Tlaloc wrote on Oct 29th, 2008, 9:19am:I have an additional question that seems related. Vivek, it seemed from your statements that you view an AB output stage as an inevitability, but I'm not sure why. If the diff pair current is being set by a noise spec and you have enough bandwidth and slewing for the max step, why use an AB stage? I am designing such an amp, and the differential settling seems very robust even when I am slew limited. I must say, that the common mode isn't nearly as clean during the slew limitation, but that shouldn't matter. We are being cautious with the bandwith, however, by settling to better than 1/2 LSB. It does make perfect sense to avoid non-linear slewing when you are not sure if the amp can settle in time, though.
Hi,
I agree with you that the noise spec is going to be decide the current, but that will be the minimum current you are going to need. However, consider the statement one encounters so often in the literature (and for which I am yet to find a convincing explanation) that one can do with a 60 dB gain opamp and still achieve 90 dB dynamic range. Of course, if the opamp were ideal (true first-order system with no slew limitation), this would be true. But there are no such opamps, atleast none that fulfil the requirement perfectly.
So I have been making mine to achieve 1/2 LSB settling. A 90 dB settling however requires a bandwidth 1.5x better than a for a 60 dB case, which will need atleast 1.5x more current. Moreover, we know that with a suitable modulator architecture such as CIFF, the requirements on the opamp settling can be reduced considerably. So designing for 1/2 LSB settling is too conservative and wasteful.
That's why I might have given the impression that I consider an AB stage as an inevitability. Of course there are other issues in real design too. Maybe I will have to make a more detailed model to figure out what gain (or in other words, settling accuracy) I need.
Regards,
Vivek