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Hi Amplifier team,
CMRR is ratio of differential gain and common mode gain. The datasheet says that it is referred to the input. What does referred to input mean here?
Regards,
Shaury
Shaury,
CMRR referred to input, RTI, means that it is measured as a ratio of the change in the input offset voltage (Vos) with the change of the input common-mode voltage (Vcm).
Thus, CMRR ~ change_Vcm/change_Vos and NOT CMRR ~ change_Vcm/change_Vout where change_Vos=change_Vout/Gain.
Below please see the examples of measuring CMRR for G=1 and G=11.
Thanks Marek! Got it. If we calculate CMRR using 20log (Ad/Acm) formula, it would give us the same value. Acm doesn't depend on the gain of the instrumentation amplifier. That's why increasing differential gain improves CMRR.