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AD7938/AD7939

Preliminary Technical Data

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of harmonics to the

fundamental. For the AD7938/ AD7939, it is defined as

()

THD

20log

−

where V1 is the rms amplitude of the fundamental and V2, V3,

V4, V5, and V6 are the rms amplitudes of the second through the

sixth harmonics.

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the

rms value of the next largest component in the ADC output

spectrum (up to fS/2 and excluding dc) to the rms value of the

fundamental. Normally, the value of this specification is

determined by the largest harmonic in the spectrum, but for

ADCs where the harmonics are buried in the noise floor, it will

be a noise peak.

Intermodulation Distortion

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

fb, any active device with nonlinearities will create distortion

products at sum and difference frequencies of mfa ± nfb where

m, n = 0, 1, 2, 3, etc. 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), while the

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

− 2fb).

The AD7938/AD7939 are 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 while 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 as 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 dBs.

Rev. PrN | Page 12 of 32

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