I was wondering, if you cascade two identical low pass sallen key filters o ne 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, f rom a 2 pole ystems to a 4 pole system, but I'm not sure, I used the follow ing website to come up with my single stage, at 140Hz....
much thanks!
Let me see if I can write this in ASCII (AND IIRC :-)...
The "shrinkage" (i.e. equivalent bandwidth) of cascaded identical filters is...
S = sqrt(2^1/N - 1)
So two identical filters (N=2), gives...
S = 0.643594
Or in numbers, two 140Hz (-3dB) cascaded filters with have a 3dB corner of 90.1Hz
(But, If you want the cascade to be Butterworth, you have to stagger the pole locations.) ...Jim Thompson
El 18-07-13 20:34, panfilero escribió:
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.
No, at the previous -3 dB point, you now have -6 dB.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
You want to do more than just displace the frequencies. You want to increase one and make it less damped, and decrease the other and make it more damped.
Better yet, you want to choose poles for the new filters based on whether you want the overall response to be Butterworth, Chebychev, Gaussian, or whatever.
one after another, do you keep the same same cut-off frequency of the sing le 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 foll owing website to come up with my single stage, at 140Hz....
What everyone else said. If you have H&H's Art of Elec. then there is a small table in the filters s ection that gives you some of the 'Gains' and frequencies for various filte r types.
George H.
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