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kataria0

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CMRR is unimportant for inverting op-amp configuration?
Mar 26^th, 2009, 11:22am

In reference to the book "Microelectronics Circuits" by Sedra Smith (second edition), I have some confusion regarding his statements on CMRR. Please see section 3.11 if you have the book. The statements are about a simple op-amp configured as inverting amplifier and non-inverting amplifier configuration.

The book says "The finite CMRR of op-amp is unimportant in the case of the inverting configuration, since the positive input reminal is grounded and hence the common-mode input signal is approximately zero. On the other hand in the non-inverting configuration the common-mode input signal is nearly equal to the applied input signal, and thus the finite CMRR of the op-amp may have to be taken into account in applications that demand high accuracy"

My confusion: Both inverting and non-inverting configurations by themselves are single-ended input configuration, so the concept of common-mode input doesn't make any sense to me. Common-mode signal riding on a differential signal makes sense to me. In other words, when I hear that the differential signal is not ideal, it to me implies that there is some common-mode signal.

I am sure I am not understanding something here since Mr. Sedra couldn't have made a wrong statement.

Thanks in advance for responding.

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raja.cedt

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Re: CMRR is unimportant for inverting op-amp configuration?
Reply #1 - Mar 26^th, 2009, 12:34pm

hi,
you are correct,but the only mistake i found is even though it single ended from opamp input point of view you have to think like is differential and CMRR is like how much common mode input signal is converted as differential signal..now even though opamp CMRR is finite,if your are using inverting amplifier then applied common mode signal is zero..so you wont get any error...but in non-inverting case always common mode signal will be ther,if you apply any input signal.so error will come.So i think now you understood..if any issue let me know.Another thing is because of the above problem you wont see any ACTIVE FILTERS with non-inverting amplifier..
thanks,
rajasekhar.

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aaron_do

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Re: CMRR is unimportant for inverting op-amp configuration?
Reply #2 - Mar 27^th, 2009, 12:57am

Sounds interesting. Basically it seems like this...

1) you want non-inverting configuration with gain of B.
2) gain of opamp is A.
3) so the signal at the inverting terminal should be in phase with the input signal and have an amplitude of B/A.
4) Thus there is a very small differential signal accross the op-amp and a potentially large common-mode signal (almost the whole input signal).
5) ideally CM gain of op amp is zero. If it is non-zero, it will contribute to the DM gain of the overall op-amp...

cheers,
Aaron

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HdrChopper

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Re: CMRR is unimportant for inverting op-amp configuration?
Reply #3 - Mar 27^th, 2009, 7:49pm

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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