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sallen key butterworth low pass filter
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Jan Audio:
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 ?
Audioguru:
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.
RandallMcRee:

--- Quote from: Jan Audio 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 ).

. . .

--- End quote ---

You probably want a Bessel lowpass for fastest settling if you are going to an ADC.
Conrad Hoffman:
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.
Jan Audio:
Thanks for the replys


--- Quote from: Conrad Hoffman on June 08, 2019, 04:58:05 pm ---You can do a 4-pole Bessel with a single opamp and all caps the same.

--- End quote ---

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
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