I'm currently working on a project where I have to create a user control panel.
The goal is to use the TSC from ST to achieve user input with 9 touchkeys and a rotary wheel,
I read all the application notes and I understand the charge transfer principle, which we are going to use.
I want to add a driven shield, because this will reduce noise. and this is where I get stuck.
My first question:
In the AN4312 they say:
"The Cs/Cx of the shield should be in the same range as the Cs/Cx of the touchkeys"
Is this the same for the rotary wheel?
If so, How does this work? Because the wheel and the keys have different values for Cs.
My second question:
My PCB design wil feature a 4 layer stack-up. The bottom layer will be a ground plane.
What's the best solution:
- The driven shield in de 2nd or 3rd layer, between the gnd-plane and the touchkeys
- The driven shield around the touchkeys, like they do in the AN4312.
- A combination of the 2 above
Any help would be appreciated!