AD7927 datasheet(10/29 Pages) AD

10 / 29 page

Data Sheet

AD7927

Rev. D | Page 9 of 28

Signal-to-(Noise + Distortion) Ratio (SINAD)

This is the measured ratio of signal-to-(noise + 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 (fS/2), excluding dc. The ratio

is dependent on the number of quantization levels in the digiti-

zation process; the more levels, the smaller the quantization

noise. The theoretical signal-to-(noise + distortion) ratio for

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

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

Thus, for a 12-bit converter, this is 74 dB. Figure 5 shows the

signal-to-(noise + distortion) ratio performance vs. input fre-

quency for various supply voltages while sampling at 200 kSPS

with an SCLK of 20 MHz.

Total Harmonic Distortion (THD)

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

fundamental. For the AD7927, it is defined as:

log

)

(

THD





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.

Figure 7 shows a graph of total harmonic distortion vs. analog

input frequency for various supply voltages, and Figure 8 shows

a graph of total harmonic distortion vs. analog input frequency

for various source impedances (see the Analog Input section).

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