The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> PSS or QPSS for SC-CMFB OTA
https://designers-guide.org/forum/YaBB.pl?num=1120552636

Message started by ethan on Jul 5^th, 2005, 1:37am

Title: PSS or QPSS for SC-CMFB OTA
Post by ethan on Jul 5^th, 2005, 1:37am

hi there,

for my fully-differentail fold-cascode OTA with SC-CMFB, the clock frequency, say, can be up to 400kHz. Currently, I set CMFB clock with 200kHz with the same sampling frequency which I will use later.

For my OTA differential inputs, they can be set up to 100kHz.

I am wondering that, in this case, whether I can use QPSS, instead of PSS, to do periodic steady-state simulation, since I thought the inputs can be various fundamental frequencies by name, besides the different names and frequencies for CMFB clocks.

Am I right? please advise.

Title: Re: PSS or QPSS for SC-CMFB OTA
Post by bunny on Jul 5^th, 2005, 4:58am

hi ethan,
I too was bugged by the same problem with switched capacitor CMFB. Why dont u try using simple transient analysis followed by ac analysis (The ac analysis should use the operating point calculated by transient analysis at some particular instance). It worked for me. Ofcourse PSS and PAC also worked for me. but the first method makes the things very simple

Title: Re: PSS or QPSS for SC-CMFB OTA
Post by ethan on Jul 5^th, 2005, 8:01am

Hi bunny,

Thank you for your suggestion.

bunny wrote on Jul 5^th, 2005, 4:58am:

hi ethan,
I too was bugged by the same problem with switched capacitor CMFB. Why dont u try using simple transient

I have tried it with conventional transient analysis with SC CMFB, it also works for common mode output voltage and Vctrl node. But when I added a small sine wave in the inputs, the output peak-to-peak swing was small, not as large as expected.

bunny wrote on Jul 5^th, 2005, 4:58am:

analysis followed by ac analysis (The ac analysis should use the operating point calculated by transient analysis at some particular instance). It worked for me.

I have tried conventional AC analysis either. But I found it was hard to control the dc operating biasing since there two half clock phases for CMFB with different DC biasing.

Would you mind describing a little bit more detailed about how you did control that "ac analysis should use the operating point calculated by transient analysis at some particular instance"?

bunny wrote on Jul 5^th, 2005, 4:58am:

>Of course PSS and PAC also worked for me. but the first method makes the things very simple

I guess I have not fully understood PSS and PAC yet.

ethan