question about CMFB- using opamp without using the CMFB

[Usman Hai]

cmfb basic

Hi all,
I have a question about SC CMFB. Is it possible that after the CMFB settles to its desired voltages, one can use the opamp without using the CMFB. Like the case in an algorithmic ADC, where opamp is used in few clock continuously without the CMFB being used.

 

[dineshbabumm]

question about CMFB

CMFB basic functioning is to ensure if the node voltage drifts from its actual voltage, CMFB will help restore it..

And CMFB takes some cycle to settle to its desirved bias voltage. So I think we cannot use the opamp without CMFB.. Especially when the gain of the op amp is very high.. Because the chances of the bias voltage drifting is very high..

 

[Usman Hai]

Re: question about CMFB

Thank you very much.
However my point is after the CMFB settled to the desired values, can I disconnect the CMFB from the opamp. It seems possible as we need biasing voltage from CMFB circuit, and if that biasing is not disturbed then I think output can have the desired voltage even we are not clocking the CMFC anymore.

Assuming that bias voltages may drift during these few cycles, and I dont have time to use CMFB again for next few cycles. In this case, should I use continuous time CMFB?

 

[dineshbabumm]

Re: question about CMFB

Thank you very much.
However my point is after the CMFB settled to the desired values, can I disconnect the CMFB from the opamp. It seems possible as we need biasing voltage from CMFB circuit, and if that biasing is not disturbed then I think output can have the desired voltage even we are not clocking the CMFC anymore.

Why we use CMFB in first place is it senses node voltages and will help restore it if it changes from the desired value right? I mean if you disconnect the CMFB once the opamp works in desired bias voltages, what will you do if the node voltage drifts after some time? Say some noise occurs in the input and it leads to change in the bias voltage at the output.. If CMFB is disconnected, then it might not be able to sense this drift and help in restore right? Thats why I feel you need to have the CMFB connected with the op amp through out..

 

[karim365]

Re: question about CMFB

Hi all,
I have a question about SC CMFB. Is it possible that after the CMFB settles to its desired voltages, one can use the opamp without using the CMFB. Like the case in an algorithmic ADC, where opamp is used in few clock continuously without the CMFB being used.