Butterworth notch filter numerical errors

I recorded my ECG with ADS1298 and loaded it using matlab to do post-processing.

However, I found a strange pheonomenon.

I used 60 Hz notch filter to reduce 60 Hz noise, but the filtered signal was distorted as shown below figure.

I think the 60 Hz componet did not reduce in that point, what is the reason? and how can I solve it?

My matlab code and raw data was attached.

Thanks.

Answer 1

[Kaashyap Pappu]

Hi,

I believe your filter is behaving as expected. You could increase the stop band attenuation further if you want to reduce the component of the 60Hz signal. Using pwelch on your raw and filtered data will show the difference before and after filtering, which is the suppression of the 60Hz component.

You could use this example to achieve the same. Assign to the variable ‘openloop’ your raw data, and change the sampling frequency to your sampling frequency. The remaining program will carry out the required processing.

Hope this helps!