Hi,

The things you have seen is just what I have seen too. RC filters are not that successful in making Low Pass filters since they dont this nice sudden cutoff FQ.

these links will get you some help designing an active filter.

http://www.microchip.com/stellent/id...cName=en010007

http://www.national.com/appinfo/amps...h_filters.html

There are many Op-amps that can be used with a single 5V supply. This way you get an active filter with very sharp cutoff FQ.

/me