AD7921ARM datasheet(12/28 Pages) AD

12 / 28 page

AD7911/AD7921

Rev. 0 | Page 12 of 28

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, and so on. Intermodulation distortion terms are

those for which neither m nor n is equal to zero. For example,

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

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

(fa − 2fb).

The AD7911/AD7921 are tested using the CCIF standard,

where two input frequencies are used (see fa and fb in the

Specifications section). In this case, the second-order terms are

usually distanced in frequency from the original sine waves,

while the third-order terms are usually at a frequency close to

the input frequencies. As a result, the second-order and third-

order terms are specified separately. The calculation of the

intermodulation distortion is as in the THD specification,

where it is defined as the ratio of the rms sum of the individual

distortion products to the rms amplitude of the sum of the

fundamentals expressed in dB.

AD7921ARM Datasheet(PDF) 12 Page - Analog Devices