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Designing a driven shield with touchkeys and a rotary wheel

Question asked by gerben de haan on Nov 15, 2017

Hello,

 

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!

 

Thanks,

Gerben

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