I have a doubt related to the PCB layout for a capacitance sensor stretchable PCB, that needs to be connected to a separate PCB provided with AD7147 for capacitance acquisition.
The sensor board is a TPU 2 layers stretchable PCB. A simple graphical representation of sensors, traces and AC Shield is given in the image below.
The structure followed for the AC Shield is the one suggested in the AD7147 PCB Guidelines technical document (apart from the fact that the AC Shield is not a plane but it replicates the trace shape (only larger)):
"If there is no space on the sensor PCB to allow routing around the sensors, use an ACSHIELD plane layer directly under the sensor layer."
My problem is the presence of noise when I touch the sensor traces. In the graph below you can note 2 peacks in the capacitance values:
- the first one (higher) appears when I touch the sensors (expected and correct)
- the second one (lower) appears when the sensor traces are touched (undesired)
My question is:
how is it possible to avoid the noise related to traces touch? Is it only a matter of improving the shielding? In that case, which is the correct way? Otherwise, what am I missing?
I was thinking of adding a dielectric film on top, with GND circle pads again on top (since my application does not contemplate the use of finger), could this be a possible structure? IDoes this have a relationship with the noise issue?
Thank you very much,