STM8T141 datasheet(27/50 Pages) STMICROELECTRONICS

27 / 50 page

STM8T141

Design guidelines

Doc ID 15699 Rev 7

27/50

Figure 20.

Connecting the shield (coaxial cable implementation)

8.2

Sensitivity adjustment

Several factors impact device sensitivity:

●

The sensing electrode material and size

●

The touch panel material and thickness

●

The board layout and in particular the sensing signal tracks

●

The value of the sampling capacitor (CS) for proximity thresholds only

●

The ground coupling of the object (finger or hand) and sensor.

●

The touch or proximity detection threshold selected by the option byte.

8.2.1

CS influence on sensitivity

In touch mode, the Cs capacitor value has no influence on the sensitivity as the thresholds

are relative to the actual reference value. In proximity mode, the Cs value allows the

sensivity to be tuned. A higher sampling capacitor value increases the resolution and the

sensitivity but also the charging time. Decreasing the sampling capacitor value therefore

decreases the sensitivity.

For more details, please refer to application note AN2966.

8.2.2

PCB layout and construction

The PCB traces, wiring, and components associated or in contact with CX pins become

touch sensitive and should be treated with caution to limit the touch area to the desired

location. As an example, multiple touch electrodes connected to a sensing channel can be

used to create control surfaces on both sides of an object.

It is important to limit the amount of stray capacitance on the CX pin. This can be done by

minimizing trace lengths and widths to achieve for higher gain without using higher values of

CS. To minimize cross-coupling, electrode traces from adjacent sensing channel should not

run close to each other for long distances. For detailed information on the impacts of the first

three factors, refer to application note AN2869.

VDD

SHLDOUT

Plastic jacket

Metal shield

Center core

Dielectric insulator

ai15527

100 k

SHIELD

SHLDIN

Coaxial cable

2 k

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