[Openeeg-list] Besselworth calculations

[Stephen M. <ste...@gm...>]

Hello,

I'm not sure if this project is still ongoing but I'm going to post and
wish for the best.

I am attempting to build an Auditory Brainstem Response (ABR) amplifier.
The circuit is very much the same as the designs for the "modularEEG"
except that the filters are different (I want 300-3kHz while the current
designs are mostly <100 Hz). This would mainly involve modifying the
"besselworth filter", see screenshot below.

My question is how was the cutoff frequency of 59Hz calculated? The
equations I've found for this are Fc = 1/(2*pi*sqrt(R232*R288*C234*C231))
which equals 152.5 Hz and the gain calculation Gain = (1+R230/R229) = 13.19x

Could someone help explain how these numbers were calculated in the
schematic? I understand that the 3rd pole of this filter is on the digital
board but how would that play into the calculations?

Thank you,
Steve Mci

[image: image.png]