Hi, erikl. Thank you for your reply.
Maybe I didn't describe my question clearly. I know that the estimate current is about 0.37mA. And I calculated the current as follows.
Vc=Vin*(1/sCs)/(Rs+1/sCs)=Vin/(sRsCs+1)=0.3sin(2*pi*50)t (assume sRsCs-->0)
and then use
Ic=Cs*(dVc/dt)
to get the result 0.3768mA.
What I really want to ask is if there would be a problem with the assumption made in the calculation. In other words, if I chose the width 0.4um, you think it will be all right. Because during the start time of the sampling, that is charge the Cs with voltage from 0 to Vc, the current can be very large. Am I right, would this be a problem? If it is so, then how can I chose the proper width. If I am wrong, plz let me know. Thank you!