High pass filter not attenuating signal at start of input
Any filter with such a low cutoff (as compared to the sample rate) will lots of time domaing ringing.
The best way of adressing this is to properly initialize the state of the filter using known properties of your system &amp; signal, but that's fairly tricky.
In your case, there may be a shit cut though: I'm guessing that you have a large bias or DC offset in your original signal. Just subtract the mean (or maybe the mean over the first samples or so, if the bias is drifting). This should subtantially reduce the ringing.