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Design >> Analog Design >> why DC gain of opamp with SC CMFB freq dependent?
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Message started by prcken on Sep 30^th, 2012, 7:37pm

Title: why DC gain of opamp with SC CMFB freq dependent?
Post by prcken on Sep 30^th, 2012, 7:37pm

Hi
I found that the open loop gain of the SC CMFB opamp is sampling frequency dependent, I use PSS+PAC analysis, it drops when sampling frequency increases, same opamp configuration with SC-CMFB, the gain drops to 69dB at fs=5MHz. the DC gain is 75dB when fs=1MHz , with continuous time CMFB is 77dB.
any idea?
thanks

Title: Re: why DC gain of opamp with SC CMFB freq dependent?
Post by BackerShu on Nov 18^th, 2012, 9:31pm

SC circuit is equivalent to Resistor. As frequency increases, equivalent resistor reduces; thus gain reduces.