Olimex EKG/EMG Lead II measurement looks OK

The Lead II wave looks OK.  I zoomed in to check out the 40Hz oscillation and found that it was actually not 40Hz, but 50Hz line frequency!

If it is indeed line frequency noise, then the problem can easily be solved, by using software or hardware 50 Hz band notch filter.  But I have no experience in using band notch filter.  So I need to google again.

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