Active Low-Pass Filter
As its name suggests, a low-pass filter passes signals with frequencies lower than some specific value, called the cut-off frequency, but blocks passage of frequencies above the cut-off. Illustrated here is an active low pass filter having a 1000 Hz cut-off frequency. You can shift the cut-off by changing C1 and C2 together. To multiply the cut-off by a factor of N, multiply the capacitances of C1 and C2 by a factor of 1/N. For example, a 2000 Hz cut-off would require 0.005 uF capacitors, while a 500 Hz cut-off calls for 0.02 uF capacitors for C1 and C2. Drive the filter directly from the output of a preceding op-amp stage for best results.
Copyright by Bill Bytheway, K7TTY February 2012