Deciding cut off frequency for low/high pass filter

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Vindhyachal Takniki

Joined Nov 3, 2014
487
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.

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SLK001

Joined Nov 29, 2011
1,543
Your requirement #4 means that no filter can be used. In other words, how much loss are you willing to suffer.

Also, what makes you think that we will know what your filters requirement should be? What OTHER signals are in your environment that you DON'T want to pass thru? How low should THEY be? If there are no other signals, then why do you need a filter?

If you want me to look at your schematic, post them as an image, not as a file that I have to download and open and face a security risk.
 

crutschow

Joined Mar 14, 2008
24,329
With a filter you will have to accept some small signal loss near the rolloff frequencies, so you have to decide how much you can tolerate (e.g. 0.1dB, 0.01dB, or ...).

And the higher the filter order (number of poles) the faster the rolloff.
For example, you can add an extra R and C at the input of the standard 2-pole Sallen-Key to make a 3-pole active filter, as shown here.
 
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PeteHL

Joined Dec 17, 2014
298
If you make the cut-off frequencies equal to 10 Hz and 240 Hz as you so far have done, then you are allowing 10 Hz and 240 Hz signals to pass, but as you say attenuated -3 dB. If you alternately make the cut-off frequencies equal to 5 Hz and 480 Hz, then 5 Hz and 480 Hz are passed by your band-pass filter which is contrary to what you say you need. Why is it important that the level of the 10 Hz and 240 Hz signals be exactly equal to that of all of the other frequency signals passed by your band-pass filter?

No matter what you do, you are going to be faced with this dilemma as there does not exist a filter that for example will not attenuate a 10 Hz signal and attenuate a 9 Hz signal by perhaps -10 dB.
 

Audioguru again

Joined Oct 21, 2019
872
I answered with this Sallen-Key second order Butterworth bandpass filter on the other website:
Sorry, I had the highpass filter with wrong values before. It is fixed now.
 

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