AD7322 datasheet(21/37 Pages) AD

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AD7322

Data Sheet

Rev. B | Page 20 of 36

–8

–2

–4

–6

±2.5V

RANGE

±10V

RANGE

±10V

RANGE

±5V RANGE

±2.5V

RANGE

±5V RANGE

0V TO +10V

RANGE

0V TO +10V

RANGE

±16.5V VDD/VSS

±12V VDD/VSS

VCC = 5V

VREF = 2.5V

Figure 40. Pseudo Input Range with VCC = 5 V

±2.5V

RANGE

±10V

RANGE

±10V

RANGE

±5V RANGE

±2.5V

RANGE

±5V RANGE

0V TO +10V

RANGE

0V TO +10V

RANGE

±16.5V VDD/VSS

±12V VDD/VSS

–8

–2

–4

–6

VCC = 3V

VREF = 2.5V

Figure 41. Pseudo Input Range with VCC = 3 V

DRIVER AMPLIFIER CHOICE

In applications where the harmonic distortion and signal-to-

noise ratio are critical specifications, the analog input of the

AD7322 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 AD7322 can handle source

impedances of up to 4.7 kΩ before the THD starts to degrade.

Due to the programmable nature of the analog inputs on the

AD7322, 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 AD7322. 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 the AD8610 can be used in the AD7322 single-

ended mode configuration.

Differential operation requires that VIN+ and the 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

AD7322. The common-mode range is determined by the REFIN/

OUT 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 the 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 42 and Figure 43. In Figure 42 the

common-mode signal is applied to the noninverting input of

the second amplifier.

VIN

V+

V–

Ω

1.5k

Ω

1.5k

Ω

1.5k

Ω

1.5k

Ω

10k

Ω

20k

Ω

VCOM

Figure 42. Single-Ended-to-Differential Configuration with the AD845

VIN

V+

V–

442

Ω

442

Ω

442

Ω

442

Ω

442

Ω

100

Ω

AD8021

442

Ω

Figure 43. Single-Ended-to-Differential Configuration with the AD8021

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