# Switched capacitor output common mode

1. ## Switched capacitor output common mode

I am designing switched capacitor biquad low pass filter. I have some questions regarding the design.

1. Cutoff frequency = 1KHz
2. 65nm LP process
3. Clock frequency is 50K

There are 2 integrator in my circuit. It is a second order filter. It is not fully differential.

My question is

How am I supposed to set dc output common mode of the circuit since the switched cap is not going to set it in DC.
Usually in normal RC integrator the output common mode voltage is set by the input common mode because of DC feedback.

I hope for your answer or any reference papers I can go through.

Thanks and regards,
Supriya

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2. ## Re: Switched capacitor output common mode

in a fully differential switch cap integrator the output common mode is set by a switch cap common mode feedback circuit. the theory of this can be applied in various ways. basically apply charge to a cap and switch the cap into your high impedance node. just remember a charged cap being applied to a high z node acts like a battery causing that node to be the voltage across the cap. I have seen continuous time c feedback integrators set their input common mode this way( cmfb for the output and a switch cap setting the input cm = to the output cm)

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3. ## Re: Switched capacitor output common mode

Originally Posted by prestonee
in a fully differential switch cap integrator the output common mode is set by a switch cap common mode feedback circuit. the theory of this can be applied in various ways. basically apply charge to a cap and switch the cap into your high impedance node. just remember a charged cap being applied to a high z node acts like a battery causing that node to be the voltage across the cap. I have seen continuous time c feedback integrators set their input common mode this way( cmfb for the output and a switch cap setting the input cm = to the output cm)
Hi Prestonee,