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    Fully Differential Opamp - Folded Cascode + Common Source SC CMFB Issues

    Hello,

    I have designed a two stage opamp - one stage is a folded cascode opamp followed by a common source stage - I am using a 65 nm process and need the extra gain of the common source stage.

    I want to use SC CMFB for each stage. What are the timing issues for doing this with these two stages? What should I keep in mind or look out for? Any resources on this set up? Any advice on the timing and clocking?

    Thanks.

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  2. #2
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    Re: Fully Differential Opamp - Folded Cascode + Common Source SC CMFB Issues

    You can use the same clock for both of the stages if they are both fully differential. In this case, you may need 10-15 clock cycles to set up the desired common mode. This is dependent on the SC CMFB switches and used caps (their time constant). In these settling clock cycles you cannot sample the output acccurately. So smaller caps and large switches are beneficial.
    Your life is all about ΔΣ, the narrow you estimate it, the precise it is ...



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