AD7475ARMZ datasheet(11/24 Pages) AD

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Data Sheet

AD7475/AD7495

Rev. D | Page 11 of 24

TERMINOLOGY

Integral Nonlinearity

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. The endpoints of

the transfer function are zero scale, a point ½ LSB below the

first code transition, and full scale, a point ½ 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 + 0.5 LSB.

Gain Error

This is the deviation of the last code transition (111. . . 110) to

(111. . . 111) from the ideal (that is, VREF − 1.5 LSB) after the

offset error has been adjusted out.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns into track mode on the

13th SCLK rising edge (see the Serial Interface section). The

track-and-hold acquisition time is the minimum time required

for the track-and-hold amplifier to remain in track mode for its

output to reach and settle to within 0.5 LSB of the applied input

signal, given a step change to the input signal.

Signal-to-Noise and Distortion Ratio (SINAD)

The measured ratio of signal-to-noise and distortion at the

output of the analog-to-digital converter (ADC). The signal is

the rms amplitude of the fundamental. Noise is the sum of all

nonfundamental signals up to half the sampling frequency

(fS/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 SINAD ratio

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



 

dB







Distortion

Noise

Signal

For a 12-bit converter, the SINAD is 74 dB.

Total Harmonic Distortion (THD)

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

the AD7475/AD7495, THD is defined as



THD

log





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

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

sum and difference frequencies of mfa ± nfb where m, n = 0, 1,

2, and 3. 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 AD7475/AD7495 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. Like THD, intermodulation distortion is calculated

as the rms sum of the individual distortion products to the rms

amplitude of the sum of the fundamentals, expressed in dBs.

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