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Data Sheet

AD7329

DRIVER AMPLIFIER CHOICE

In applications where the harmonic distortion and signal-to-

noise ratio are critical specifications, the analog input of the

AD7329 should be driven from a low impedance source. Large

source impedances significantly affect the ac performance of the

ADC and can necessitate the use of an input buffer amplifier.

When no amplifier is used to drive the analog input, the source

impedance should be limited to low values. The maximum source

impedance depends on the amount of THD that can be tolerated

in the application. The THD increases as the source impedance

increases and performance degrades. Figure 21 and Figure 22

show graphs of the THD vs. the analog input frequency for

various source impedances. Depending on the input range and

analog input configuration selected, the AD7329 can handle

source impedances of up to 4 kΩ before the THD starts to

degrade.

Due to the programmable nature of the analog inputs on the

AD7329, the choice of op amp used to drive the inputs is a

function of the particular application and depends on the input

configuration and the analog input voltage ranges selected.

The driver amplifier must be able to settle for a full-scale step to

a 13-bit level, within 0.0122%, in less than the specified acquisition

time of the AD7329. An op amp such as the AD8021 meets this

requirement when operating in single-ended mode. The AD8021

needs an external compensating NPO type of capacitor. The

AD8022 can also be used in high frequency applications where

a dual version is required. For lower frequency applications, op

amps such as the AD797, AD845, and AD8610 can be used with

the AD7329 in single-ended mode configuration.

Table 8. Typical AC Performance Using Different Op Amps

in Single-Ended Mode, ±10 V Input Range

Parameter

No Buffer

AD845

AD8021

AD8610

SNR (dB)

74.24

74.03

73.78

73.88

SNRD (dB)

72.42

74.88

72.11

71.98

THD (dB)

−77.05

−75.95

−77.04

−76.47

Table 9. Typical AC Performance Using Different Op Amps

in Differential Mode, ±10 V Input Range

Parameter

No Buffer

AD845

AD8021

AD8610

SNR (dB)

77.16

76.81

76.95

76.76

SNRD (dB)

76.50

76.02

76.78

75.89

THD (dB)

−84.91

−83.74

−90.55

−83.24

Differential operation requires that VIN+ and VIN− be simulta-

neously driven with two signals of equal amplitude that are 180°

out of phase. The common mode must be set up externally to the

AD7329. The common-mode range is determined by the REFIN/

REFOUT voltage, the VCC supply voltage, and the particular amplifier

used to drive the analog inputs. Differential mode with either an ac

input or a dc input provides the best THD performance over a

wide frequency range. Because not all applications have a signal

preconditioned for differential operation, there is often a need

to perform a single-ended-to-differential conversion.

This single-ended-to-differential conversion can be performed

using an op amp pair. Typical connection diagrams for an op

amp pair are shown in Figure 45 and Figure 46. In Figure 45,

the common-mode signal is applied to the noninverting input

of the second amplifier.

Rev. C | Page 23 of 38

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