AN2869 datasheet(24/43 Pages) STMICROELECTRONICS

24 / 43 page

Surface electrode design

AN2869

24/43

Doc ID 15298 Rev 6

3.5

Specific recommendations

3.5.1

LEDs and sensors

In some cases, a hole needs to be inserted in the sensor electrode to create a back-lighting

touchkey (see Figure 17). This is a very common solution which does not involve a

sensitivity dip in the middle of the sensor electrode as the electric field tends to close over

above the hole. As the sensor area decreases, there is a corresponding decrease in

sensitivity.

Figure 17.

Back-lighting touchkey

3.5.2

Driven shield

The principle of a driven shield is to drive the shield plane with the same signal as the

electrode.

There are several advantages to using a driven shield instead of a grounded shield:

●

The parasitic capacitance between the electrode and the shield no longer needs to be

charged. This cancels the effect on the sensitivity.

●

A driven shield is useful for certain applications where shielding may be required to:

–

Protect the touch electrodes from a noise source

–

Remove touch sensitivity from the cable or track between the electrode and the

sensing MCU.

–

Increase system stability and performance when a moving metal part is close to

the electrode.

MS18976V1

Field lines cover the hole

AN2869 Datasheet(PDF) 24 Page - STMicroelectronics