Common mode rejection ratio for cascaded differential amplifier stages

A simple equation for calculating the total common mode rejection ratio, CMRR/sub T/, for cascaded differential amplifier stages is given, and it is applied to several typical cases in instrumentation circuits. Two factors for each stage, C and D, are defined, and it is shown that, in general, CMRR/sub T/ can be calculated by adding the reciprocals of the equivalent CMRR for each stage, which is defined as the product of its C factor and the product of D factors for the preceding stages. The acceptability of the approximate formulas is verified, and application examples based on typical situations in instrumentation circuits are given. It is shown that the assumptions made to obtain such a simplified equation are acceptable.<
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