how to define Vcm in pipeline AD design?

[cloudius]

vcm ota

i've read lots of pipeline A/D papers.Vcm appears oftenly in comparators ,OTA and S/H circuit schematics.
and i'v got confused at the definition and magnitude of Vcm.

Is Vcm a fixed value under certain VDD??
or Vcm should be caculated form circuit??
has Vcm relateto OTA,S/H or other blocks??

thx

 

[pfd001]

Vcm is the common model voltage. it is a fixed value, normally, it is VDD/2

 

[jerryzhao]

The VCM is common voltage only. You analyze the pipeline ADC need not care this voltage. Normally it is vdd/2, Vrefp(or vtop) = Vcm +x(normally x=0.5v). Vrefn(or Vbot) = Vcm-x(normally x=0.5v).
We can get Vcm by two resistors divide the VDD.