Tlaloc wrote on Sep 9th, 2008, 8:48pm:First off, the major drawback of chopping is a greatly increased slewing requirement. Let's assume that your capacitor gain in your integrator without chopping is 1/4. For every full scale (FS) worth of input signal, you output is 1/4*FS. So, without chopping, if your input is FS and your output is 3/4*FS, you output would still only need to step by 1/4*FS (with ideal rail-to-rail outputs) to bring the output to +FS. With chopping enabled, you would need to drive the output from 3/4*FS to -FS, which is a huge difference in output step. Since most OTA's enter a slewing condition during these large transients, chopping exacts a huge slewing burden. As an aside, continuous time chopping exacts a huge bandwidth burden.
I am just flip-flop the OTA in the integrator, so the integrator output should be exactly the same as the case without chopping. Why is there the difference of the output step.
Quote:If your OTA does not settle properly, either through bandwidth limitations or slewing limitations, that shows up as a gain reduction. This is simply due to the fact that the charge on your caps are not what you expect them to be. There would always be less charge, so therefore it would be indistinguishable from an integrator with lower gain.
OK。 I think that's right.
Quote:I don't know why you say that the error voltage on the summing junctions would be a noise-shaped signal, though. The very-low frequency errors--i.e. offset, 1/f noise, cap mismatch, and some charge injection--would be up-converted to the chopping frequency. However, this is not a noise signal. It can be explicitly defined by the charge equations, and hence, I would not agree that it would move the quantization noise into the signal band.
I think the output of integrator 1 is noise shaped ((1-z^-1)^3*(z^-1/(1-z^-1))). The voltage at the OTA virtual ground is just 1/A of the OTA output, thus is noise-shaped as well. If it is chopped, it will be moved by fs/2, and mess up the signal band. Hope that clarify what I want to say.