AN2869 datasheet(12/43 Pages) STMICROELECTRONICS

12 / 43 page

Main capacitive sensing guidelines

AN2869

12/43

Doc ID 15298 Rev 6

Table 3.

Dielectric constants of common materials used in a panel construction

Sensitivity

A useful parameter to consider with panel material and thickness (T) is the electric field

equivalent vacuum thickness TV.

Equation 1

where t is the thickness of the dielectric.

TV is the thickness of vacuum with an electric field conduction equivalent to that of the

material. The smaller it is, the easier the field can reach through. Panels with the same TV

make keys with identical sensitivity. This works for both directions of course and may be

used to evaluate the touch sensitivity from the back side of the application.

For a panel built from a stack of different materials, it is possible to add the vacuum

equivalent thickness of each layer:

Equation 2

Each material has an influence on the sensitivity. So the equation can be used when, for

example, the electrodes are on the bottom surface of the PCB substrate, then the thickness

and

R of the substrate will be also factors of the global sensitivity.

Material

Air

1.00059

Glass

4 to 10

Sapphire glass

9 to 11

Mica

4 to 8

Nylon

Plexiglass

3.4

Polyethylene

2.2

Polystyrene

2.56

Polyethylene terephthalate (PET)

3.7

FR4 (fiberglass + epoxy)

4.2

PMMA (Poly methyl methacrylate)

2.6 to 4

Typical PSA

2.0 - 3.0 (approx.)

------

VSTACK

()

V layers

()

∑

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