switched capacitor integrator satuartion and low dynamic range

[pankaj jha]

Hello everyone !!!
I am simulating a switched capacitor based integrator in Cadence. the problem is that the integrator saturates for very low values of input voltages. Also if i give a sinewave input of 100mV amplitude (riding over 0.9V dc; for 180nm technlogy), the output shows a sinewave which never goes below 0.9V.

Can somebody expalin the reason why???

 

[pfd001]

For integrator, it can be regarded as adder. If the input voltage is DC, the output will acculate, and then it will saturate. If the input is sine wave, there is positive and negative signals, it will no saturate