Cascade Sallen Key Filters

[panfilero]

I was wondering, if you cascade two identical low pass sallen key filters one after another, do you keep the same same cut-off frequency of the singlesallen key filter? I'm thinking yes, that you just increase your poles, from a 2 pole ystems to a 4 pole system, but I'm not sure, I used the following website to come up with my single stage, at 140Hz....

http://sim.okawa-denshi.jp/en/OPstool.php


much thanks!

 

[Wimpie]

El 18-07-13 20:34, panfilero escribió:

I was wondering, if you cascade two identical low pass sallen key filters one after another, do you keep the same same cut-off frequency of the single sallen key filter? I'm thinking yes, that you just increase your poles, from a 2 pole ystems to a 4 pole system, but I'm not sure, I used the following website to come up with my single stage, at 140Hz....

http://sim.okawa-denshi.jp/en/OPstool.php


much thanks!

Hello,

The overall -3 dB frequency for the two-stage will be the -1.5 dB
frequency of a single stage (you add two).

If you want the same overall -3 dB (or other value) bandwidth, you
need to displace the frequencies somewhat (one down and one up). You
will get better passband flatness and better out of band rejection.

 

[George Herold]

I was wondering, if you cascade two identical low pass sallen key filtersone after another, do you keep the same same cut-off frequency of the single sallen key filter? I'm thinking yes, that you just increase your poles,from a 2 pole ystems to a 4 pole system, but I'm not sure, I used the following website to come up with my single stage, at 140Hz....



http://sim.okawa-denshi.jp/en/OPstool.php





much thanks!

What everyone else said.
If you have H&H's Art of Elec. then there is a small table in the filters section that gives you some of the 'Gains' and frequencies for various filter types.

George H.