From An Introduction to Mixed-Signal IC Test and Measurement
8.5 SIGNAL REJECTION TESTS
8.5.1 Common-Mode Rejection Ratio
A number of signal rejection specifications are common to analog and sampled channel testing. Signal rejection tests are those which measure a channel's ability to prevent an undesired signal from propagating to the channel's output. The undesired signal may originate in the power supply, in another supposedly separate circuit, or in the channel itself.
An FFT analysis of the output of an amplifier contains the following spectral amplitudes:
FFT Spectral Bin
62 (2 31)
93 (3 31)
124 (4 31)
155 (5 31)
In addition, the total RMS value of the output signal is 0.95601 V. Calculate S/2 nd, S/3 rd, S/THD and S/THD+N.
A multitone test signal consists of a sum of three 1.0-V RMS sine waves, one at 0.9 kHz, another at 2.1 kHz, and the third at 5.3 kHz. Determine the frequencies of all third-order intermodulation frequencies.
85.63, 55.31, 53.84, 46.79 dB.
5.1, 3.3, 11.5, 9.7, 3.9, 0.3, 7.1, 3.5 kHz.
One such signal rejection test, common-mode rejection ratio (CMRR), is a measurement of how well a channel with a differential input can reject a common-mode signal. Ideally, a differential input circuit produces an output equal to GV diff, where V diff = IN P ? IN N is the differential input voltage and G is the gain of the input circuit. Provided that IN P and
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