help,common mode voltage swing in fully diffrential opamp

q0w1e2r3 said:

hi,everyone
when i'm dealing with the simulation of a fully differential opamp in spectre,a puzzle baffle me.The schemetic and result is as follows.
Common mode voltage selected by me equals 900mV,i use sc common mode feedback and telescope opamp for simplicity.i use this closed feedback(T&H) to achieve doubling the input voltage accurately.the clock is non-overlapping.
When i simulate in spectre i find the diffrence mode volatge doubled ,that's right.
but the common mode voltage swings from 730mV to 900mV.Why?is that becuse of dc offset or something else?how can i overcome it.in addition ,if i cascade series of such circiut as pipeline ADC,is this harmful?(assuming using fully differential opamp and common mode input range is wide enough)
Added after 2 minutes:

On a fully differential amplifier the common mode voltage is fixed by the input potential on COM pin. That's the advantage of this kind of amplifier.
Before simulating so much (and with some errors) I suggest you to read carefully the theory of the fully differential amplifier and the circuit topology.
You may do it following those links:

a good fully differential amplifier datasheet:
https://focus.ti.com/docs/prod/folders/print/ths4121.html

the theory of Texas fully differential amplifier:
**broken link removed**

an Analog Devices very good fully differential OA and circuit topology:
https://www.analog.com/UploadedFiles/Data_Sheets/671129650AD8132_e.pdf

I hope those will help you.

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