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AD872A

Complete 12-Bit 10 MSPS Monolithic A/D Converter

Analog Devices 

AD872A

The wide input bandwidth and superior dynamic performance

of the input THA make the AD872A suitable for undersam-

pling applications where the input frequency exceeds half the

sample frequency. The input THA is designed to recover rap-

idly from input overdrive conditions, returning from a 50%

overdrive in less than 40 ns.

Because of the THA’s exceptionally wide input bandwidth,

some users may find the AD872A is sensitive to noise at fre-

quencies from 10 MHz to 50 MHz that other converters are

incapable of responding to. This sensitivity can be mitigated by

careful use of the differential inputs (see previous paragraphs).

Additionally, Figure 18 shows how a small capacitor (10 pF-

20 pF for 50 Ω terminated inputs) may be placed between VINA

and VINB to help reduce high frequency noise in applications

where limiting the input bandwidth is acceptable.

؎1V

10 OR 20pF

1 VINA

AD872A

2 VINB

The AD872A’s reference input impedance is equal to 5 kΩ

(± 20%), and its effective noise bandwidth is 10 MHz, with a

referred-to-input noise gain of 0.8. For example, the internal

reference, with an rms noise of 28 µV (using an external 1 µF

capacitor), contributes 24 µV (0.05 LSB) of noise to the transfer

function of the AD872A.

The full-scale peak-to-peak input voltage is a function of the

reference voltage, according to the equation:

(VINA = VINB ) Full Scale = 0.8 × (VREF – REF GND )

Note that the AD872A’s performance was optimized for a 2.5 V

reference input: performance may degrade somewhat for other

reference voltages. Figure 20 illustrates the S/(N+D) perfor-

mance vs. reference voltage for a 1 MHz, –0.5 dB input signal.

Note also that if the reference is changed during a conversion,

all three conversions in the pipeline will be invalidated.

75

70

65

Figure 18. Optional High Frequency Noise Reduction

The AD872A will contribute its own wideband thermal noise.

As a result of the integrated wideband noise (0.26 LSB rms,

referred-to-input), applying a dc analog input may produce

more than one code at the output. A histogram of the ADC

output codes, for a dc input voltage, will be between one and

three codes wide, depending on how well the input is centered

on a given code and how many samples are taken. Figure 8

shows a typical AD872A code histogram, and Figure 9 illus-

trates the AD872A’s transition noise.

REFERENCE INPUT

The nominal reference input should be 2.5 V, taken with re-

spect to REFERENCE GROUND (REF GND). Figure 19 il-

lustrates the equivalent model for the reference input: there is

no clock or signal-dependent activity associated with the refer-

ence input circuitry, therefore, no “kickback” into the reference.

REF IN 1

AD872A

5k⍀

(؎20%)

REF GND 2

AVSS

Figure 19. Equivalent Reference Input Circuit

However, in order to realize the lowest noise performance of the

AD872A, care should be taken to minimize noise at the refer-

ence input.

60

55

50

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5

REFERENCE INPUT VOLTAGE – V

Figure 20. S/(N+D) vs. Reference Input Voltage,

fIN = 1 MHz, FS = 10 MHz

Table II summarizes various 2.5 V references suitable for use

with the AD872A, including the onboard bandgap reference

(see REFERENCE OUTPUT section).

REF43B

AD680JN

Internal

Table II. Suitable 2.5 V References

Drift (ppm/؇C)

Initial Accuracy %

6 (max)

0.2

10 (max)

0.4

30 (typ)

0.4

If an external reference is connected to REF IN, REF OUT

must be connected to +5 V. This should lower the current in

REF GND to less than 350 µA and eliminate the need for a

1 µF capacitor, although decoupling the reference for noise

reduction purposes is recommended.

Alternatively, Figure 21 shows how the AD872A may be driven

from other references by use of an external resistor. The exter-

nal resistor forms a resistor divider with the on-chip 5 kΩ resis-

tor to realize 2.5 V at the reference input pin (REF IN). A trim

potentiometer is needed to accommodate the tolerance of the

AD872A’s 5 kΩ resistor.

REV. A

–11–

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