2nd order low pass filter

[gehan_s]

Hi all,

I was going through these two ways of implementing a second order low pass filter. One is a Sallen - Key configuration while the other is just two low pass filters connected in series. I simulated the AC analysis of both of them and got the same response. What are the advantages and disadvantages of the two?

Thanks in advance !!!!!!!!!!!!!!

 

[crutschow]

A Sallen-Key has feedback that allows a sharper rolloff at the corner frequency then two single-pole filters in series. Your Sallen-Key filter is not properly desired to do this (where did you get those component values?). If you change C3 in the Sallen-Key filter to 440nF you will see the increased sharpness in the rolloff point.

The advantage of two RC filters in series is that they are totally passive and don't necessarily require any active device for their operation. The Sallen-Key filter does require an amplifier.

An easy way to design active filters is with the free FilterPro program from Texas Instrument.