DIFFERENTIAL AMPLIFIER

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References: 3

Most amplifiers deal with signals which exist between a signal wire and ground. Differential amplifiers, which are widely used in ECG, EEG, EMG, and pressure amplifiers are intended to amplify the difference in voltage between two signal lines and should ignore any voltage which exists simultaneously on the two signal lines (the common mode signal). The most common common-mode signal is 50 Hz (or 60 Hz) pick-up which may be several volts in some situations, whereas the differential signal may be measured in microvolts in the case of the EEG. Thus a good differential amplifier must have a high common mode rejection ratio (CMRR) of the order of 80 to 100 dB.

CMRR is degraded if the source impedance is different on each signal lead (such as in the case of skin electrodes with unequal quality of application) and so CMRR is usually quoted for a particular source impedance inequality (e.g. 5000 [O]).