2nd order low pass filter

Thread Starter


Joined Sep 17, 2012
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 !!!!!!!!!!!!!!



Joined Mar 14, 2008
You really should post only in one forum initially and then wait a reasonable time for a reply before posting in another forum with the identical question. ;)


Joined Oct 29, 2011
I would recommend studying a good filter reference book, or if you look through the TI web site, they have app noted that would answer this question. If R1=R2, and C1=C2, then the Sallen-Key network isn't buying you much over a passive implementation except for isolation. With the passive version, R2-C2 loads down the R1-C1 stage, changing the attenuation slightly.

For the Sallen-Key approach, making the feedback capacitor larger than the grounded cap will produce a Q value that is larger. The passive approach limits the Q to around 0.25, but w/ Sallen-Key, Q values of 2, 3, 5 or even higher can be attained. If the feedback cap is raised to double original value, while the grounded cap is halved, the Q value will go to 1.0, as opposed to the present value of 0.50 (Sallen-Key). A good text explains this in detail.

In general the 2 networks are not the same, but if the Sallen-Key is not fully utilized, it's advantage over passive may be slight. Adjusting the Q value of the active network provides optimum results.