Author Topic: sallen key butterworth low pass filter  (Read 3050 times)

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Offline Jan AudioTopic starter

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sallen key butterworth low pass filter
« on: June 08, 2019, 01:57:03 pm »
Hi, i need a lowpass at 18K ( not above 20K ) for filtering audio before going into ADC.
I was thinking i try the sallen key butterworth for best results ( single supply ).

So i found some calculators online, you can fill a frequency and it says the parts.
The outcome is the C1 is double the C2 capacitor.
Is the result bad if the C1 is not double the C2 ?

Sofar i calculated the resistors should be 5K6, capacitor C1 2,2nf & C2 1,0nf ( available values ).

edit : i found this simulator http://sim.okawa-denshi.jp/en/OPstool.php
it gives fc = 19161.114418375[Hz] in total.

I also was looking for a 4th order filter, and all the sudden all 4 capacitors are different, why is that ?
Does 2 second order filters on a frequency give another frequency ?, that is new for me.
And these capacitor values they output are not available.

So how do i find out online what is the best 18K lowpass without scope FFT ?
« Last Edit: June 08, 2019, 02:15:30 pm by Jan Audio »
 

Offline Audioguru

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Re: sallen key butterworth low pass filter
« Reply #1 on: June 08, 2019, 02:44:56 pm »
If the gain of the opamp is 1 and the resistor values are the same then the feedback capacitor value MUST be double the value of the capacitor to ground to produce a Butterworth response and at the calculated cutoff frequency. Simply use two capacitors in parallel for the doubled value.

One second-order Butterworth filter has a cutoff frequency at -3dB (0.707 times the flat level) 0.707 times is half the power that sounds like only a little less level.
Two second order Butterworth filters in series double the amount of level reduction causing a muddy non-Butterworth response and a cutoff frequency lower than is calculated.
 

Offline RandallMcRee

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Re: sallen key butterworth low pass filter
« Reply #2 on: June 08, 2019, 04:40:36 pm »
Hi, i need a lowpass at 18K ( not above 20K ) for filtering audio before going into ADC.
I was thinking i try the sallen key butterworth for best results ( single supply ).

. . .

You probably want a Bessel lowpass for fastest settling if you are going to an ADC.
 

Offline Conrad Hoffman

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Re: sallen key butterworth low pass filter
« Reply #3 on: June 08, 2019, 04:58:05 pm »
You can do a 4-pole Bessel with a single opamp and all caps the same. First, you need to give how many dB down you can live with at a couple points around 18 kHz. You may find that even with 4 poles, the slope isn't enough. You'll either have too much getting through at 20 kHz, or do too much damage to the frequency response below 18 kHz.
 

Offline Jan AudioTopic starter

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Re: sallen key butterworth low pass filter
« Reply #4 on: June 08, 2019, 06:33:45 pm »
Thanks for the replys

You can do a 4-pole Bessel with a single opamp and all caps the same.

That would be great, only i can not find a schematic.


This site here gives the same schematic for bessel as the sallen key butterworth :
https://www.changpuak.ch/electronics/Bessel_Lowpass_active_24dB.php
 

Offline Benta

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Re: sallen key butterworth low pass filter
« Reply #5 on: June 08, 2019, 09:14:03 pm »
You can do a 4-pole Bessel with a single opamp and all caps the same. First, you need to give how many dB down you can live with at a couple points around 18 kHz. You may find that even with 4 poles, the slope isn't enough. You'll either have too much getting through at 20 kHz, or do too much damage to the frequency response below 18 kHz.

Please show this circuit/schematic. It would be of greatest interest to all here.
 

Offline Conrad Hoffman

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Re: sallen key butterworth low pass filter
« Reply #6 on: June 08, 2019, 10:00:40 pm »
I wrote a calculator for it here- http://www.conradhoffman.com/chsw.htm fifth item down. The design gets some criticism, but I've built many and they work well. You'll want to sim it in LTSpice or similar. I believe the Bessel filter (at least this one) is defined more by phase shift, so you'll probably enter a lower frequency than the -3dB target. Click on Bessel then start with the 18 kHz and try an ISF of 0.0555 to get all 1000 pF caps.
 

Offline Benta

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Re: sallen key butterworth low pass filter
« Reply #7 on: June 08, 2019, 10:11:45 pm »
I wrote a calculator for it here- http://www.conradhoffman.com/chsw.htm fifth item down. The design gets some criticism, but I've built many and they work well. You'll want to sim it in LTSpice or similar. I believe the Bessel filter (at least this one) is defined more by phase shift, so you'll probably enter a lower frequency than the -3dB target. Click on Bessel then start with the 18 kHz and try an ISF of 0.0555 to get all 1000 pF caps.

Sorry, but .zip files turn me off.
Apart from that, please provide calculations about what you're postulating. "I believe the..." etc. doesn't really suffice.

 

Offline MagicSmoker

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Re: sallen key butterworth low pass filter
« Reply #8 on: June 08, 2019, 10:51:18 pm »
...
This site here gives the same schematic for bessel as the sallen key butterworth :
https://www.changpuak.ch/electronics/Bessel_Lowpass_active_24dB.php

That's because the Bessel, Butterworth and Chebyshev* all use the same schematic, it is the ratio of the RC networks that determines the response.

Generally speaking, for audio applications you want the Bessel or Butterworth response; avoid the Chebyshev and Elliptic.


* - but not the Elliptic (aka Cauer).

 

Offline floobydust

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Re: sallen key butterworth low pass filter
« Reply #9 on: June 09, 2019, 01:36:34 am »
OP are sure you don't need a brickwall filter? Slope from 18kHz to 20kHz sound very steep.
 

Offline MarkF

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Re: sallen key butterworth low pass filter
« Reply #10 on: June 09, 2019, 03:59:16 am »
If you want a LC Filter Design on-line calculator, I use this one.  Butterworth and Chebyshev series or shunt calculations.

   https://www-users.cs.york.ac.uk/~fisher/lcfilter/
 

Offline soldar

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Re: sallen key butterworth low pass filter
« Reply #11 on: June 09, 2019, 09:44:52 am »
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Conrad Hoffman

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Re: sallen key butterworth low pass filter
« Reply #12 on: June 09, 2019, 07:36:11 pm »
I wrote a calculator for it here- http://www.conradhoffman.com/chsw.htm fifth item down. The design gets some criticism, but I've built many and they work well. You'll want to sim it in LTSpice or similar. I believe the Bessel filter (at least this one) is defined more by phase shift, so you'll probably enter a lower frequency than the -3dB target. Click on Bessel then start with the 18 kHz and try an ISF of 0.0555 to get all 1000 pF caps.

Sorry, but .zip files turn me off.
Apart from that, please provide calculations about what you're postulating. "I believe the..." etc. doesn't really suffice.

Can't really help you then, but you can go to the source I used- Function Circuits: Design and Applications, 1976, part of the Burr-Brown electronics series. Out of print, but commonly available used.
 

Offline Benta

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Re: sallen key butterworth low pass filter
« Reply #13 on: June 09, 2019, 10:38:21 pm »
If you can't work without .zip files, fine, that's your karma.
But does the link provided by soldar represent the topology you're talking about?

 

Offline Conrad Hoffman

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Re: sallen key butterworth low pass filter
« Reply #14 on: June 10, 2019, 12:27:19 am »
If you can't work without .zip files, fine, that's your karma.
But does the link provided by soldar represent the topology you're talking about?

Yes, same filter, but with his addition of damping. The reason I use the zip files is because I can't just put up an exe file. Well, I can, but most people couldn't (and shouldn't) download it. The math for the filter is unwieldy, so a numerical solution is used. Either way, it's not something you'd want to do by hand.
 

Offline Jan AudioTopic starter

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Re: sallen key butterworth low pass filter
« Reply #15 on: June 10, 2019, 11:26:43 am »
I do not know if it helps but I found this: https://consort3.wordpress.com/2018/06/10/4th-order-single-op-amp-low-pass-filter/

That is exactly what i need, thank you.

OP are sure you don't need a brickwall filter? Slope from 18kHz to 20kHz sound very steep.

Yes i would like to have it, do you know a schematic ?

That's because the Bessel, Butterworth and Chebyshev* all use the same schematic, it is the ratio of the RC networks that determines the response.

Oh, i normally always change resistors and capacitors from original schematics, can i add my name to it now ?, funny.
 


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