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Simulators >> Circuit Simulators >> PAC simulation on OTA with SC CMFB
https://designers-guide.org/forum/YaBB.pl?num=1414636974

Message started by petitidiot on Oct 29^th, 2014, 7:42pm

Title: PAC simulation on OTA with SC CMFB
Post by petitidiot on Oct 29^th, 2014, 7:42pm

Dear guys,
I have been confused by a problem for a long time.
I have a fully differential OTA with switched capacitor common mode feedback. I want to find the dc gain. The first way is to run ac simulation with flying capacitors having initial conditions. The simulation result is 64dB, which is the same with the sc cmfb replaced by ideal ct cmfb. However, if i run the pac, the dc gain is only 46dB. The gm is 1uS. The flying cap is 100fF. The clk is 20KHz. I use alternative network to settle the output, so the effective switch cap resistor may read 1/2fC = 250M. The calculated gain is near the pac simulation result. The textbook said that the switch cap may provide loading to the output impedance. I am not sure does it also counts for the resistive loading? As such, which simulation, ac or pac, reflects the real situation?
Thanks!

Title: Re: PAC simulation on OTA with SC CMFB
Post by Ken Kundert on Oct 29^th, 2014, 11:21pm

How did you measure DC gain?

Title: Re: PAC simulation on OTA with SC CMFB
Post by petitidiot on Oct 30^th, 2014, 12:34am

Dear Kundert,
When I measured the DC gain, first I ran a short transient and recorded the transient differential voltage over the switch capacitors. Then I put that value in the initial conditions of those caps and ran an AC analysis. By this means, I could get the result the same with ideal continuous time cmfb. However, I could get this DC gain by PAC. I am not sure if the resistve loading effect of sc has to be taken into consideration.

Title: Re: PAC simulation on OTA with SC CMFB
Post by Ken Kundert on Oct 30^th, 2014, 2:36am

The most accurate approach would probably be to use pstb analysis to compute the openloop gain and extract the DC value. And you should do so in the actual circuit where the amplifier will be operating. I do not recommend using AC analysis. It is forcing you to make unnecessary assumptions and approximations.

-Ken