CMRR in open loop and closed loop configuration of opamp

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anubhavsingh244

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For an opamp in open look configuration,


Here Ad should be equal to open loop gain of opamp and


Now consider the closed loop config of opamp (negative feedback used):


Again,
However, Ad is closed loop gain here (according to Sedra Smith) and


In terms of open loop gain,
A is open loop gain here.

So Adm is open loop gain in the first case and Adm is closed loop gain in the second case. How to know which gain to use in CMRR?

Reference: Microelectronic Circuits (Seventh Edition) by Sedra Smith
 

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CMRR is an input referred parameter and shouldn't depend on circuit gain. For several reason it's not practical to measure CMRR in open loop:
Open loop gain is not specified and expected to vary with supply voltage, temperature and between exemplars. Also biasing in linear range is quite difficult.

Instead you prefer to measure CMRR in closed loop with well defined gain. Ratio of output voltage to differential input voltage variation can be easily calculated.
 

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Can you please provide the page numbers? I find it rather strange. I want to read the original pages.
 

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If you mean to test CMRR physically on test equipment then you need some auxillary hardware. There are old app notes about such methods which I have posted here before, but not stored on this phone.

In simulation you can put it in closed loop and "sniff" the input difference with a vcvs to get "open loop" numbers from two operating points. I have found stb unreliable without warning, so I use more primitive, inspectable setups.
 

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Simple Op Amp Measurements | Analog Devices

Op amps are often used in high precision analog circuits, so it is important to measure their performance accurately.
www.analog.com



Using Matlab to eval Aol and Fdbk ratio accuracy on CMRR quite revealing (the reason folks use
IA's which are laser trimmed in manufacturing to get performance) -




The above info might help.


Regards, Dana.
 

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