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CMRR of 2 stage differential input and differential output op-amp

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Chinmaye

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Dear all,
I am trying to find the CMRR of 2 stage diff input and diff output opamp in cadence. To find differential gain, I provide 0.5V to positive input and -0.5V to the negative input. Then I plot (Vo+ - Vo-)/(Vi+ - Vi-) over a range of frequencies to get the plot of differential gain over a range of frequencies. To calculate the common mode gain, I give 1V to both the inputs and plot the magnitude of either Vo+ or Vo-. The calculated common mode gain is not matching with the plotted gain. What mistake am I making here?
 

You should short the inputs and connect it to a voltage source. Put in the appropriate DC voltage, put in AC Magnitude as 1 and sweep the frequency through an AC simulation.
If there are no systematic mismatches, then Vo+ and Vo- will show identical output. Else you will see a difference in the two outputs. That is your common mode to differential gain.
 

Here is alternative approach using MatLab. You can evaluate effect of
finite AOL and fdbk factor on CM effectiveness.


1612875814041.png



Regards, Dana.
 
If you are using Virtuoso, use ideal_balun element from analogLib.
How to use it, you can learn from this forum, designer's guide forum or simply virtuoso help.
 
You can use two input source one vsin for differential and one vdc for common mode. The vdc for common mode can have an AC value which you can set to 1 for common mode gain to calculate. Then connect the p,n of ideal_balun to p,n of the inputs and connect "d" of ideal_balun to differential and "c" to common mode input.
Now you can calculate the common mode (CM) gain when you set AC of differential to 0 and set AC of CM to 1.
A better way is to use "XF" simulation. Then you specify your output and when you use direct plot, you can choose which input is used for this output (either CM or differential). Also you can put the expression in calculate (Vout_idff - Vout_CM) in dB and get CMRR.
 

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