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What's All This CMRR Stuff, Anyhow? (Download)

Jan. 31, 2025
Bob Pease explains common-mode-rejection ratio.

Read this article online.

Recently, many people have asked me about how to test op amps for common-mode rejection ratio (CMRR), which is defined as the delta of the offset voltage (Vqs) versus the common-mode voltage (Vcm). The first thing I tell them is how not to measure CMRR. If you drive a sine wave or triangle wave into point A, it seems like the output error, as seen by a floating scope, will be (N+1) times (Vcm divided by the CMRR).

But that’s not quite true: you will see (N+1) times (the CM error plus the gain error). So, at moderate frequencies where the gain is rolling off and the CMRR is still high, you will see mostly the gain error, and your curve of CMRR vs. frequency will look just as bad as the Bode plot. That's because with this circuit, that's just what you will be seeing!