Hi nxing,
the answer is simple: During the sample phase the feedback capacitor must be discharged to be prepared to the next period (with charge transfer).
However, this assumes that the charge from the last period has been transferred before to the next stage which acts as a sample-and-hold stage (sample phase of the second stage). OK?
LvW
That means, you never can treat a S/C integrator as a separat and isolated stage because, in reality, it always is operated in conjunction with some other circuitry.