AN2869 datasheet(10/43 Pages) STMICROELECTRONICS

10 / 43 page

Main capacitive sensing guidelines

AN2869

10/43

Doc ID 15298 Rev 6

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).

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

AN2869 Datasheet(PDF) 10 Page - STMicroelectronics