How to get the phase margin of CMFB in Cadence?

[princerock]

cmfb simulation cadence

Hi,

I designed a fully differential op amp with CMFB. There is a requirement about the phase margin of CMFB in the specs. Then how can I get this from cadence simulation?

Thank you for your help.

princerock

 

[suria3]

cmfb phase margin cadence

Hi,

To measure the PM of the CMFB loop, 1st break the loop at any point, let say you break the loop at where the dc points are extracted between two resistors at the output to the error amplifer. Then, u can use the Middle Brook method to inject the AC in the loop. Once you break the loop, the input of the AC should be given at the high impedance node and output at the low impedance node.

Then, run the simulatuon fpr AC analysis. Then,open calculator in cadence spectre, click Vf point at output and input then divide (this gives you gain). Then click Vf and point at output and click phase ( this gives you phase). So, now you have two graphs overlap which are gain and phase. From here you gain measure PM easily.


Regards,
Suria