2nd order low pass filter

Discussion in 'General Electronics Chat' started by gehan_s, Apr 15, 2013.

  1. gehan_s

    Thread Starter Active Member

    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 !!!!!!!!!!!!!!
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  2. crutschow


    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. ;)
  3. cabraham

    Active Member

    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.