Yes, ideally Vout+ and Vout- are complementary, especially when it come to the signal being processed. Yes the signal is also digital.
Unfortunately other signals (unwanted) also appear, such as clock feedthrough, power supply/substrtae noise etc. and then can add to both Vout+ and Vout- simultaneously, therefore making Vout+ not the exact complement of Vout-. These effects also result due to non-ideal comparator design, rather than improper design.
Therefore to help cancel these unwanted (mostly even order, of which DC can be though of "0" order, whcih is the point sheldon made) effects, and get the best out of the fft, the fft should be done differentially.
Of course i am an RF/Analog IC person, so my judgement is always somewhat blurred :P
vivkr wrote on Apr 21st, 2006, 1:48am:Hi ACWWong,
I don't quite understand the point you make. It is clear that one needs to look at differential
outputs when dealing with analog signals such as the integrator outputs to cancel the common-mode
and even harmonics,
However, if a comparator is working correctly, then Vo+ is always the complement of Vo-, and these
2 signals are digital, and contain exactly the same information, except for an inversion. So, there can be
no common-mode or even harmonics on these. To have even harmonics on differential comparator outputs,
you would need a situation where sometimes Vo+ = Vo- and not its complement. This would imply
improper comparator operation. So I really wonder how one would see errors unless one was doing the
calculations incorrectly.
Could you elaborate your point further?
Regards
Vivek