Touch Sense Design with Driven Shield

hamdi_esen · ‎2013-08-20

Posted on August 20, 2013 at 09:54

Hello,

I need to design a 20 pin capacitive touch board and I am planning to add a driven shield. But it seems that there is no detailed design guidelines about the topic. I could use some help at least about key subjects like design constraints.

I use STM32F051 Cortex-M0 microcontroller by the way.

Thanks, Hamdi.

Thierry GUILHOT · ‎2013-08-20

Posted on August 20, 2013 at 15:00

Hi,

The driven shield or active shield is a specific electrode which is surrounding the normal touchkey electrodes and sensing tracks. It optimizes the touchkey sensitivity and improve the touchkey immunity to water by reducing the corresponding touchkey parasitic capacitance to ground. Practically, the driven shield electrode is connected to a GPIO of an analog I/O group which is dedicated to the shield. The second GPIO of this goup is connected to the shield sampling capacitor. The driven shield electrode is designed either with a plain or hatched plane using a clearance distance with the touchkey electrode of 0.5mm to 1mm. We also recommend to use a clearance distance to ground of 1mm minimum to limit the corresponding parasitic capacitance.

The shield sampling capacitor and serial resistor must be adjusted to ensure that the driven shield charge curve is as close as possible to the ones of the touchkey electrodes it is shielding. A good starting point is to consider that the The Cs/Cx of the driven shield should be in the same range as the Cs/Cx of the touchkey electrodes. Using Csshield = k.Cskey usually gives good results. Note that the Cs of the driven shield does not need to be a high grade capacitor. Any type should work. The Rsshield should be in the range of Rskey/k.

Several examples of a driven shield are available on the evaluation boards offering a touch sensing function. For instance, you can refer to the STM32303C-EVAL (MB1019).

Below is also detailed an implementation:

Best Regards,

Thierry,

hamdi_esen · ‎2013-08-20

Posted on August 20, 2013 at 16:49

Hi Thierry

,

Thanks for the detailed explanation. I have to ask one more question;

I need to use a ground plane on the bottom side to provide some noise immunity. Is it ok if there is ground plane underneath the touchkeys, or should I cut out the plane so that ground plane is only placed where there is no touchkey on top? And what would be the effect when the ground is hatched or not?

Best regards,

Hamdi

Thierry GUILHOT · ‎2013-08-20

Posted on August 20, 2013 at 18:07

Hi Hamdi,

It is not recommended to place a ground plane underneath the touchkeys as it will reduce their sensitivity. In your case, I would rather suggest to place a driven shield plane on the bottom layer, underneath the touchkeys and around them to move away the ground plane. The driven shield will improve the noise immunity without degrading the touch sensitivity.

If you do not have the choice, you can place a hatched ground plane underneath the touchkey with a 10% copper mesh. This will minimize the loss of sensitivity. To get some more design guidelines, please refer to AN2869.

Best Regards,

Thierry,

gates.nathan · ‎2018-09-11

Hello Thierry and others

Could you clarify 2 things for me.

Is it best to cover the board on both sides with an active shield if its possible? So beneath the electrodes/sensors and their racks, as well as surrounding the tracks and sensors/electrodes?
Should the active shield be a solid plane, or hatch? And what clearance should there be between electrodes and the active shield?

Thank you

Nathan