AN3431

Principle of operation

Doc ID 019017 Rev 1

5/18

1

Principle of operation

In both traditional and PresSense capacitive sensing technologies, the change in

capacitance caused by the user is measured by a sensor. In traditional capacitive sensing

technology, the sensor pad forms one side of a parallel capacitor and the user’s finger forms

the other side. If the user brings his finger closer to the pad, the distance (d) between his

capacitance (C) increases.

Equation 1: Parallel plate capacitor

Where

In PresSense technology, the concept of the parallel plate capacitor still applies, except that

the second plate is not the finger of the user but a conductive material with a fixed potential

suspended over the sensor pad. When a user applies a force to the conductive material

above the sensor pad, a slight local deflection is created in the material. Consequently, the

distance (d) between the two plates alters.

The conductive material, used as a top cover, must be kept at a fixed potential. In some

cases, the external metallic part of the system must be connected to earth for safety

reasons. The conductive top cover can be connected to earth but, in this case, the electronic

touch sensing must be referred to earth.

Figure 1 shows a cross section of a typical PresSense button when it is unpressed.

Figure 1.

PresSense technology: cross section of an unpressed button

Figure 2 shows a cross section of a typical PresSense button when it is pressed.

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