Value of data become near zero after filter

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This is my original data with green line showing the average of 30sets of data, blue line showing average plus five times std, black showing average minus five times std and red line showing bad data.

After I apply the butterworth highpass filter which I show the code below

[d,c] = butter(order,fcl / (fs / 2),'high');

dataout_h = filter(d,c,dataset);

datahigh = dataout_h;

The average data becomes nearly zero and does anyone know the reason?

[Adam]

Depends entirely on what order and fcl are I imagine.

Answer 1

[Ayham Zedan]

The answer is in your question. You are high filtering your data. In otherwords, removing low frequency components of your data.

The average of the data is basically the DC value of your data (non changing part), corresponding to a frequency of 0Hz. which is a part of the low frequency components removed by your high pass filter, it is actually the lowest "frequency" of your data.