I am trying to simulate delta-sigma modulator with HSIM (it is said it is faster than Spectre). However when I look at the voltage across the sampling cap, there is around 0.5 mV numerical error (Spectre gives right result). It seems like the voltage keeps going to the right value at first, but never makes the last step. I try to tweak the simulation options, but cannot make it work. Here is the option I used:

.param HSIMOUTPUT=psfbin
.param HSIMSPEED=0
.param HSIMSPICE=3

Any suggestions? Thanks a lot.