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Design >> Analog Design >> Strange second order harmonic
https://designers-guide.org/forum/YaBB.pl?num=1272407821

Message started by Tlaloc on Apr 27^th, 2010, 3:37pm

Title: Strange second order harmonic
Post by Tlaloc on Apr 27^th, 2010, 3:37pm

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?