# LowPass Filter coefficient calculation

Question asked by Alvis on Mar 11, 2018
Latest reply on Mar 18, 2018 by Alvis

Hi all,

I am checking the Low Pass Filter coefficient to use on my micro-controller,

I have researched some topic, like https://ez.analog.com/message/21166?commentID=21166#comment-21166

I use the excel to verify my formula is correct or not (check with the SigmaStudio compiler window).

I can not get the correct A1 and A2 value but I can get the correct B0, B1 and B2, so I don't know where is wrong.

like this example:

excel:

Fs: 48000

f0: 1000Hz

Gain: 0

Q: 1.41

boost: 0

and the

omega(w0): (2 * Pi() * f0 ) / Fs = 0.130899694

sin(w0): 0.130526192

cos(w0): 0.991444861

alpha( this is from the SigmaStudio Filter Wiki):

alpha = sin(w0) / (2 * Q) = 0.046285884

like below pic:

from the above calculation,

I can get the similar B0/B1/B2:

SigmaStudio:

from mine excel:

but to check the A1 and A2, the value is not correct, I follow the Wiki formula and the reference topic,

so I am not sure which is the correct answer. I don't know my formula is wrong or not.

Here is mine A1 and A2 formula:

A1 =  -2 * cos(w0) = -1.982889723

A2 = 1 - alpha = 0.953714116

from the SigmaStudio compiler window A1 and A2 value:

A1= 1.8951700925827

A2 = -0.911523461341858

from the above I can sure is completely different.