Common-mode rejection ratio (CMRR) is defined as the rejection of the ADC output to an applied common-mode input voltage. The common-mode input is 60 Hz with a peak-to-peak amplitude equal to the specified absolute input voltage range. The standard deviation (RMS) value of the ADC output is calculated and scaled to volts. In order to measure CMRR, record two ADC readings. The first reading (VA) is with no common-mode input signal. The first reading represents the baseline ADC noise. The second reading (VB) is with the common-mode input applied. The second reading represents the combination of the ADC baseline noise plus the increased RMS noise caused by the common-mode input. The ADC baseline noise is extracted from the combined noise to yield the noise induced by the common-mode input voltage. The CMRR measurement is described by Equation 3:
For gains > 1, add 6 dB of compensation value for each binary increase of gain.