Capacitive Sensor Output on nRF52 DK Depends on Power Source

[matomback]

I am working on a capacitive sensing application using the SAADC module and csense code on the nRF52 DK  (SDK v14.2). I use the recommended hardware configuration, with a charging output P0.26 connected to each pad through 100k resistors, and each pad wired to an ADC input (please see setup below).

I have all eight ADC pins responding to touch correctly when the board is powered via USB connection. However, when I switch to battery using the same hardware configuration (coin cell on DK, also LiPo on a custom board I am testing), I am unable to get a consistent (or barely any) response on any of the pins.

It appears that the change in capacitance from touch is dramatically reduced when on battery, and setting the threshold to lower values does not help to fix. Is there any workaround for this? Any information on running csense code from CR2032/LiPo instead of USB power? I found this thread that seemed to reference the same issue but no solution was provided: https://devzone.nordicsemi.com/f/nordic-q-a/31114/problem-of-capacitive-sensor-output-on-nrf52-dk-board 

Thank you

[awneil]

Probably because the USB is earthed (via the PC) - whereas the battery isn't ... ?

[matomback]

That's what I'm thinking.. so any idea if there's a way to get the application to work powered by a battery/wirelessly? It should be possible, as it is with other boards/libraries using the same kind of hardware configuration. Can't find anything on Nordic's support for this issue though.

[awneil]

matomback said:

Can't find anything on Nordic's support for this issue though

I guess it's not really to do with Nordic - it's a basic sensor design thing?

[Andreas]

Hi,

 

This might be a long shot, but would it at all help to add a ground fill around the sensor. As an attempt to increase the capacitive coupling between the finger and the local ground on the board.

 

Best regards,

Andreas

[matomback]

Hi Andreas,

Thanks for your response and sorry for the delay, things are very busy this time of year!

I was able to get it working and responding with normal sensitivity after including a ground plane beneath the sensors. I have seen other cap sensing designs with ground planes but had not seen anything on the Nordic documentation specifying this so I did not think to include at first.

Best,

Matt