HdrChopper
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Hi,
When talking about CMRR you are in principle talking about the opamp, and not its external cricuitry. Therefore, since any opamp has a differential input, it does make sense to talk about CM and DM signals. Now, since in the inverting configuration the + input is grounded, assuming a nearly ideal opamp then the - input will also be grounded and no CM signals will be applied to the differential opamp inputs. On the other hand, when considering a non-inverting amplifier, the applied signal has always a CM component since the - input will "follow" the + input if loop gain is high...thus you have a CM signal. This is the scenario Aaron described in 4) As he also stated in 5), a finite CMRR will be seen as a closed loop gain error (just think of a small signal applied to the + input at different CM levels: since CMRR is finite the ouput signal at two different input CM levels will be different)
Regards Tosei
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