DocID024847 Rev 2

11/26

AN4316

Tuning of the thresholds

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4

Tuning of the thresholds

This section provides recommendations on how to select reliable thresholds. Depending on

application use cases, some recommendations will have to be adapted.

Capacitive touch sensing applications are sensitive to earth coupling. The parameter

tunning must be done in the same environment as the final application. The hardware tool

connected to the application may change the earth coupling. This is especially true for

example in battery power applications. ST is providing a galvanic insulated hardware tool

(ST-Link/ISOL) to minimize this effect.

4.1

Use of a standard test finger

In order to build an application working with the widest range of people finger

characteristics, we propose to use a standard test finger which will give a worst case but

also will allow to repeat the test without human dependency, such as finger size, pressure

and contact area, skin conductivity, etc... To perform repeatable test, it is recommended to

use an electrically conductive pen-shape tool with a flat rubber end. The flat end made of

conductive rubber, allows to touch the touchkey with a constant contact surface.

The operator performing the validation will take care to center the contact area on the

touchkey.

Additionally, it makes sense to plan a final validation with a panel of users.

A picture of a standard finger is provided below.

Figure 4. Standard 8mm diameter finger

Refer to Appendix A for a recommended standard test finger.

4.2

Threshold definitions

4.2.1

Touchkeys thresholds

To tune the detection thresholds, it is first necessary to determine the sensitivity of each

touchkey. In order to do so: