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.phpit 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 ?