Additional information on Bio-feedback module (Patreon)

Here is a photo and schematic PDF and a supplemental video of the common mode noise.

https://youtu.be/Wt5gPtadqM0 

Common mode noise is noise that flows from sources other than the GND line.

The linked video shows how noise increases when touching the wall. That noise is the 60 Hz household power frequency.

The household power supply noise flows from the TENS pad through the body and through the wall to the module.