Choosing RC values for Active LPF to match signal input impedance
Few pure voltage sources are to be found in real circuits. One often considers a voltage source to be accompanied by a source resistance \$ R_s\$. This resistance might be found in a sensor's data sheet.

Source resistance might also arise from output resistance of a previous stage. The model of this low-pass filter then becomes:







simulate this circuit &ndash; Schematic created using CircuitLab
This stage may be part of a chain. Its output resistance at Vout is usually considered to be very low, thanks to the op-amp. However, at higher frequencies where op-amp gain drops off, output resistance may not be so very low.