Common Mode Rejection Ration (CMRR)
We’ve touched on common mode voltage and how its magnitude and frequency can degrade the performance of an isolation amplifier. Now we’ll discuss the differential amplifiers ability to reject common mode voltage, a.k.a. its Common-mode rejection ratio (CMRR).
In theory, with identical signals connected simultaneously to the (+) and (-) leads of a differential amplifier, the output should be 0V. In reality, some small voltage will always be present. This is an error voltage that will ride on top of the expected output. A differential amplifier’s CMRR is an indicator of just how much error voltage might be present on the output.
The common-mode rejection ratio is calculated as follows:
CMRR = 20 LOG(Vin(cm)/Vout) dB
Vin(cm) = the voltage common to both the + and – leads of the amplifier
So for example, assuming that you have 5V connected to the + and – leads of a differential amplifier and you see an output of 158µV, your CMRR is 90dB.
By arranging the formula, with 5V common to the + and – leads of the amplifier and a known CMRR of 100dB, we know that the output will be ±50µV:
Vout = VCM/10CMRR/20
As these examples illustrate, the higher the CMRR, the less common voltage is passed through to the output, resulting in a cleaner signal.