AN2869 datasheet(29/43 Pages) STMICROELECTRONICS

29 / 43 page

AN2869

Surface electrode design

Doc ID 15298 Rev 6

29/43

Figure 23.

Spring and foam picture (both are not compressed)

3.5.4

PCB and layout

Sensor track length and width

The parasitic capacitance of a track depends on its length and its width. Besides that, a long

track can create an antenna effect which may couple noise. So, the main rule to keep in

mind is that the shorter and thinner the track is, the smaller the parasitic capacitance.

It is recommended to route the tracks as thin as the PCB technology allows and shorter than

10 cm for standard or flexible PCBs.

Sensor track routing

The main goal, when laying out the PCB, should be to minimize the interactions between

elements or, if they cannot be minimized, to make them uniform for all capacitive elements.

Although the touch sensing controller algorithms, used to acquire touchkey, linear sensor

and rotary sensor signals, take into account that the capacitance of each array is different, it

is good practice to keep things as balanced as possible.

AN2869 Datasheet(PDF) 29 Page - STMicroelectronics