Help me understand touchRead grounding requirements?

[amowry]

I'm making a USB MIDI device with capacitive touchpads. If I'm NOT grounded to the chassis it works great under the following conditions: It's plugged into a PC or it's plugged into an ipad with or without headphones. However, if it's plugged into an ipad and the ipad is connected via the headphone jack to amplified computer speakers or stereo system that is plugged into mains with a two-prong cord (i.e. not grounded), then I must ground myself to the chassis in order for the touchpads to work reliably. Could someone explain to me why this is? It's not a big deal for my to ground myself to the chassis, but I'd like to understand it better.

If it helps, my device has springs on the PCB that touch foil dots (the sensors) on the inside of the plastic exclosure. The pads for the springs are surrounded by ground hatch and have ground hatch under them on the opposite layer.

Thanks for your help!

 

[Constantin]

Leakage from the power supply leading to interference, is my guess. Grounded power supplies are available.

I notice this too if I'm using a laptop with a well grounded floor in the EU. A two prong PS leads to "vibrations" if you touch the chassis while a proper three-pin grounded plug results in minimal/no leakage currents.

 

[amowry]

Thank you, that sounds similar to what I'm getting--there are rapid oscillations in the touchpad readings, like the speakers are injecting noise back into the ipad through the common ground.