AN3150 datasheet(22/25 Pages) STMICROELECTRONICS

Part #	AN3150

Description	Guidelines to optimize proximity detection with STM8T141 and STM8T143 capacitive sensors

PDF	25 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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22 / 25 page

Front Panel

AN3150

22/25

DocID17048 Rev 3

Front Panel

Each material type is defined with a dielectric permittivity constant, known as epsilon (

εr).

The higher

εr, the easier the electrical field is transmitted through the material.

If the proximity distance without any panel is tair0, it becomes tair1 when a panel is inserted

between the electrodes and the finger.

Equation 2 gives the formula at equivalent sensitivity tuning (same electrode, same CS, and

same threshold).

Equation 2

Example

If Tair0 = 5 cm and we insert a 5 mm glass panel (

εr = 10) by gluing it to the electrode PCB

with 0.5 mm of pressure sensitive adhesive (

εr = 3), then

Tair1 = 5 - (0.5/10 + 0.05/3) = 4.93 mm.

Note that the total distance between the finger and the electrode has increased due to the

good dielectric performance of the glass panel.

Figure 17. Influence of panel on detection distance

Tair1

Tair0 Σ T εr

⁄

()

–

Detection range

Tair0

Tair1

PCB

Electrode copper

Glue

Non conductive panel

AN3150 Datasheet(PDF) 22 Page - STMicroelectronics