In response to: How can I deal with 3rd HD in a 2nd SigmaDelta Modulator
It’s possible that the OTA is limiting you. Doing a matlab model of the sigma-delta and running it should tell you whether it is a circuit problem or a system-level problem. My hunch is that it’s a system-level problem: the noise-shaping filter doesn’t have enough gain at 24 kHz to suppress the 3rd order distortion.
Seems like the sigma-delta loop doesn’t have a lot of gain where you want it. I would suggest putting in some noise-transfer zeros (noise-shaping filter poles). This would move the nulls off dc and closer to where you want them. You may need to go to a 3rd-order sigma-delta to do this (one NTF zero at dc and two complex ones at 24 kHz or so).
The other alternative, as stated before, is to improve the linearity of the quantizer: go to a multibit.