4th Order Butterworth Filter - Page 1
Forum section: electronics / beginners

author: Bob McCloy
author_rank: Newbie
timestamp: July 08, 2019, 09:41:41 am
content: Hi, I am currently trying to design a low pass, 4th order butterworth filter with a -3dB frequency of around 4kHz. The overall circuit is closely based on this open source design...http://microfluidics.utoronto.ca/gitlab/dstat/dstat-hardwareHere is an image their 4th Order Butterworth filter section:To get the values I chose to use the Analog Devices filter wizard available here:https://www.analog.com/designtools/en/filterwizard/And this is the circuit design and values it gave me:I have a question regarding the 1kΩ resistors connected to the inverting input of both opamps used in the open source design, that are not present in the Analog Devices design, from my understanding the input impedance of an opamp is already very large so wouldn't these 1kΩ resistors be insignificant?Also, I would like to do the circuit analysis of the 4th order butterworth filter to get the frequency response and confirm that the values provided by Analog Devices filter wizard do work, am I correct in my method that I could convert the systems to their Leplacian equivalents and then treat each 2nd Order Filter as a seperate system, work out thier transfer functions, and then multiply those two 2nd order transfer functions together to get the overall 4th order equation as a function of the complex variable 's'. Then replace 's' with 'jω' to get the frequency response?Thank you in advance!

author: fcb
author_rank: Super Contributor
timestamp: July 08, 2019, 10:55:09 am
content: The MAX5443 won't drive the lower loads of your filter without some distortion effects. Use a buffer or increase the impedance of your first filter section. Low value resistors like you have spec'd will also waste power without significant benefits to your noise floor.The 1K's on the MAX4477 inverting inputs have several effects, they could be there to damp out spurious oscillations and/or reduce DC offset (as this is DC coupled).

author: MarkF
author_rank: Super Contributor
timestamp: July 08, 2019, 03:48:07 pm
content: I can not help you with the active solution.But if you want to try a passive solution, I use this on-line tool:https://www-users.cs.york.ac.uk/~fisher/lcfilter/

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 08, 2019, 05:08:50 pm
content: QuoteCorrect. It is indeed that straightforward.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2019, 01:57:45 am
content: Like fcb said, some operational amplifiers misbehave if their inverting input is driven with a low impedance. In this case though, the 1k series resistors are not there for bias current compensation because they do not match the resistance seen at the non-inverting inputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2019, 12:57:51 pm
content: Use this resource:http://sim.okawa-denshi.jp/en/Fkeisan.htmYou will need to go off the path slightly, to design a higher order filter. If 3rd order is satisfactory for your purposes, you can eliminate an op-amp, and use the solution given (prefer MFB, by the way -- less sensitive to component values than SK).Tim

author: Bob McCloy
author_rank: Newbie
timestamp: July 16, 2019, 02:17:45 pm
content: Sorry for the late reply,Thank you all for the information. I appreciate the help. I'll go away and look over these resources and see how I go.Thanks again!

author: Bob McCloy
author_rank: Newbie
timestamp: July 16, 2019, 02:27:39 pm
content: Quote from: MarkF on July 08, 2019, 03:48:07 pmHa thats cool. A University of York link. I am currently studying Electronics there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2019, 02:45:37 pm
content: Cool! Make sure they know to keep that account active, it's a useful resource!Tim