AD7991 datasheet(16/28 Pages) AD

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AD7991/AD7995/AD7999

Rev. 0 | Page 16 of 28

TERMINOLOGY

Signal-to-Noise and Distortion (SINAD) Ratio

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 the nonfundamental signals excluding dc,

up to half the sampling frequency (fS/2). The ratio is dependent

on the number of quantization levels in the digitization process:

the more levels, the smaller the quantization noise. The theoretical

SINAD ratio for an ideal N-bit converter with a sine wave input

is given by

Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB

Therefore, SINAD is 49.92 dB for an 8-bit converter, 61.96 dB

for a 10-bit converter, and 74 dB for a 12-bit converter.

Total Harmonic Distortion (THD)

The ratio of the rms sum of harmonics to the fundamental. For

the AD7991/AD7995/AD7999, 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 sixth harmonics.

Peak Harmonic or Spurious Noise

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. Typically, 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, the

largest harmonic may be 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 equals 0. For example,

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

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

(fa − 2fb).

The AD7991/AD7995/AD7999 are tested using the CCIF standard,

where two input frequencies near the maximum 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 intermodulation distortion is, like the THD

specification, the ratio of the rms sum of the individual distortion

products to the rms amplitude of the sum of the fundamentals,

expressed in decibels.

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 sine wave signal to all unselected input channels and

then determining the degree to which the signal attenuates in

the selected channel with a 10 kHz signal. The frequency of the

signal in each of the unselected channels is increased from 2 kHz

up to 92 kHz. Figure 14 shows the worst-case across all four

channels for the AD7991.

Full-Power Bandwidth

The input frequency at which the amplitude of the reconstructed

fundamental is reduced by 0.1 dB or 3 dB for a full-scale input.

Integral Nonlinearity

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. The endpoints are

at zero scale (a point 1 LSB below the first code transition) and

full scale (a point 1 LSB above the last code transition).

Differential Nonlinearity

The difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

The deviation of the first code transition (00 … 000 to

00 … 001) from the ideal—that is, AGND + 1 LSB.

Offset Error Match

The difference in offset error between any two channels.

Gain Error

The deviation of the last code transition (111 … 110 to

111 … 111) from the ideal (that is, VREF − 1 LSB) after

the offset error has been adjusted out.

Gain Error Match

The difference in gain error between any two channels.

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