sallen key butterworth low pass filter - Page 1
Forum section: electronics / projects

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 08, 2019, 01:57:03 pm
content: 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 simulatorhttp://sim.okawa-denshi.jp/en/OPstool.phpit gives fc = 19161.114418375 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 ?

author: Audioguru
author_rank: Super Contributor
timestamp: June 08, 2019, 02:44:56 pm
content: 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.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 08, 2019, 04:40:36 pm
content: Quote from: Jan Audio on June 08, 2019, 01:57:03 pmYou probably want aBessellowpass for fastest settling if you are going to an ADC.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 08, 2019, 04:58:05 pm
content: 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.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 08, 2019, 06:33:45 pm
content: Thanks for the replysQuote from: Conrad Hoffman on June 08, 2019, 04:58:05 pmThat 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

author: Benta
author_rank: Super Contributor
timestamp: June 08, 2019, 09:14:03 pm
content: Quote from: Conrad Hoffman on June 08, 2019, 04:58:05 pmPlease show this circuit/schematic. It would be of greatest interest to all here.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 08, 2019, 10:00:40 pm
content: I wrote a calculator for it here-http://www.conradhoffman.com/chsw.htmfifth 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.

author: Benta
author_rank: Super Contributor
timestamp: June 08, 2019, 10:11:45 pm
content: Quote from: Conrad Hoffman on June 08, 2019, 10:00:40 pmSorry, 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.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 08, 2019, 10:51:18 pm
content: Quote from: Jan Audio on June 08, 2019, 06:33:45 pmThat'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).

author: floobydust
author_rank: Super Contributor
timestamp: June 09, 2019, 01:36:34 am
content: OP are sure you don't need a brickwall filter? Slope from 18kHz to 20kHz sound very steep.

author: MarkF
author_rank: Super Contributor
timestamp: June 09, 2019, 03:59:16 am
content: 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/

author: soldar
author_rank: Super Contributor
timestamp: June 09, 2019, 09:44:52 am
content: 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/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 09, 2019, 07:36:11 pm
content: Quote from: Benta on June 08, 2019, 10:11:45 pmCan'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.

author: Benta
author_rank: Super Contributor
timestamp: June 09, 2019, 10:38:21 pm
content: 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?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 10, 2019, 12:27:19 am
content: Quote from: Benta on June 09, 2019, 10:38:21 pmYes, 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.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 10, 2019, 11:26:43 am
content: Quote from: soldar on June 09, 2019, 09:44:52 amThat is exactly what i need, thank you.Quote from: floobydust on June 09, 2019, 01:36:34 amYes i would like to have it, do you know a schematic ?Quote from: MagicSmoker on June 08, 2019, 10:51:18 pmOh, i normally always change resistors and capacitors from original schematics, can i add my name to it now ?, funny.