Sample-and-hold or switched capacitor noise simulation in SPICE?

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I'm trying to simulate a sample and hold structure (actually an active
pixel sensor with correlated double sampling) in HSPICE. Specifically,
I'm trying to simulate noise levels. It seems that spice has a
limitation in that noise simulations are a subset of ac simulations,
which are based around linearized circuit models, instead of transient
simulations, which are needed to turn switches on in the proper
sequence and get the right results. Any ideas on what I should do? Is
it possible to use spice for this or do I need to find another CAD
package? Similar problems would arise with standard switched capacitor
circuits too. Thanks for any help,
Matt Cohen