Common-mode rejection ratio (CMRR) specifies the ability of the ADC to reject common-mode input signals. CMRR is expressed as dc and ac parameters. For measurement of CMRR (dc), three common-mode test voltages equal to AVSS + 50 mV, (AVDD1 + AVSS) / 2, and AVDD1 – 50 mV are applied with the inputs externally shorted together. The maximum change of the ADC offset voltage is recorded versus the change in common-mode test voltage. Equation 11 shows how CMRR (dc) is computed.

where:

• ΔV_CM = Change of dc common-mode test voltage
• ΔV_OS = Change of corresponding offset voltage

For the measurement of CMRR (ac), an ac common-mode signal is applied at various test frequencies at 95% full-scale range. An FFT is computed from the ADC data with the common-mode signal applied. As shown in Equation 7, the nine largest amplitude spurious frequencies in the frequency spectrum are summed as powers and related to the amplitude of the common-mode test signal.

where:

• V_CM (RMS) = Common-mode input signal amplitude
• V_O (RMS) = Root-sum-square amplitude of spurious frequencies = √(V₀ ² + V₁ ² + ...V₈ ²)