Problem using bandpass IIR filter

Hello, I am trying to use a bandpass IIR filter to filter by EEG data which is collected at a sampling rate of 1000. I am using the following code to do it, bpfilter = designfilt('bandpassiir', 'StopbandFrequency1',5,'PassbandFrequency1', 8, 'PassbandFrequency2', 35,'StopbandFrequency2', 40, 'StopbandAttenuation1', 60, 'PassbandRipple', 1, 'StopbandAttenuation2', 60, 'SampleRate', 1000);

But after filtering and plotting the spectrum I am getting a straight line parallel to the x-axis and zero amplitude. Whereas when I used FIR, I obtained a good spectrum. It is just that since my amplitudes matter hence I don't want to use an FIR filter due to its undulations in bandpass frequencies.

Answer 1

[Star Strider]

I would normally suggest using the filtfilt function, however the filters returned by designfilt appear to be more appropriate to the filter function (as the documentation recommends). That appeared to work well when I used it with a signal I synthesized to test it.

Try this:

FilteredEKG = filter(bpfilter, EKG);

with appropriate changes to use your variables.

[Ankit Roy]

Thank You Star Strider for the answer. Actually it turned out that I was doing something silly elsewhere. Otherwise it is fine.

[Star Strider]

As always, my pleasure.