AN2869 datasheet(14/43 Pages) STMICROELECTRONICS

14 / 43 page

Main capacitive sensing guidelines

AN2869

14/43

Doc ID 15298 Rev 6

2.2.5

Metal chassis

A metal chassis behind a touch sensor is a good path to the ground and tends to reduce the

sensitivity of the touch response in case there is a significant area of overlap. Such a

metallic surface must never be electrically floating as it makes the whole product unstable in

terms of touch detection. This is also applicable for any conductive decorative feature close

to the sensor.

Metal chassis and decorative items must be grounded or connected to the driven shield (see

Section 3.5.2: Driven shield) if it is implemented.

Metallic paints can be an issue if they contain conductive particles. Low particle density

paint is recommended.

2.2.6

Air gap

Due to its dielectric constant, air can be used as an isolator. An air gap reduces the touch

sensitivity when it is in the touch side stack. However, in some conditions, air can be useful

to reduce the ground loading in the non-touch side stack. Such ground loading can be due

to the metal chassis or an LCD. For instance, when designing a touch-screen solution, an air

gap of 0.5 mm to 1 mm between the LCD and the touch sensor is recommended. Air gaps

also help to reduce the sensitivity of the back side of a portable device.

2.2.7

Transfer of an electrode from PCB to the front panel

It is possible to use a conducting cylinder or a compressed spring to achieve a transfer of an

electrode from a PCB to the front panel. Please refer to Section 3.5.3 or Section 4.4.1 for

further information.

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