Sheldon,
Thanks for your reply.
You are right. "Changing individual capacitors can change the transfer function."
But I changed the sampling capacitor, feedback capacitor and integration capacitor at the same time in my circuit so that the coefficients of the transfer function should keep unchanged.
The SNR value change is consistent with the sampling capacitor change (2pF->200fF). Even though this conclusion is correct, I just don't understand why it happens in this "tran" analysis. As I know, the "tran" analysis does NOT include device noise (thermal, 1/f....) so I am not supposed to see the noise floor change on the fft plots here. Do you have any other thoughts? Thank you!
Dashun
sheldon wrote on Oct 21st, 2010, 2:15am:Xuedashun,
You are trying to implement some transfer function, for example,
Y(z) = z^-2X(z) + (1-z^-1)^2 E(z).
Changing individual capacitors can change the transfer function.
Are the simulation results for your circuit consistent with the
results for the equivalent discrete time transfer function that you
have implemented by changing the capacitor value?
Best Regards,
Sheldon