Thanks vivek and Tlaloc for your answers...
vivkr wrote on Sep 23rd, 2008, 11:27pm:In principle, this should function as a lowpass filter. I remember having had to build an analog moving average filter to reduce white noise in input samples, but cannot remember any details, as I was still a greenhorn then. However, we allowed good settling at each sample.
If in this case you allowed for good settling therefore the LPF action is very light, almost approaching the ideal T&H circuit where full settling is allowed. I'm actually looking for the opposite: a heavy filtering action.
vivkr wrote on Sep 23rd, 2008, 11:27pm:You will surely have plenty of distortion in your case, since you are sizing your switch in a such a way that it has too large an Ron, or to put it another way, your RC time constant is too large, and the R is nonlinear (since a switch is used).
Why not use a passive, linear R and a C for your filter. It would appear to me that the sampling part is anyway redundant since you are relying on averaging across multiple cycles.
I'm actually thinking of using a very low Ron switch in series with a linear R and C (option 3 in the attached figure), in such a way the LPF time constant is set by linear components, and the switch just provides the holding action, which I require in my application.
In fact I´m looking to not increase the base band white noise power and at the same time hold the incoming signal every Tck. Therefore I need to provide some anti-aliasing means before sampling and I was wondering if merging the LPF (supposedly for preventing aliasing) and the sampling circuit would fulfill both requirements...
Obviously having a simple LPF and the T&H circuit with a buffer in the middle would do it (option 1 in the attached figure), but the whole point is about saving the buffer's area and merging both the filter and the T&H circuit....
vivkr wrote on Sep 23rd, 2008, 11:27pm:As for white noise, the basic truth remains - total white noise power integrated = (kT/C).
I agree, but my problem is not the total integrated white noise, but rather how that power ends up distributed over frequency....
[/quote]
Or you could use a switched cap R into a C. Then your pole would be more stable across process and you wouldn't have a DC voltage drop.
[/quote]
Tlaloc:
Using a switched cap is not an option since - as I described above - I 'd like to provide some antialiasing means and sample the signal in the same S&H circuit....if this were possible... that´s my question :-/
Finally,
I think option 2 does provide antialiasing means before sampling the signal. Although the time constant of the filter changes during the sampling time (Ch in parallel with C1), the anti- aliasing action takes place....so, can this option also be reduced to option 3?
Many thanks
Tosei