Weird charge/discharge with multimeter

Community PCB Design
1

Hi all,

I was probing a custom PCB and simultaneously had it hooked to an oscilloscope and noticed some weird behavior. When I touch the first multimeter probe to any pin, I see an oscillation (always drop in voltage first) on the oscilloscope [photo below]. Thought this was weird behavior, so I tried it on a sparkfun Redboard Artemis – same thing. Then I tried simply touching the board with the multimeter probe not attached to the multimeter–same result. These boards were hooked up first to power supply, then to battery.

What exactly is this, and how can I mitigate it? I would expect ESD to have a positive voltage spike. Interestingly, if I touch exposed metal on the PCB a second time, the oscillation does not occur, leading me to think there is some sort of charging going on…

Thank you for the insights–lots to learn!

IMG_2037.JPG
IMG_2037.JPG4032×3024 2.97 MB

2

If the PIn that is not controlled/set as INPUT or OUTPUT you are measuring just some high impedance leakage voltage value.

3

derekp:
These boards were hooked up first to power supply, then to battery.

Is the board connected to the power supply or to the battery when you see this? It looks like there is some energy discharge going on which could be caused by having the board not grounded to the same reference as the probe you are using. It could also be static discharge, but the perfect oscillatory nature would look more like a lightning waveform (type 3) - in any case; it looks like you have different ground potentials.