query regarding CMRR graph

[rishabh_31ec]

Hi,
As I have already mentioned that I have designed 4-stage NGCC opamp in TSMC 180nm process using Cadence and I got all my specification except CMRR. When I have simulated my opamp I have found that CMRR curve increasing with frequency but it should be decrease as frequency increases. I am confused what is exact problem ?
I have attached my CMRR graph and Test bench for CMRR analysis.

 

[erikl]

Seems you display the ratio dB20(power_supply_voltage/output_voltage) which of course is positive.

For the CMRR you should display the ratio dB20(VF("/OUT")/VF("/Vcm")) , i.e. dB20(output_voltage/common_mode_voltage) instead, which should be <<1, resp. a negative result in dB20 log. presentation.

 

[rishabh_31ec]

Thanx erikl,
I think dB20(VF("/OUT")/VF("/Vcm")) is Common Mode Range not CMRR and I referred Holberg lecture in which CMRR is dB20(VF("/VCM)/VF("/OUT")). CMRR must be as high as possible...Then where is the problem in my result?

 

[erikl]

... I referred Holberg lecture in which CMRR is dB20(VF("/VCM)/VF("/OUT")). CMRR must be as high as possible...

Ok. Seems both versions (yours and its inverse) are in use in various books and papers.

Then where is the problem in my result?

As mentioned above: you didn't plot dB20(VF("/VCM")/VF("/OUT")) but dB20(VF("/V+")/VF("/OUT")).