I've got one of those ECG boards off Ebay and am having a problem with mains hum appearing on the output. I can't understand where it's coming from (airborne in some form I guess) and therefore how to get rid of it. Initially I assumed the hum would disappear when the board was mounted in a screened metal box, but it hasn't made much improvement and the noise is very evident on the scope trace (see pictures). This *is* mains interference as it works out at exactly mains frequency. It's even there if I attach the DC input leads to a9V battery instead so it's not coming from the PSU. The links below show the noisy trace and the set-up as I've currently implemented it. I'm dealing here with high impedance, low level signals approximating to a dynamic mic in character I would imagine, so it's a PITA. Any suggestions?