Question about common-mode input in SC circuits

[ee484]

opampinput parasitic cap

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 does the input common-mode is defined at the OTA in that case?
My guess is that one phase it defines the common-mode, then parasitic capacitor holds the common-mode value in the other phase and amplifiis only the differential signals??


Thank you for your all comments.

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

Yours,
BJ

 

[sunking]

CM input of SC circuit

There is a cap to hold it.
You can reference some book about cmfb, such as allen: cmos analog circuit design

 

[ee484]

Re: CM input of SC circuit

Which cap are you talking about?
Sampling cap? feedback cap? or input parasitic cap?
My question was ...since there is no DC path that holds input CM correctly, I understand that there should be something that provides input common-mode.

I'll appreciate if you can be more specific.
Thanks

 

[benchen]

Re: CM input of SC circuit

the CM input of a OTA is the ouput CM of its previous OTA, Is that right?

 

[daisyinfall]

Re: CM input of SC circuit

Hi,ee484,have you designed an intergrator?In my opinion ,the intergrator has a similar problem,and it is dc opened.
can u put out the schematic?

 

[zenisle]

CM input of SC circuit

common mode feed back can use resisitor

 

[mists]

CM input of SC circuit

in switched capacitor circuit, in general, it use two non-overlap phase clock, one phase is sample, the other is intergrate or amplicate. so the opamp can use sample clock to set CM input voltage.

 

[ee484]

Re: CM input of SC circuit

So, far nobody gave me definite answer....
If anyone knows it....or know where I can find the solid answer...for example references...I'll appreciate it!!!
Thanks


How does OTA input cm level is defined in amplification phase? Is that a parasitic cap that is charged in sampling phase??