AD7939BCPZ datasheet(14/36 Pages) AD

14 / 36 page

AD7938/AD7939

Data Sheet

Rev. C | Page 14 of 36

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns to track mode at the end

of conversion. The track-and-hold acquisition time is the time

required for the output of the track-and-hold amplifier to reach

its final value, within ±½ LSB, after the end of conversion.

Signal-to-Noise and Distortion Ratio (SINAD)

This is the measured ratio of signal-to-noise and distortion at

the output of the ADC. The signal is the rms amplitude of the

fundamental. Noise is the sum of all nonfundamental signals up

to half the sampling frequency (fSAMPLE/2), excluding dc. The

ratio is dependent on the number of quantization levels in the

digitization process; the more levels, the smaller the

quantization noise.

The theoretical signal-to-noise and distortion ratio for an ideal

N-bit converter with a sine wave input is given by

SINAD = (6.02 N + 1.76) dB

Thus, for a 12-bit converter, SINAD is 74 dB, and for a 10-bit

converter, it is 62 dB.

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

log

where:

V1 is the rms amplitude of the fundamental.

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 fSAMPLE/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 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 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 intermodulation distortion is

calculated per the THD specification, 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.

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