Have you checked the phase margin of your common-mode regulation? It might be very low so that there is a lot of ringing when the common mode changes quickly as it probably happens in your SC circuit. In your continuous-time circuit, the common mode is probably not changing so that you don't see the problem there.

I also doubt the problem is caused by the instability of my CMFB loop.
But I really don't know how to check the CMFB loop' phase margin of my OP schematic .
I tried to break up the connection between CMFB and the main OP, and give a AC signal input to the broken place of main OP , and then watch the output from CMFB. However, the differential pairs of CMFB worked in the linear region.
so i cannot get the right infromation from it. Can anyone explain how to check the stability of my CMFB loop in detail?

by the way, here is a figure of the output of a simple SC_amplifier using the OP. It is the differential output signals of the OP that oscillated.

May be it is simply more load capacitance (degrading phase or gain margins) when you use the opamp as a building block for the SC filter as opposed to the continuous time approach?

Tosei

Hi Frank,

I certainly was not clear enough, but you're absolutely right and actually that is what I assumed by suggesting a higher load cap might be causing stability problems.
There might be an "almost dominant" pole in the amplifier topology whose stability easily degrades as soon as you connect a certain load capacitance. However I insist on suggesting that in case the amplifier is not unbalanced the stability problems should be associated to the differential loop.

Tosei