AD7490 datasheet(12/29 Pages) AD

12 / 29 page

Data Sheet

AD7490

Rev. D | Page 11 of 28

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 is a

noise peak.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa

and fb, any active device with nonlinearities creates distortion

products at the 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 are 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 AD7490 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, and 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.

AD7490 Datasheet(PDF) 12 Page - Analog Devices