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Removing 60Hz from ECG using digital band stop filter

2015-01-12 16:20  
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Let’s make a filter, which filters off the 60Hz frequency from ECG signal. As we know American power supply is 60Hz. This is common noise in biomedical signals, while they are powered from industrial power supply. This type of noise can be defined easily and can be filtered as parameters of noise are known.
Here is one example of how to implement FIR filter using mathematical tools, like Matlab. This can be done by using microcontroller, like ARM or even ARM, because the frequencies are up to 1 KHz.
Initial conditions:

f0=60Hz – pover supply frequency;
fs=500Hz – sampling rate;
frequencies who define complex zeros:

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we get w0=0.754;
Positions of complex zeros:

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Zeros and poles in z plane

System Function

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From it we can calculate filter coefficients:

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And filter coefficients:

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Also we know that:

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And here we get filter characteristics:

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We have band stop filter at 60Hz and its jam at 60Hz is -300dB. Bellow is filter structure:

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Now using this filter we can filter ECG signal:

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As you can see this is simple FIR filter. In other words there is nothing more than average function which doesn’t need much of resources. The other benefit of FIR filter is that we can get linear Phase characteristics, there fore filtered signal isn’t distorted because of different spectra frequency delay. As result wee see that ECG become more smooth. FIR filter


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