I currently have a thd problem with a fully differential, chopped, resistive feedback PGA that I'm designing, and I don't know if it is a circuit or a simulator issue.  The PGA passes THD spec without mismatch, and the 3rd order harmonic is dominant, as it should be.  If I put in 3sigma worth of mismatch, the 2nd order harmonic becomes dominant.  If I turn off the chopping the 2nd order drops away again.

I am chopping at the input and at an internal low impendence node, so I am not chopping around the resistors.  Because of this, it should not be a settling issue.  I can slow down the chopping with the mismatch, and the 2nd order harmonic gets worse, which supports the notion that it's not a settling issue.

Finally, I can run QPSS (spectreRF vs. spectre transient) with chopping and mismatch, and the thd looks beautiful again.  After seeing this, I tried cranking on the tolerances even more for the transient run, but there was no difference.

I certainly would like to believe the QPSS and say that the transient is giving a bogus report, but I have no idea why that would be the case.  Since I don't know if it is a simulator issue or a circuit issue, I wasn't even certain which forum to place, but this seemed the most likely of places.

Anyone have any ideas?