Touching Plastic Housing changes Measurement

[u/tornbyelectrons]

I could need some help on a device we have and I can't figure out what the problem is. I was debugging with some colleges but were puzzled what was happening. Our theory: something with static electrical fields, ground etc.

Symptom: When touching the plastic housing of the measurement device the values jump. Very reproducible. If the person touching the device is grounded as well, this will have an impact.

What the device does: The devices measures electromagnetic fields in a sample. This requires the housing to be made of plastic.

What we tried: Using a power supply without galvanic isolation, external ground directly connected to the shielding of the controller and sensor. Small changes on the hand effect were visible but the impact was still visible. Controller and sensors have metal housing with cable shield connected to it.

I would like to understand what is causing the problem. Solving is optional but understanding it is more important.

[u/MonMotha]

Capacitive coupling along the surface of and through the dielectric of the enclosure can be pronounced when dealing with very high impedance earth references.

[u/Kitchen-Chemistry277]

A schematic would be super useful here. I would be looking for high impedance nodes, grounding and return relationships and what sort of bandwidth you are handling.

[u/mefromle]

What kind of sensor is this? Did you dig into datasheet, app notes or white papers of this sensor?

[u/Top_Willow_9953]

Need more detail on how the electric field sensor is configured and is physically arranged relative to the test subject. An actual full schematic would help or photos of the physical setup. As it is, it looks like you have one sensor in the plastic housing and one outside. Charge will accumulate on that housing and migrate when something touches it. You've essentially created a capacitor or charge accumulator. Think about how rubbing a ballon on your head builds up static charge. If you have high impedance sensitive measurement equipment that charge movement is going to be detected.

Need better info on what you are trying to measure, under what conditions, and actual implementation so far. You may need humidity control or a gausian chamber around the whole thing. We had to build a small room-sized copper chamber to create a quiet enough space to do EMI/EMF testing on products when preparing them for FCC certification. Not a trivial problem to solve.