4-pole single opamp lopass filters - diyAudio
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Old 16th October 2010, 09:53 PM   #1
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA
Default 4-pole single opamp lopass filters

I've always been pleased at how well 4-pole filters work, especially for noise reduction and such. It's even nicer if you can do a single opamp version. The math for it is a PITA, though an excellent numeric solution was published by Burr-Brown in their old, and out of print, Function Circuits book, albeit in Fortran. Here's my minimalist conversion that hopefully will be of use to somebody. It's the 4th item down.
I may be barking up the wrong tree, but at least I'm barking!
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Old 16th October 2010, 10:13 PM   #2
jcx is online now jcx  United States
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sensitivity to component tolerance is usually poor when you try to pile on poles, Q

even cascaded Sallen-Key biquad can become a problem holding tolerance in the higher Q's sections of high order filters

3rd order with passive RC in front of a biquad section is probably the best compromise with the input RC knocking down RF before the op amp input - the multiple feedback low pass biquad is esp good if signal inversion is OK
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Old 17th October 2010, 12:27 AM   #3
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Location: Canandaigua, NY USA
Certainly a valid concern, though I'd say it's an issue building any high order filter, regardless of topology, at least if one stays in the analog domain. Once the values are calculated it would be good advice to run some simulations and see what happens as components vary. IMO, getting close tolerance components isn't too difficult, or one can just measure. Where I'd really want to pay attention is if two high order filters had to sum correctly, say for a crossover.
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Old 2nd November 2011, 07:52 PM   #4
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Join Date: Mar 2010
Default Higher order crossovers

Dear Conrad and jcx -

You are both correct above re: component sensitivity. Douglas Self's "The Design of Active Crossovers" (out recently) is masterful in it's coverage of the above and so much more. Highly recommended reading.

Best, Speakerman
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Old 2nd November 2011, 07:59 PM   #5
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Most of the time resistors an capacitors of "usual" tolerance from the same lot all measure very close, so it's not too bad to compensate so long as you know what the actual values are...
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Old 2nd November 2011, 08:18 PM   #6
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I have a small range of 0.3% and fewer 0.5% capacitors to use as references.
How would I minimise errors when used as a reference to calibrate a capacitance meter?
Temperature, humidity, handling, RF & other interferences? What else would/could have an effect on repeatability and absolute accuracy?

Would 1% absolute accuracy be within reach?
Would 1% absolute accuracy be sufficient for the higher Q filter that we are being told may be "off" by quite a bit?
regards Andrew T.
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