Recently I encountered some problems with PAC simulations so I decided to test the PAC simulation.
I built the simple RC circuit with the R replaced by a switched capacitor C_R as shown in the following figure:


Vin -------(phi 1) -------(phi 2) ----------Vout
                            |
                            |
                         ------
                         ------  C_R
                            |
                            |
                         ------
                          gnd

phi1 and phi2 are non-overlapping clocks controlling two NMOS switches respectively.  Vout is connected to the ground (gnd) through another capacitor (not shown).
To verify the simulation issues, I constructed three similar circuits as follows (all with same input Vin and clock signals):
Circuit (1): NMOS model set 1, Vout = vosci
Circuit (2): NMOS model set 2, Vout = vosc
Circuit (3):  Switched capacitor CR is replaced by its equivalent resistor R, Vout = vorc.

PSS simulations are similar for vosci and vosc while vorc is simply a dc voltage as expected.  However, when proceeding to perform the PAC simulation, I obtained the following results (see attached).
From the graphs, it can be seen that vosci (Circuit 1) and vorc are as expected from simple RC circuit but vosc (Circuit 2) is very odd with a gain of below -100dB.  
As the only difference between Circuit (1) and Circuit (2) is the NMOS model (BSIM3), could anyone help me solve the weird PAC results mystery?  Thanks!

Hi Frank,
Thanks for replying.  I have absolutely no clue which of the BSIM3 model parameter(s) cause such PAC problem.  I tried to compare the two sets of NMOS BSIM3 models and found that the one in Circuit (1) uses version 3.2 and Circuit (2) uses version 3.3.  Also, there are some differences between the two models but majority of the parameters are defined (although the values are slightly different).
Could it be due to some missing parameters in the BSIM3 version 3.3 in Circuit (2)?

Version 3.2 is rather old; there were some bugs fixed in intermediate versions.  You could also try just changing the version number (try 3.24).

Did you try some basic dc i/v curves over the voltage range you're working with?

-Geoffrey
(Post #1000!)