manodipan wrote on May 20th, 2008, 4:34am:Hi Guys,
I am facing some problem for offset compensation in my circuit.The circuit is explained below:
It is a switched capacitor circuit where the charge transfer is done by a comparator and current source.The sampling phase is same like conventional opamp based case,but in transfer phase the comparator decides when to start and stop the transfer.There are two phses of transfer namely coarse and fine phase.Now all the components are modelled in VerilogA.Now due to the finite delay of the comparator there is some offset at output because of finite time when comparator detects and some action is taken.Now if input is some pulse,for some current source setting(fixed) offset is same and can becompensated by properly adjusting the period of fine transfer.but when input is a sinusoid,offset changes along with signal and the system is to be compensated for this varying offset.The circuit is single ended and the compensation should preferably done by the same comparator.
Now we can't use output or input offset storage technique,because it requires an opamp...so i am puzzled about what to do????can u guys giv e me some idea to compensate for this circuit??Thanks in advance.
Looks to me like a basic problem when using this structure. I would try one of the two things:
1. Try to speed up the comparator. This runs into power and is also not easy since your scheme obviously depends on the comparator being able to realize a very large gain.
2. If possible, try to reduce the current of the current source. This is going to work when your input signal in moving relatively slowly, which it would seem is not the case since
you worry about the signal-dependent offset.
3. Alternatively, if the signal-dependent offset dominates over the fixed comparator offset, you can try to use a replica comparator which does the same thing as the main comparator and see
if you can devise some sort of cancellation scheme (I don't have the details in my mind but I imagine that it would be possible). The residual fixed offset and offset difference between the 2 comparators
should be harmless in comparison.
Hope this helps.
Regards
Vivek