2nd order low pass filter

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gehan_s

Joined Sep 17, 2012
38
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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crutschow

Joined Mar 14, 2008
25,133
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. ;)
 

cabraham

Joined Oct 29, 2011
82
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.

Claude
 
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