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Design >> Analog Design >> input cm dc defintion in switched cap circuits
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Message started by ee484 on Jul 29^th, 2006, 8:46am

Title: input cm dc defintion in switched cap circuits
Post by ee484 on Jul 29^th, 2006, 8:46am

Hi, I would like to ask you guys about common-mode of the input of OTA used in switched capacitor circuits.
For example, the gain of 2 circuit (typically used in a stage of pipelined ADC) has no DC path at the input of OTA. However, we are comfortable with using it. How is the input common-mode is defined at the OTA in that case? (in amplification phase in gain of 2)
My guess is that one phase it defines the input common-mode, then (input) parasitic capacitor holds the common-mode value in the other phase and amplifiis only the differential signals?? In other words, this will work only in high-speed SC circuits?

Thank you for your all comments.

If you have solid reference discuss about this, I'll appreciate it.

Yours,
BJ

Title: Re: input cm dc defintion in switched cap circuits
Post by loose-electron on Aug 1^st, 2006, 7:29pm

In a switched capacitor system, the common mode usually gets determined by the prior stage common mode voltage.

And that, in a fully differential system is determined by the CMFB reference of that amplifiers ouput stage.

If you have a system where the input of an amplifier is "open switch floating," give some serious thought to swapping the two inputs to a common mode refernce voltage instead.

As a general rule, I never let inputs to an amplifier "float" because that can get unpredicable. (Especially true with high leakage 90nm - 45nm CMOS) It may simulate nicely, but expect a rude awakinging when you get it in the lab.
:'(

A lot of the academic textbooks don't mention that the inputs should be swapped to center scale DC reference points when unused. It is kind of like all the literature on bandgaps does not bother to deal properly with startup circuits.

In a similar fashion a lot of books illustrate ground referenced designs. That is suicide from a noise perspective, which is why the world does everything fully differential if at all possible.

THe devil is always in the details.