AN4312 datasheet(10/36 Pages) STMICROELECTRONICS

10 / 36 page

Main capacitive sensing guidelines

AN4312

10/36

DocID024815 Rev 1

Figure 4. Silver printing on PET

More and more applications need a flex PCB or FFC/FPC(b) to interconnect circuitry; it is

suitable, provided that the overall application is mechanically stable. Furthermore, the FPC

tracks will be part of the touch sensor. So if the flex moves a little bit, even a few

micrometers, the capacitance to its surroundings will definitely change and might be

significant, causing false touch detections or drops in sensitivity. Putting the flex in close

proximity to a metal chassis or other signals, or on top of noisy circuitry, can cause problems

as well (loss of sensitivity or spurious detection).

Figure 5. Flexible PCB (FPC)

b. FFC = Flat Flexible Conductor, FPC = Flexible Printed Circuit

Table 2. Potential application problems with flex PCB placement

When the flex PCB is in close proximity to...

...the following can occur.

...the ground or to a metal chassis connected to

the ground.

...the sensitivity is reduced.

... a floating metal object or to a floating metal

chassis

... the object or the chassis conducts the touch to

the electrode

... a source of noise

... the acquisition will be strongly perturbed and

so the touchkey will become non-usable

AN4312 Datasheet(PDF) 10 Page - STMicroelectronics