AN2869 datasheet(39/43 Pages) STMICROELECTRONICS

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AN2869

Projected sensor design

Doc ID 15298 Rev 6

39/43

Figure 34.

Ground floods around Tx

Ground plane at the bottom is recommended to:

●

prevent false detection from the back side

●

shield Rx track from Tx one

●

increase the sensitivity

●

improve the directivity

For water immunity, avoid ground plane at top layer, and reduce it at the bottom by using

hatched plane.

Tx routing

To route the Tx signal efficiently, the most important thing to respect and almost the only one

is the RC time constant rule. But keep in mind that signals which switch rapidly (more than

tens of kHz) such as high speed communication signals, LCD or LED drive signals must be

routed far away from Tx.

The Tx track is less sensitive than the Rx track so it can be put on any layer of the PCB but

Rx tracks should be considered when routing it to ensure a good design.

Rx routing

On the other hand, the Rx track is very sensitive due to the capacitance of the sensor. A

false detection may occur if some guidelines are not followed. The most obvious is to route it

far from the sensor itself, e.g. on another layer.

Another one is to avoid placing ground near the Rx track which reduces the sensitivity.

Then, when Rx and Tx are very close (about less than 10 mm), an electric field is also

generated and a finger which roams here can generate a false touchkey detection (see

Figure 35).

MS18991V2

Rx1

Rx4

Rx3

Rx2

Tx (bottom)

GND

Clearance Tx to GND = 1 mm

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