Leakage through isolated linear supply causing MAX31855 error?

[ledtester]

I'm using a LM317 to heat a heatsink. Attached to the heatsink is a K-type thermocouple which is being monitored by a MAX31855 module (ebay variety) and a Teensy. The Teensy is powered via USB.

I didn't use a thermal insulating pad on the 317 -- after all, I want it to heat the heatsink. I've noticed that if I connect a lead from my linear power supply to the 317 (either positive or negative) the temperature readings are way off -- even with no power applied and the power supply switch off.

I know the MAX31855 needs to be isolated. I guess though that my isolated linear power supply isn't isolated enough. I'm wondering what exactly is going on.

[ledtester]

Well, I figured out something - apparently it was my ethernet cable.

Powering the laptop via mains vs. battery didn't make a difference, but the reading error went away once I unplugged my laptop's ethernet cable.