Choosing the PSoC 4 CapSense Overlay – KBA227569

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Materials with relative permittivity between 2.0 and 8.0 are well suited for CapSense® overlay, such as glass, acrylic and PET film. Conductive materials interfere with the electric field pattern. Therefore, you should NOT use conductive materials(such as metal material) for overlay. You should also avoid using conductive paints on the overlay.

Table 1 Relative Permittivity of Overlay Materials

Material

Relative Permittivity

Air

1.0

Formica

4.6 – 4.9

Glass (Standard)

7.6 – 8.0

Glass (Ceramic)

6.0

PET Film (Mylar®)

3.2

Polycarbonate (Lexan®)

2.9 – 3.0

Acrylic (Plexiglas®)

2.8

ABS

2.4 – 4.1

Wood Table and Desktop

1.2 – 2.5

Gypsum (Drywall)

2.5 – 6.0

The thickness depends on different applications and overlay materials. The following table lists the recommended maximum thickness of acrylic overlay for different CapSense widgets.

Table 2 Maximum Thickness of Acrylic Overlay

Widget

Maximum thickness (mm)

Button

5

Slider

5

Touchpad

0.5

For other materials, convert the thickness with using relative permittivity.

For example, the recommended maximum thickness of glass (ceramic) should be

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Then we can get the recommended maximum thickness of glass (ceramic)

Table 3 Maximum Thickness of Glass (Ceramic) Overlay

Widget

Maximum thickness (mm)

Button

10.71

Slider

10.71

Touchpad

1.071

Reference Document:

 

AN85951 - PSoC® 4 and PSoC® 6 MCU CapSense® Design Guide