AN4312 datasheet(12/36 Pages) STMICROELECTRONICS

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Main capacitive sensing guidelines

AN4312

12/36

DocID024815 Rev 1

Sensitivity

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

equivalent vacuum thickness TV.

Equation 2

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 touchkeys 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 3

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

2.2.4

Mechanical construction and PCB to panel bonding

In order to ensure stable touch detection, the PCB must always be at the same place on the

panel. The slightest variation, even as small as 100 microns, may lead to differences in the

signal which can be detected. This must be avoided to ensure the integrity of the touch

detection. The panel and other elements of the device must not be moved, or only as little as

possible, by the user’s finger. To avoid this kind of problem, glue, compression, co-convex

surfaces can be used to mechanically stabilize the PCB and the panel very close together.

In the list of the different ways to achieve this, we can put: heat staking plastic posts,

screws, ultrasonic welding, spring clips, non-conductive foam rubber pressing from behind,

etc.

PMMA (Poly methyl methacrylate)

2.6 to 4

Typical PSA

2.0 - 3.0 (approx.)

Table 3. Dielectric constants of common materials used in a panel construction

Material

εR

-----

TVSTACK

()

TV layers

()

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