AN4312 datasheet(25/36 Pages) STMICROELECTRONICS

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AN4312

Surface sensor design

3.4.3

Rotary sensor with central touchkey

It is possible to locate a touchkey in the centre of a rotary sensor. This touchkey has a lower

sensitivity compared to other single touchkeys. To reduce the loss of sensitivity induced by

the center touchkey on the rotary sensor, it is recommended to place the center touchkey

and rotary sensor electrodes on the same acquisition bank. The pattern of the central

touchkey must be as symmetrical as possible so that the loading effect on the rotary sensor

is also symmetrical.

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 20, Figure 21 and Figure 22). 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 20. Back-lighting touchkey

Figure 21. PCB 3D top view

MS18976V1

Field lines cover the hole

AN4312 Datasheet(PDF) 25 Page - STMicroelectronics