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Data Sheet

AD7321

Rev. B | Page 15 of 36

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of crosstalk

between any two channels. It is measured by applying a full-scale,

100 kHz sine wave signal to all unselected input channels and

determining the degree to which the signal attenuates in the

selected channel with a 50 kHz signal. Figure 14 shows the worst-

case across all eight channels for the AD7321. The analog input

range is programmed to be the same on all channels.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and

fb, any active device with nonlinearities creates distortion

products at sum and difference frequencies of mfa ± nfb, where

m, n = 0, 1, 2, 3…. Intermodulation distortion terms are those

for which neither m nor n are equal to 0. For example, the

second-order terms include (fa + fb) and (fa − fb), whereas the

third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and

(fa − 2fb).

The AD7321 is tested using the CCIF standard where two input

frequencies near the top end of the input bandwidth are used.

In this case, the second-order terms are usually distanced in

frequency from the original sine waves, whereas the third-order

terms are usually at a frequency close to the input frequencies.

As a result, the second- and third-order terms are specified

separately. The calculation of the intermodulation distortion is

per the THD specification, where it is the ratio of the rms sum

of the individual distortion products to the rms amplitude of

the sum of the fundamentals expressed in decibels.

PSR (Power Supply Rejection)

Variations in power supply affect the full-scale transition but

not the linearity of the converter. Power supply rejection is the

maximum change in the full-scale transition point due to a

change in power supply voltage from the nominal value (see the

Typical Performance Characteristics section).

CMRR (Common-Mode Rejection Ratio)

CMRR is defined as the ratio of the power in the ADC output at

full-scale frequency, f, to the power of a 100 mV sine wave

applied to the common-mode voltage of the VIN+ and VIN−

frequency, fS, as

CMRR (dB) = 10 log (Pf/PfS)

where Pf is the power at frequency f in the ADC output, and PfS

is the power at frequency fS in the ADC output (see Figure 17).

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