Notch Filter for 60 Hz about ecg-acquisition-classification HOT 1 OPEN

Hi AGG2017,

Looking at the code, we can calculate the coefficients of the notch filter for power lines at 60Hz, assuming a sampling frequency of 500Hz (from arduino.ino).

H(z) = (z-1)/(z-a) where a is within 0-1, the closer a is to 1 the tighter the notch.
to normalize DC unity we multiply H(z) by (1+a)/2
our wn is fo/fc = (60/500)2pi = 0.75398
rotating poles and zeros of z and plugging into H(z)
with a = 0.99 (close to 1 as we want a tight notch since we only want to get rid of 60Hz)
and wn = 0.75398
H(z) = ((1+a)/2) ((z^2) - 2zcos(wn) +1)/ ( (z^2) - 2azcos(wn) + (a^2))
H(z) = (0.995 - 1.999z^(-1) + 0.995z^(-2) ) / (1- 1.979z^(-1) + 0.9801z^(-2))

Solving the coefficients using inverse z transform

y[n] - 1.979 y[n-1] + 0.9801 y[n-2] = 0.995 x[n] - 1.999 x[n-1] + 0.995 x[n-2]

b coeff = 0.995, -1.999, 0.995
a coeff = 1, -1.979, 0.9801

I would try:

y = 0.995 * x - 1.999* x1 + 0.995 * x2 + 1.979 * y1 - 0.9801 * y2;

for a 60Hz notch filter

from ecg-acquisition-classification.