DocID15625 Rev 2

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AN2970

Capacitive sensing overview

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Capacitive sensing overview

1.1

Sensing electrode capacitance

A capacitance exists between any reference point relative to ground as long as they are

electrically isolated. If this reference point is a sensing electrode, it helps to think of it as a

capacitor. The positive electrode of the capacitor is the sensing electrode, and the negative

elerode is formed by the surrounding area (virtual ground reference, labeled 1 in Figure 1).

Figure 1. Coupling with hand increases the capacitance of the sensing electrode

When a conductive object is brought into proximity of the sensing electrode, the coupling

between the object and the electrode increases, together with the capacitance of the

sensing electrode relative to ground. For example, a human hand will increase the sensing

electrode capacitance as it approaches it. Touching the dielectric panel that protects the

electrode increases its capacitance significantly.

The sensing electrode can be made of any electrically conductive material, such as copper

on PCBs, or transparent conductive material like Indium Tin Oxide (ITO) deposited on glass

or Plexiglas.

1.2

Principles of charge transfer

charge-transfer equivalent hardware and charge-transfer sequence for a given channel.

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Sensing electrode

Lower capacitance

Higher capacitance

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