how to define the common mode input voltage of OTA used in S/H and MDAC ?

[hsiangleung]

i come across a problem on the selection of common mode input voltage. The OTA (shown in fig1)is used to implement the SC S/H circuit in pipeline ADc and the subsequent MDACs. The supply voltage is 1.2V and the output common mode vlotage is supposed to be 0.6V. If I choose the Vcm,in also around 0.6V, the input transistor could hardly turn on for the PMOS Vth absolute value is near 0.5V.
So in applications of S/H and residue amplifier,how can we choose the Vcm,in. In such analog circuit, is the Vcm,in decided by the applied input signal common mode level? or it is decided by the vcm in fig2?

 

[erikl]

... or it is decided by the vcm in fig2?

Yes, and via the CMFB feedback voltage(s). The CMFB amp also uses V_CM,i .