SBASAM0B March 2024 – November 2024 ADS127L18
PRODMIX
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 are applied with the inputs externally shorted together. These test voltages are equal to AVSS + 50mV, (AVDD1 + AVSS) / 2, and AVDD1 – 50mV. The maximum change of the ADC offset voltage is recorded versus the change in common-mode test voltage. Equation 12 shows how CMRR (dc) is computed.
where:
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. Equation 7 shows that the nine largest amplitude spurious frequencies in the frequency spectrum are summed as powers. These frequencies are then related to the amplitude of the common-mode test signal.
where: