AD7329 datasheet(23/40 Pages) AD

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AD7329

Rev. 0 | Page 23 of 40

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, 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

Buffer

AD845

AD8021

AD8610

±10 V

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

Buffer

AD845

AD8021

AD8610

±10 V

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

simultaneously 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.

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