Basically what you need is a fairly high order active filter either a bessel or chebyshev on the input to limit the input spectrum to below half the sampling frequency in order to prevert the appearance of artifacts due to undersampling.
Google is our friend in times like these although i would just mention a linear small signal simulation package i use sometimes called winlap,its freeware and its not spice but its got some nice features such as the abilitly to extract the pole-zero map for any input circyit and convert them into extended fraction tranfer functions and so on.