/NOPB - National Semiconductor

July 28, 2008

ADC12020

12-Bit, 20 MSPS, 185 mW A/D Converter with Internal

Sample-and-Hold

General Description

The ADC12020 is a monolithic CMOS analog-to-digital con-

verter capable of converting analog input signals into 12-bit

digital words at 20 Megasamples per second (MSPS), mini-

mum. This converter uses a differential, pipeline architecture

with digital error correction and an on-chip sample-and-hold

circuit to minimize die size and power consumption while pro-

viding excellent dynamic performance. Operating on a single

5V power supply, this device consumes just 185 mW at 20

MSPS, including the reference current. The Power Down fea-

ture reduces power consumption to 40 mW.

The differential inputs provide a full scale input swing equal

to 2VREF with the possibility of a single-ended input. Full use

of the differential input is recommended for optimum perfor-

mance. For ease of use, the buffered, high impedance, single-

ended reference input is converted on-chip to a differential

reference for use by the processing circuitry. Output data for-

mat is 12-bit offset binary.

This device is available in the 32-lead LQFP package and will

operate over the industrial temperature range of −40°C to

+85°C.

Features

■Internal sample-and-hold

■Outputs 2.35V to 5V compatible

■Pin compatible with ADC12010, ADC12040, ADC12L063

and ADC12L066

■TTL/CMOS compatible input/outputs

■Power down mode

■On-chip reference buffer

Key Specifications

■Resolution 12 Bits

■Conversion Rate 20 MSPS (min)

■DNL ±0.35 LSB (typ)

■ENOB (fIN = 10.1 MHz) 11.3 bits (typ)

■Supply Voltage +5V ±5%

■Power Consumption, 20 MHz 185 mW (typ)

Applications

■Image Processing Front End

■Instrumentation

■PC-Based Data Acquisition

■Fax Machines

■Waveform Digitizers

■Sonar/Radar

■DSP Front Ends

Connection Diagram

20051701

TRI-STATE® is a registered trademark of National Semiconductor Corporation.

ADC12020 12-Bit, 20 MSPS, 185 mW A/D Converter with Internal Sample-and-Hold

Ordering Information

Industrial (−40°C ≤ TA ≤ +85°C) Package

ADC12020CIVY 32 Pin LQFP

ADC12020CIVYX 32 Pin LQFP Tape and Reel

ADC12040EVAL 40 Msps Evaluation Board

Block Diagram

20051702

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ADC12020

Pin Descriptions and Equivalent Circuits

Pin No. Symbol Equivalent Circuit Description

ANALOG I/O

2VIN+

Non-Inverting analog signal Input. With a 2.0V reference voltage,

the differential input signal level is 2.0 VP-P centered on VCM.

3VIN−

Inverting analog signal Input. With a 2.0V reference voltage the

ground-referenced input signal level is 2.0 VP-P centered on VCM.

This pin may be connected to VCM for single-ended operation, but

a differential input signal is required for best performance.

1VREF

Reference input. This pin should be bypassed to AGND with a 0.1

µF monolithic capacitor. VREF is 2.0V nominal and should be

between 1.0V to 2.2V.

These pins are high impedance reference bypass pins. Connect a

0.1 µF capacitor from each of these pins to AGND. DO NOT LOAD

these pins.

31 VRP

32 VRM

30 VRN

DIGITAL I/O

10 CLK

Digital clock input. The range of frequencies for this input is 100

kHz to 30 MHz (typical) with guaranteed performance at 20 MHz.

The input is sampled on the rising edge of this input.

11 OE

OE is the output enable pin that, when low, enables the TRI-

STATE® data output pins. When this pin is high, the outputs are in

a high impedance state.

8 PD

PD is the Power Down input pin. When high, this input puts the

converter into the power down mode. When this pin is low, the

converter is in the active mode.

14–19, 22–

27 D0–D11

Digital data output pins that make up the 12-bit conversion results.

D0 is the LSB, while D11 is the MSB of the offset binary output

word. Output levels are TTL/CMOS compatible.

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ADC12020

Pin No. Symbol Equivalent Circuit Description

ANALOG POWER

5, 6, 29 VA

Positive analog supply pins. These pins should be connected to a

quiet +5V voltage source and bypassed to AGND with 0.1 µF

monolithic capacitors located within 1 cm of these power pins, and

with a 10 µF capacitor.

4, 7, 28

AGND The ground return for the analog supply.

DIGITAL POWER

13 VD

Positive digital supply pin. This pin should be connected to the

same quiet +5V source as is VA and bypassed to DGND with a 0.1

µF monolithic capacitor in parallel with a 10 µF capacitor, both

located within 1 cm of the power pin.

9, 12

DGND The ground return for the digital supply.

21 VDR

Positive digital supply pin for the ADC12020's output drivers. This

pin should be connected to a voltage source of +2.35V to +5V and

be bypassed to DR GND with a 0.1 µF monolithic capacitor. If the

supply for this pin is different from the supply used for VA and VD,

it should also be bypassed with a 10 µF tantalum capacitor. VDR

should never exceed the voltage on VD. All bypass capacitors

should be located within 1 cm of the supply pin.

20 DR GND

The ground return for the digital supply for the ADC12020's output

drivers. This pin should be connected to the system digital ground,

but not be connected in close proximity to the ADC12020's DGND

or AGND pins. See Section 5 (Layout and Grounding) for more

details.

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ADC12020

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.

VA, VD6.5V

VDR ≤VD+0.3V

|VA–VD|≤ 100 mV

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

0.3V)

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

Package Input Current (Note 3) ±50 mA

Package Dissipation at TA = 25°C See (Note 4)

ESD Susceptibility

Human Body Model (Note 5) 2500V

Machine Model (Note 5) 250V

Soldering Temperature,

Infrared, 10 sec. (Note 6) 235°C

Storage Temperature −65°C to +150°C

Operating Ratings (Notes 1, 2)

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

Supply Voltage (VA, VD) +4.75V to +5.25V

Output Driver Supply (VDR) +2.35V to VD

VREF Input 1.0V to 2.4V

CLK, PD, OE −0.05V to (VD + 0.05V)

VIN Input −0V to (VA − 0.5V)

VCM 1.0V to 4.0V

|AGND–DGND| ≤100mV

Package Thermal Resistance

Package θJA

32-Lead LQFP 79°C / W

Converter Electrical Characteristics

Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = +5V, VDR = +3.0V,

PD = 0V, VREF = +2.0V, fCLK = 20 MHz, tr = tf = 3 ns, CL = 25 pF/pin. Boldface limits apply for TJ = TMIN to TMAX: all other limits

TA = TJ = 25°C (Notes 7, 8, 9)

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

STATIC CONVERTER CHARACTERISTICS

Resolution with No Missing Codes 12 Bits (min)

INL Integral Non Linearity (Note 11) ±0.55 +1.7 LSB (max)

-1.4 LSB (min)

DNL Differential Non Linearity ±0.35 +0.9 LSB (max)

-1.0 LSB (min)

GE Gain Error ±0.05 2.9 %FS (max)

Offset Error (VIN+ = VIN−) −0.04 ±1.75 %FS (max)

Under Range Output Code 0 0

Over Range Output Code 4095 4095

DYNAMIC CONVERTER CHARACTERISTICS

FPBW Full Power Bandwidth 0 dBFS Input, Output at −3 dB 100 MHz

SNR Signal-to-Noise Ratio

fIN = 1 MHz, VIN = −0.5 dBFS 70 dB

fIN = 4.4 MHz, VIN = −0.5 dBFS 70 dB

fIN = 10.1 MHz, VIN = −0.5 dBFS 70 67 dB (min)

SINAD Signal-to-Noise and Distortion

fIN = 1 MHz, VIN = −0.5 dBFS 70 dB

fIN = 4.4 MHz, VIN = −0.5 dBFS 70 dB

fIN = 10.1MHz, VIN = −0.5 dBFS 69 67 dB (min)

ENOB Effective Number of Bits

fIN = 1 MHz, VIN = −0.5 dBFS 11.3 dB

fIN = 4.4 MHz, VIN = −0.5 dBFS 11.3 dB

fIN = 10.1 MHz, VIN = −0.5 dBFS 11.3 10.8 dB (min)

THD Total Harmonic Distortion

fIN = 1 MHz, VIN = −0.5 dBFS −83 dB

fIN = 4.4 MHz, VIN = −0.5 dBFS −83 dB

fIN = 10.1 MHz,

VIN = −0.5 dBFS

+25°C −83 -74 dB (min)

+85°C -71 dB (min)

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ADC12020

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

SFDR Spurious Free Dynamic Range

fIN = 1 MHz, VIN = −0.5 dBFS 86 dB

fIN = 4.4 MHz, VIN = −0.5 dBFS 86 dB

fIN = 10.1 MHz,

VIN = −0.5 dBFS

+25°C 86 71 dB (min)

+85°C 68 dB (min)

IMD Intermodulation Distortion fIN = 4.7 MHz and 5.3 MHz, each = −7 dBFS −73 dBFS

REFERENCE AND ANALOG INPUT CHARACTERISTICS

VCM Common Mode Input Voltage VA/2 V

CIN

VIN Input Capacitance (each pin to

GND) VIN = 2.5 Vdc + 0.7 Vrms

(CLK LOW) 8 pF

(CLK HIGH) 7 pF

VREF Reference Voltage (Note 13) 2.00 1.0 V (min)

2.4 V (max)

Reference Input Resistance 100 MΩ(min)

DC and Logic Electrical Characteristics

Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = +5V, VDR = +3.0V,

PD + 0V, VREF = +2.0V, fCLK = 20 MHz, tr = tf = 3 ns, CL = 25 pF/pin. Boldface limits apply for TJ = TMIN to TMAX: all other limits

TA = TJ = 25°C (Notes 7, 8, 9)

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

CLK, PD, OE DIGITAL INPUT CHARACTERISTICS

VIN(1) Logical “1” Input Voltage VD = 5.25V 2.0 V (min)

VIN(0) Logical “0” Input Voltage VD = 4.75V 1.0 V (max)

IIN(1) Logical “1” Input Current VIN = 5.0V 10 µA

IIN(0) Logical “0” Input Current VIN = 0V −10 µA

CIN Digital Input Capacitance 5 pF

D0–D11 DIGITAL OUTPUT CHARACTERISTICS

VOUT(1) Logical “1” Output Voltage IOUT = −0.5 mA VDR = 2.5V 2.3 V (min)

VDR = 3V 2.7 V (min)

VOUT(0) Logical “0” Output Voltage IOUT = 1.6 mA, VDR = 3V 0.4 V (max)

IOZ TRI-STATE Output Current VOUT = 2.5V or 5V 100 nA

VOUT = 0V −100 nA

+ISC Output Short Circuit Source Current VOUT = 0V −20 mA (min)

−ISC Output Short Circuit Sink Current VOUT = VDR 20 mA (min)

POWER SUPPLY CHARACTERISTICS

IAAnalog Supply Current PD Pin = DGND, VREF = 2.0V

PD Pin = VDR

32.3

41 mA (max)

IDDigital Supply Current PD Pin = DGND

PD Pin = VDR, fCLK = 0

3.3

3.5

4.5 mA (max)

IDR Digital Output Supply Current PD Pin = DGND, CL = 0 pF (Note 14)

PD Pin = VDR, fCLK = 0

mA (max)

Total Power Consumption PD Pin = DGND, CL = 0 pF (Note 15)

PD Pin = VDR, fCLK = 0

178

227 mW

PSRR1+ Power Supply Rejection Ratio Rejection of Positive Full-Scale Error with

VA = 4.75V to 5.25V 69 dBFS

PSRR1− Power Supply Rejection Ratio Rejection of Positive Full-Scale Error with

VA = 4.75V to 5.25V 51 dBFS

PSRR2 Power Supply Rejection Ratio Rejection of Power Supply Noise with

10 MHz, 250 mVP-P riding on VA

48 dBFS

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ADC12020

AC Electrical Characteristics

Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = +5V, VDR = +3.0V,

PD = 0V, VREF = +2.0V, fCLK = 20 MHz, tr = tf = 3 ns, CL = 25 pF/pin. Boldface limits apply for TJ = TMIN to TMAX: all other limits

TA = TJ = 25°C (Notes 7, 8, 9, 12)

Symbol Parameter Conditions Typical

(Note 10)

Limits

(Note 10)

Units

(Limits)

fCLK1Maximum Clock Frequency 20 30 MHz (min)

fCLK2Minimum Clock Frequency 100 kHz

tCH Clock High Time 20 ns (min)

tCL Clock Low Time 20 ns (min)

tCONV Conversion Latency 6Clock Cycles

tOD Data Output Delay after Rising CLK Edge VDR = 2.5V 11 16.8 ns (max)

VDR = 3.0V 11 16.8 ns (max)

tAD Aperture Delay 1.2 ns

tAJ Aperture Jitter 2 ps rms

tDIS Data outputs into TRI-STATE Mode 4 ns

tEN Data Outputs Active after TRI-STATE 4 ns

tPD Power Down Mode Exit Cycle 0.1µF capacitors on pins 30, 31, 32 500 ns

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed

specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test

conditions.

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

Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND, or VIN > VA), 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 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 listed above will be reached only when the 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 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is 220 pF discharged through 0Ω.

Note 6: The 235°C reflow temperature refers to infrared reflow. For Vapor Phase Reflow (VPR), the following Conditions apply: Maintain the temperature at the

top of the package body above 183°C for a minimum 60 seconds. The temperature measured on the package body must not exceed 220°C. Only one excursion

above 183°C is allowed per reflow cycle.

Note 7: The inputs are protected as shown below. Input voltage magnitudes above VA or below GND will not damage this device, provided current is limited per

(Note 3). However, errors in the A/D conversion can occur if the input goes above VA or below GND by more than 100 mV. As an example, if VA is 4.75V, the

full-scale input voltage must be ≤4.85V to ensure accurate conversions.

20051707

Note 8: To guarantee accuracy, it is required that |VA–VD| ≤ 100 mV and separate bypass capacitors are used at each power supply pin.

Note 9: With the test condition for VREF = +2.0V (4VP-P differential input), the 12-bit LSB is 977 µV.

Note 10: Typical figures are at TA = TJ = 25°C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing

Quality Level).

Note 11: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive and negative

full-scale.

Note 12: Timing specifications are tested at TTL logic levels, VIL = 0.4V for a falling edge and VIH = 2.4V for a rising edge.

Note 13: Optimum performance will be obtained by keeping the reference input in the 1.8V to 2.2V range. The LM4051CIM3-ADJ (SOT-23 package) is

recommended for this application.

Note 14: IDR is the current consumed by the switching of the output drivers and is primarily determined by 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(C0 x f0 + C1 x f1 +....C11 x f11) where VDR is the output driver power

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

Note 15: Excludes IDR. See note 14.

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ADC12020

Specification Definitions

APERTURE DELAY is the time after the rising edge of the

clock to when the input signal is acquired or held for conver-

sion.

APERTURE JITTER (APERTURE UNCERTAINTY) is the

variation in aperture delay from sample to sample. Aperture

jitter manifests itself as noise in the output.

CLOCK DUTY CYCLE is the ratio of the time during one cycle

that a repetitive digital waveform is high to the total time of

one period. The specification here refers to the ADC clock

input signal.

COMMON MODE VOLTAGE (VCM) is the d.c. potential

present at both signal inputs to the ADC.

CONVERSION LATENCY See PIPELINE DELAY.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

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

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE

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

Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02

and says that the converter is equivalent to a perfect 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.

Gain Error = Pos. Full Scale Error − Neg. Full Scale Error

GAIN ERROR is the deviation from the ideal slope of the

transfer function. It is the difference between the input that

cause a transition from all zeros to the first code.

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

viation of each individual code from a line drawn from negative

full scale (½ LSB below the first code transition) through pos-

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

deviation of any given code from this straight line is measured

from the center of that code value.

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 intermodulation

products to the total power in the original frequencies. IMD is

usually expressed in dBFS.

MISSING CODES are those output codes that will never ap-

pear at the ADC outputs. The ADC12020 is guaranteed not

to have any missing codes.

NEGATIVE FULL SCALE ERROR is the difference between

the actual first code transition and its ideal value of ½ LSB

above negative full scale.

OFFSET ERROR is the difference between the two input

voltages (VIN + –VIN−) required to cause a transition from code

2047 to 2048.

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

clock before the data update is presented at the output pins.

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. Data for any given sample

is available at the output pins the Pipeline Delay plus the Out-

put Delay after the sample is taken. New data is available at

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

pipeline delay.

POSITIVE FULL SCALE ERROR is the difference between

the actual last code transition and its ideal value of 1½ LSB

below positive full scale.

POWER SUPPLY REJECTION RATIO (PSRR) is a measure

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

voltage. For the ADC12020, PSRR1 is the ratio of the change

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

supply voltage, expressed in dB. PSRR2 is a measure of how

well an a.c. signal riding upon the power supply is rejected at

the output.

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

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

sum of all other spectral components below one-half the sam-

pling frequency, not including harmonics or dc.

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

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

input signal to the rms value of all of the other spectral com-

ponents below half the clock frequency, including harmonics

but excluding dc.

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-

ence, expressed in dB, between the desired signal amplitude

to the amplitude of the peak spurious spectral component,

where a spurious spectral component is any signal present in

the output spectrum that is not present at the input and may

or may not be a harmonic.

TOTAL HARMONIC DISTORTION (THD) is the ratio, ex-

pressed in dBc, of the rms total of the first nine harmonic

levels at the output to the level of the fundamental at the out-

put. THD is calculated as

where f1 is the RMS power of the fundamental (output) fre-

quency and f2 through f10 are the RMS power in the first 9

harmonic frequencies.

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ADC12020

Timing Diagram

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Output Timing

Transfer Characteristic

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FIGURE 1. Transfer Characteristic

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ADC12020

ADC12020 Typical Performance Characteristics VA = VD = 5V, VDR = 3.0V, fCLK = 20 MHz,

fIN = 10.1 MHz, VREF = 2.0V unless otherwise stated

DNL

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DNL vs. Temperature

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DNL vs. Clock Duty Cycle

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DNL vs. Sample Rate

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INL

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INL vs. Temperature

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ADC12020

INL vs. Clock Duty Cycle

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INL vs. Sample Rate

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SNR vs. Temperature

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SNR vs. Clock Duty Cycle

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SNR vs. Sample Rate

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SNR vs. FIN

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ADC12020

SNR vs. VREF

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THD vs. Temperature

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THD vs. Clock Duty Cycle

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THD vs. Sample Rate

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THD vs. FIN

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THD vs. VREF

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ADC12020

SINAD vs. Temperature

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SINAD vs. Clock Duty Cycle

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SINAD vs. Sample Rate

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SINAD vs. FIN

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SINAD vs. VREF

20051745

SFDR vs. Temperature

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ADC12020

SFDR vs. Clock Duty Cycle

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SFDR vs. Sample Rate

20051748

SFDR vs. FIN

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SFDR vs. VREF

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TOD vs. VDR

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Spectral Response, 1.1 MHz Input

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ADC12020

Spectral Response, 44 MHz Input

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Spectral Response, 10.1 MHz Input

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Spectral Response

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IMD @ F1 = 9.5MHz, F2 = 10.5MHz

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ADC12020

Functional Description

Operating on a single +5V supply, the ADC12020 uses a

pipeline architecture and has error correction circuitry to help

ensure maximum performance. The differential analog input

signal is digitized to 12 bits.

The reference input is buffered to ease the task of driving that

pin. The output word rate is the same as the clock frequency.

The analog input voltage is acquired at the rising edge of the

clock and the digital data for a given sample is delayed by the

pipeline for 6 clock cycles.

A logic high on the power down (PD) pin reduces the con-

verter power consumption to 40 mW.

Applications Information

1.0 OPERATING CONDITIONS

We recommend that the following conditions be observed for

operation of the ADC12020:

4.75V ≤ VA ≤ 5.25V

VD = VA

2.35V ≤ VDR ≤ VD

100 kHz ≤ fCLK ≤ 30 MHz

1.0V ≤ VREF ≤ 2.4V

1.0V ≤ VCM ≤ 4.0V

1.1 Analog Inputs

The ADC12020 has two analog signal inputs, VIN+ and VIN−.

These two pins form a differential input pair. There is one ref-

erence input pin, VREF.

1.2 Reference Pins

The ADC12020 is designed to operate with a 2.0V reference,

but performs well with reference voltages in the range of 1.0V

to 2.4V. Lower reference voltages will decrease the signal-to-

noise ratio (SNR). Increasing the reference voltage (and the

input signal swing) beyond 2.4V will degrade THD for a full-

scale input.

It is very important that all grounds associated with the refer-

ence voltage and the input signal make connection to the

ground plane at a single point to minimize the effects of noise

currents in the ground path.

The three Reference Bypass Pins (VRP, VRM and VRN) are

made available for bypass purposes. These pins should each

be bypassed to ground with a 0.1 µF capacitor. Smaller ca-

pacitor values will allow faster recovery from the power down

mode, but may result in degraded noise performance. DO

NOT LOAD these pins.

1.3 Signal Inputs

The signal inputs are VIN+ and VIN−. The input signal, VIN, is

defined as

VIN = (VIN+) – (VIN−)

Figure 2 shows the expected input signal range.

Note that the common mode input voltage range is 1V to 3V

with a nominal value of VA/2. The input signals should remain

between ground and 4V.

The Peaks of the individual input signals (VIN+ and VIN−)

should each never exceed the voltage described as

VIN+, VIN− = VREF + VCM

to maintain THD and SINAD performance.

20051711

FIGURE 2. Expected Input Signal Range

The ADC12020 performs best with a differential input with

each input centered around VCM. The peak-to-peak voltage

swing at both VIN+ and VIN− each should not exceed the value

of the reference voltage or the output data will be clipped. The

two input signals should be exactly 180° out of phase from

each other and of the same amplitude. For single frequency

inputs, angular errors result in a reduction of the effective full

scale input. For a complex waveform, however, angular errors

will result in distortion.

For angular deviations of up to 10 degrees from these two

signals being 180 out of phase, the full scale error in LSB can

be described as approximately

EFS = 4096 ( 1 - sin (90° + dev))

Where dev is the angular difference, in degrees, between the

two signals having a 180° relative phase relationship to each

other (see Figure 3). Drive the analog inputs with a source

impedance less than 100Ω.

20051712

FIGURE 3. Angular Errors Between the Two Input Signals

Will Reduce the Output Level

For differential operation, each analog input signal should

have a peak-to-peak voltage equal to the input reference volt-

age, VREF, and be centered around a common mode voltage,

VCM.

TABLE 1. Input to Output Relationship—Differential Input

VIN+VIN−Output

VCM − VREF/2 VCM + VREF/2 0000 0000 0000

VCM − VREF/4 VCM + VREF/4 0100 0000 0000

VCM VCM 1000 0000 0000

VCM + VREF/4 VCM − VREF/4 1100 0000 0000

VCM + VREF/2 VCM − VREF/2 1111 1111 1111

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ADC12020

TABLE 2. Input to Output Relationship—Single-Ended

Input

VIN+VIN−Output

VCM − VREF VCM 0000 0000 0000

VCM − VREF/2 VCM 0100 0000 0000

VCM VCM 1000 0000 0000

VCM + VREF/2 VCM 1100 0000 0000

VCM + VREF VCM 1111 1111 1111

1.3.1 Single-Ended Operation

Single-ended performance is lower than with differential input

signals. For this reason, single-ended operation is not rec-

ommended. However, if single ended-operation is required,

and the resulting performance degradation is acceptable, one

of the analog inputs should be connected to the d.c. common

mode voltage of the driven input. The peak-to-peak differen-

tial input signal should be twice the reference voltage to

maximize SNR and SINAD performance (Figure 2b). For ex-

ample, set VREF to 1.0V, bias VIN− to 1.0V and drive VIN+ with

a signal range of 0V to 2.0V.

Because very large input signal swings can degrade distortion

performance, better performance with a single-ended input

can be obtained by reducing the reference voltage when

maintaining a full-range output. and indicate the input to out-

put relationship of the ADC12020.

1.3.2 Driving the Analog Inputs

The VIN+ and the VIN− inputs of the ADC12020 consist of an

analog switch followed by a switched-capacitor amplifier. The

capacitance seen at the analog input pins changes with the

clock level, appearing as 8 pF when the clock is low, and 7

pF when the clock is high. Although this difference is small, a

dynamic capacitance is more difficult to drive than is a fixed

capacitance, so choose the driving amplifier carefully. The

LMH6550, the LMH6702 and the LM6628 are good amplifiers

for driving the ADC12020.

The internal switching action at the analog inputs causes en-

ergy to be output from the input pins. As the driving source

tries to compensate for this, it adds noise to the signal. To

prevent this, use an RC at each of the inputs, as shown in

Figure 5 and Figure 6. These components should be placed

close to the ADC because the input pins of the ADC is the

most sensitive part of the system and this is the last opportu-

nity to filter the input. For undersampling applications, the

capacitors should be eliminated.

The LMH6550 and the LMH6552 are excellent devices for

driving the ADC12020, especially when single-ended to dif-

ferential conversion with d.c. coupling is necessary. An ex-

ample of the use of the LMH6550 to drive the analog input of

the ADC12020 is shown in Figure 5.

For high frequency, narrow band applications, a transformer

is generally the recommended way to drive the analog inputs,

as shown in Figure 6.

1.3.3 Input Common Mode Voltage

The input common mode voltage, VCM, should be in the range

of 0.5V to 4.0V and be of a value such that the peak excur-

sions of the analog signal does not go more negative than

ground or more positive than 1.2 Volts below the VA supply

voltage. The nominal VCM should generally be equal to

VREF/2, but VRM can be used as a VCM source as long as

VCM need not supply more than 10 µA of current. Figure 5

shows the use of the VRM output to drive the VCM input of the

LMH6550. The common mode output voltage of the

LMH6550 is equal to the VCM input input voltage.

2.0 DIGITAL INPUTS

The digital TTL/CMOS compatible inputs consist of CLK,

OE and PD.

2.1 CLK

The CLK signal controls the timing of the sampling process.

Drive the clock input with a stable, low jitter clock signal in the

range of 100 kHz to 30 MHz with rise and fall times of less

than 3ns. The trace carrying the clock signal should be as

short as possible and should not cross any other signal line,

analog or digital, not even at 90°.

If the CLK is interrupted, or its frequency too low, the charge

on internal capacitors can dissipate to the point where the

accuracy of the output data will degrade. This is what limits

the lowest sample rate to 100 ksps.

The duty cycle of the clock signal can affect the performance

of the A/D Converter. Because achieving a precise duty cycle

is difficult, the ADC12020 is designed to maintain perfor-

mance over a range of duty cycles. While it is specified and

performance is guaranteed with a 50% clock duty cycle, per-

formance is typically maintained over a clock duty cycle range

of 40% to 60%.

The clock line should be terminated at its source in the char-

acteristic impedance of that line. It is highly desirable that the

the source driving the ADC CLK pin only drive that pin. How-

ever, if that source is used to drive other things, each driven

pin should be a.c. terminated with a series RC to ground, as

shown in Figure 4, such that the resistor value is equal to the

characteristic impedance of the clock line and the capacitor

value is

where tPR is the signal propagation rate down the clock line,

"L" is the line length and ZO is the characteristic impedance

of the clock line. This termination should be as close as pos-

sible to the ADC clock pin but beyond it as seen from the clock

source. Typical tPD is about 150 ps/inch (60 ps/cm) on FR-4

board material. The units of "L" and tPD should be the same

(inches or centimeters).

Take care to maintain a constant clock line impedance

throughout the length of the line. Refer to Application Note

AN-905 or AN-1113 for information on setting and determin-

ing characteristic impedance.

2.2 OE

The OE pin, when high, puts the output pins into a high

impedance state. When this pin is low the outputs are in the

active state. The ADC12020 will continue to convert whether

this pin is high or low, but the output can not be read while the

OE pin is high.

The OE pin should NOT be used to multiplex devices together

to drive a common bus as this will result in excessive capac-

itance on the data output pins, reducing SNR and SINAD

performance of the converter. See Section 3.0.

2.3 PD

The PD pin, when high, holds the ADC12020 in a power-down

mode to conserve power when the converter is not being

used. The power consumption in this state is 40 mW with a

20 MHz clock and the output data pins are undefined in this

17 www.national.com

ADC12020

mode. The data in the pipeline is corrupted while in the power

down mode.

The Power Down Mode Exit Cycle time is determined by the

value of the capacitors on pins 30, 31 and 32. These capac-

itors loose their charge in the Power Down mode and must

be charged by on-chip circuitry before conversions can be

accurate.

3.0 DATA OUTPUTS

The ADC12020 has 12 TTL/CMOS compatible Data Output

pins. Valid offset binary data is present at these outputs while

the OE and PD pins are low. While the tOD time provides in-

formation about output timing, a simple way to capture a valid

output is to latch the data on the falling edge of the conversion

clock (pin 10).

Be very careful when driving a high capacitance bus. The

more capacitance the output drivers must charge for each

conversion, the more instantaneous digital current flows

through VDR and DR GND. These large charging current

spikes can cause on-chip noise that can couple into the ana-

log circuitry, degrading dynamic performance. Adequate pow-

er supply bypassing and careful attention to the ground plane

will reduce this problem. Additionally, bus capacitance be-

yond that specified will cause tOD to increase, making it diffi-

cult to properly latch the ADC output data. The result could

be an apparent reduction in dynamic performance.

To minimize noise due to output switching, minimize the load

currents at the digital outputs. This can be done by connecting

buffers (74AC541, for example) between the ADC outputs

and any other circuitry. Only one driven input should be con-

nected to each output pin. Additionally, inserting series resis-

tors of 100Ω at the digital outputs, close to the ADC pins, will

isolate the outputs from trace and other circuit capacitances

and limit the output currents, which could otherwise result in

performance degradation. See Figure 4.

While the ADC12020 will operate with VDR voltages down to

1.8V, tOD increases with reduced VDR. Be careful of external

timing when using reduced VDR.

20051713

FIGURE 4. Simple Application Circuit with Single-Ended to Differential Buffer

20051714

FIGURE 5. Differential Drive Circuit of Figure 4

www.national.com 18

ADC12020

20051715

FIGURE 6. Driving the Signal Inputs with a Transformer

4.0 POWER SUPPLY CONSIDERATIONS

The power supply pins should be bypassed with a 10 µF ca-

pacitor and with a 0.1 µF ceramic chip capacitor within a

centimeter of each power pin. Leadless chip capacitors are

preferred because they have low series inductance.

As is the case with all high-speed converters, the ADC12020

is sensitive to power supply noise. Accordingly, the noise on

the analog supply pin should be kept below 100 mVP-P.

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

supply voltages, not even on a transient basis. Be especially

careful of this during turn on and turn off of power.

The VDR pin provides power for the output drivers and may be

operated from a supply in the range of 2.35V to VD (nominal

5V). This can simplify interfacing to 3V devices and systems.

DO NOT operate the VDR pin at a voltage higher than VD.

5.0 LAYOUT AND GROUNDING

Proper grounding and proper routing of all signals are essen-

tial to ensure accurate conversion. Maintaining separate ana-

log and digital areas of the board, with the ADC12020

between these areas, is required to achieve specified perfor-

mance.

The ground return for the data outputs (DR GND) carries the

ground current for the output drivers. The output current can

exhibit high transients that could add noise to the conversion

process. To prevent this from happening, the DR GND pins

should NOT be connected to system ground in close proximity

to any of the ADC12020's other ground pins.

Capacitive coupling between the typically noisy digital circuit-

ry and the sensitive analog circuitry can lead to poor perfor-

mance. The solution is to keep the analog circuitry separated

from the digital circuitry, and to keep the clock line as short as

possible.

Digital circuits create substantial supply and ground current

transients. The logic noise thus generated could have signif-

icant impact upon system noise performance. The best logic

family to use in systems with A/D converters is one which

employs non-saturating transistor designs, or has low noise

characteristics, such as the 74LS, 74HC(T) and 74AC(T)Q

families. The worst noise generators are logic families that

draw the largest supply current transients during clock or sig-

nal edges, like the 74F and the 74AC(T) families. In high

speed circuits, however, it is often necessary to use these

higher speed devices. Best performance requires careful at-

tention to PC board layout and to proper signal integrity

techniques.

The effects of the noise generated from the ADC output

switching can be minimized through the use of 47Ω to 100Ω

resistors in series with each data output line. Locate these

resistors as close to the ADC output pins as possible.

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.

This is because of the skin effect. Total surface area is more

important than is total ground plane volume.

Generally, analog and digital lines should cross each other at

90° to avoid crosstalk. To maximize accuracy in high speed,

19 www.national.com

ADC12020

high resolution systems, however, avoid crossing analog and

digital lines altogether. It is important to keep clock lines as

short as possible and isolated from ALL other lines, including

other digital lines. Even the generally accepted 90° crossing

should be avoided with the clock line as even a little coupling

can cause problems at high frequencies. This is because oth-

er lines can introduce jitter into the clock line, which can lead

to degradation of SNR. Also, the high speed clock can intro-

duce noise into the analog chain.

Best performance at high frequencies and at high resolution

is obtained with a straight signal path. That is, the signal path

through all components should form a straight line wherever

possible.

20051716

FIGURE 7. Example of a Suitable Layout

Be especially careful with the layout of inductors. Mutual in-

ductance can change the characteristics of the circuit in which

they are used. Inductors should not be placed side by side,

even with just a small part of their bodies beside each other.

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 pins and ground or to the reference input

pin and ground should be connected to a very clean point in

the analog ground plane.

Figure 7 gives an example of a suitable layout. A single

ground plane is recommended with separate analog and dig-

ital power planes. The analog and digital power planes should

NOT overlap each other. All analog circuitry (input amplifiers,

filters, reference components, etc.) should be placed over the

analog power plane. All digital circuitry and I/O lines should

be placed over the digital power plane. Furthermore, all com-

ponents in the reference circuitry and the input signal chain

that are connected to ground should be connected together

with short traces and enter the ground plane at a single point.

All ground connections should have a low inductance path to

ground.

6.0 DYNAMIC PERFORMANCE

To achieve the best dynamic performance, the clock source

driving the CLK input must be free of jitter. Isolate the ADC

clock from any digital circuitry with buffers, as with the clock

tree shown in Figure 8.

As mentioned in Section 5.0, it is good practice to keep the

ADC clock line as short as possible and to keep it well away

from any other signals. Other signals can introduce jitter into

the clock signal, which can lead to reduced SNR perfor-

mance, and the clock can introduce noise into other lines.

Even lines with 90° crossings have capacitive coupling, so try

to avoid even these 90° crossings of the clock line.

20051717

FIGURE 8. Isolating the ADC Clock from other Circuitry

with a Clock Tree

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 100 mV beyond the supply rails (more than

100 mV below the ground pins or 100 mV above the supply

pins). Exceeding these limits on even a transient basis may

cause faulty or erratic operation. It is not uncommon for high

speed digital components (e.g., 74F and 74AC devices) to

exhibit overshoot or undershoot that goes above the power

supply or below ground. A resistor of about 50Ω to 100Ω in

series with any offending digital input, close to the signal

source, will eliminate the problem.

www.national.com 20

ADC12020

Do not allow input voltages to exceed the supply voltage, even

on a transient basis. Not even during power up or power

down.

Be careful not to overdrive the inputs of the ADC12020 with

a device that is powered from supplies outside the range of

the ADC12020 supply. 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 flows

through VDR and DR GND. These large charging current

spikes can couple into the analog circuitry, degrading dynam-

ic performance. Adequate bypassing and maintaining sepa-

rate analog and digital areas on the pc board will reduce this

problem.

Additionally, bus capacitance beyond that specified will cause

tOD to increase, making it difficult to properly latch the ADC

output data. The result could, again, be an apparent reduction

in dynamic performance.

The digital data outputs should be buffered (with 74ACQ541,

for example). Dynamic performance can also be improved by

adding series resistors at each digital output, close to the

ADC12020, which reduces the energy coupled back into the

converter output pins by limiting the output current. A reason-

able value for these resistors is 100Ω.

Using an inadequate amplifier to drive the analog input.

As explained in Section 1.3, the capacitance seen at the input

alternates between 8 pF and 7 pF, depending upon the phase

of the clock. This dynamic load is more difficult to drive than

is a fixed capacitance.

If the amplifier exhibits overshoot, ringing, or any evidence of

instability, even at a very low level, it will degrade perfor-

mance. A small series resistor and shunt capacitor at each

amplifier output (as shown in Figure 5 and Figure 6) will im-

prove performance. The LMH6550 , the LMH6702 and the

LMH6628 have been successfully used to drive the analog

inputs of the ADC12020.

Also, it is important that the signals at the two inputs have

exactly the same amplitude and be exactly 180º out of phase

with each other. Board layout, especially equality of the length

of the two traces to the input pins, will affect the effective

phase between these two signals. Remember that an opera-

tional amplifier operated in the non-inverting configuration will

exhibit more time delay than will the same device operating

in the inverting configuration.

Operating with the reference pins outside of the specified

range. As mentioned in Section 1.2, VREF should be in the

range of

1.0V ≤ VREF ≤ 2.4V

Operating outside of these limits could lead to performance

degradation.

Using a clock source with excessive jitter, using exces-

sively 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 and SINAD performance.

21 www.national.com

ADC12020

Physical Dimensions inches (millimeters) unless otherwise noted

32-Lead LQFP Package

Ordering Number ADC12020CIVY

NS Package Number VBE32A

www.national.com 22

ADC12020

Notes

23 www.national.com

ADC12020

Notes

ADC12020 12-Bit, 20 MSPS, 185 mW A/D Converter with Internal Sample-and-Hold

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