Author Topic: Deciding cut off frequency for low/high pass filter  (Read 988 times)

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Offline Vindhyachal.taknikiTopic starter

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Deciding cut off frequency for low/high pass filter
« on: October 24, 2019, 12:03:22 pm »
1. I have a application in which need to get 10Hz-240Hz signal. So first made a 240hz LFP & then connected 10Hz HPF after that. Ckt is attached. Res/cap values are taken from TI workbench, nearest available will add later.
2. I was reading that when designing a filter, at fc gain is at 3db lower.
3. I need to measure 10Hz to 240Hz, so at my 10Hz signal with 3db lower & similarly 240Hz signal will be 3db lower.
4. So what is I want no gain loss at 10Hz or 240Hz.  How much lower frequency should I select than 10Hz, so that at 10hz there is no loss.
Similarly for 240Hz, how much higher frequency should i select so that there is no signal loss at 240Hz.

5. is there any general rull of thumb for this?
6. Similarly for sample signal at 240Hz with MCU adc, what is frequency for adc sampling to be selected. Generally I have select 10 times atleast. But is there any general rule of thumb for this also.
 

Offline pwlps

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Re: Deciding cut off frequency for low/high pass filter
« Reply #1 on: October 24, 2019, 01:23:17 pm »
I don't know if there is a rule but there are online Sallen-Key filter calculators able to output the Bode plot e.g.
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
so you can adjust your values for the shape you want.
 

Offline StillTrying

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Re: Deciding cut off frequency for low/high pass filter
« Reply #2 on: October 24, 2019, 01:26:08 pm »
As drawn that circuit needs + and - supplies.
I don't know any more about the analogue filters than you do. :) https://www.electronics-tutorials.ws/filter/second-order-filters.html
.  That took much longer than I thought it would.
 

Offline Audioguru again

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Re: Deciding cut off frequency for low/high pass filter
« Reply #3 on: October 24, 2019, 05:54:44 pm »
The second-order filters produce a simple 12dB per octave slope. But near the cutoff frequency there is a more gradual change.
Simply simulate the circuit to see how much extra frequency you need for the amount of level loss you can tolerate.

Most Sallen-Key second order Butterworth filters with a gain of 1 use same value parts except the feedback part is half/double the other part.

You said you want no loss at 10Hz and 240Hz then the bandpass will be enormous. I simulated it with a small amount of loss here:
EDIT: This simulation has an error with R2. The sim is corrected in my next post.
« Last Edit: October 28, 2019, 01:09:13 pm by Audioguru again »
 

Offline Vindhyachal.taknikiTopic starter

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Re: Deciding cut off frequency for low/high pass filter
« Reply #4 on: October 28, 2019, 04:58:18 am »
thanks for input 5-400hz gives desired results in my case
 

Offline Audioguru again

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Re: Deciding cut off frequency for low/high pass filter
« Reply #5 on: October 28, 2019, 01:07:17 pm »
Sorry, my simulation had an error (R2), here is the correct simulation:
 


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