From An Introduction to Mixed-Signal IC Test and Measurement


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

RMS Voltage


0.9560 V

62 (2 31)

0.05 mV

93 (3 31)

1.64 mV

124 (4 31)

0.04 mV

155 (5 31)

1.04 mV

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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Products & Services
Instrumentation Amplifiers
Instrumentation amplifiers are differential amplifiers that have been optimized for use with DC signals. They are characterized by high gain, high common mode rejection ratio (CMRR), and high input impedance.
Signal Converters
Signal converters contain inputs for one type of signal and outputs of another. Features can also include filtering and amplification or attenuation.
Signal Conditioners
Signal conditioners provide amplification, filtering, converting, and other processes required to make sensor output suitable for reading by computer boards.
RF Frequency Multipliers
RF frequency multipliers are nonlinear devices that produce an output signal with a frequency that is larger than the frequency of a corresponding input signal by a predetermined factor.
Data Acquisition
Data acquisition is the digitizing and processing of multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes. Signal conditioning includes the amplification, filtering, converting, and other processes required to make sensor output suitable for rereading by computer boards.

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