## 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.

Exercises

1.

An FFT analysis of the output of an amplifier contains the following spectral amplitudes:

FFT Spectral Bin

RMS Voltage

31

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.

2.

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.

 1 85.63, 55.31, 53.84, 46.79 dB. 2 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

##### Products & Services
Signal Isolators
Signal isolators provide electrical (galvanic) isolation between the input and output circuits. They couple the signal to the output through a transformer or optical isolator. Signal isolators also break the direct electrical (galvanic) path between two or more loop points. They are used to protect against dangerous measured-variable voltages and provide increased protection from surges and spikes.
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.
Signal Amplifiers
Signal amplifiers accept signals from sensors and other devices and amplify them to levels suitable for further processing or digitization by computer elements.

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