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CMRR calculation
Oct 07^th, 2005, 12:38am

All,
I need help me in testbench for CMRR calculation for an op-amp. I shorted the input with DC bias and gave common AC. Until certain freq the CM gain (Acm) is in fraction (output is less than input).
From the definition of CMRR = 20log (Adiff/Acm), which is
20log Adiff - 20 log Acm = +ve value - (-ve value) = big +ve value
seems unrealistic to me.

regards,
-Bharat

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sheldon

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Posts: 751

Re: CMRR calculation
Reply #1 - Oct 9^th, 2005, 7:05am

Bharat,

Have you simulated CMRR over process variations?
Some parameters are sensitive to mismatch, one
example would be CMRR. Unless you account for the
effect of mismatch on the parameter then the results
will appear unrealistically high, particularly for fully
differential designs. So add then the effect of mismatch
to the models and run a Monte Carlo simulation, the
results will probably look more realistic.

Best Regards,

Sheldon

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avlsi

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singapore

Re: CMRR calculation
Reply #2 - Jan 31^st, 2006, 2:12am

Hi bharath,

What ever u have done appears quite right to me. For Example,

if u take Adiff/Acm,u must get a large positive value ( ideally infinity,as Acm must be ideally 0)
if u take Acm/Adiff,u must get a value near 0. ( ideally zero,as Acm must be ideally 0)

In practical,we take Acm must be around -60 dB. i.e., (around Acm=(Voc/Vic)=0.001). So now u verify ur results in this fashion.

Bye,
AVLSI.

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bunny New Member Offline huh...analog design.. Posts: 8	Re: CMRR calculation Reply #3 - Feb 12^th, 2006, 4:59am Sheldon, "So add then the effect of mismatch to the models and run a Monte Carlo simulation, the results will probably look more realistic." Can u please elaborate on this point and tell me exactly how to add the effect of mismatch in spectre. Any documentation in this regard will be highly thankful Thanq
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