AN2869 datasheet(8/43 Pages) STMICROELECTRONICS

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Main capacitive sensing guidelines

AN2869

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Doc ID 15298 Rev 6

Main capacitive sensing guidelines

2.1

Overview

A surface or projected capacitive sensor is generally made up of the following different

layers:

●

A fiberglass PCB

●

A set of electrodes made of a copper pad

●

A panel made of glass, Plexiglas, or any non-conductive material

●

A silk screen printing

Figure 2.

Example of capacitive sensor construction

2.2

Construction

2.2.1

Substrates

The substrate is the base material carrying the electrodes.

A substrate can be chosen among any non-conductive material, in practice, PCB materials

(e.g. FR4, CEM-1), acrylics like Polyethylene Terephthalate (PET), or Polycarbonate can be

used. Glass is also an excellent material for this purpose.

Note:

For the projected materials (technologies) listed above, it is not recommended to use a

relative permittivity (

R) that is too high.

In many cases, the substrate which is used in electronic application will also work well for

capacitive sensing. Special care is required to avoid materials which can retain water

contained in the atmosphere (e.g. hygroscopic material such as paper based).

Unfortunately, this would modify

R (relative permittivity) with environmental conditions.

MS18973V1

Glass/plexiglass panel

Silkscreen printing

Copper pad (Cu)

electrode

Fiberglass PCB

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