AN4316 Datasheet, PDF (11/26 Pages) STMicroelectronics – Tuning a STMTouch-based application

AN4316
4
Tuning of the thresholds
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
4.2
4.2.1
Refer to Appendix A for a recommended standard test finger.
069
Threshold definitions
Touchkeys thresholds
To tune the detection thresholds, it is first necessary to determine the sensitivity of each
touchkey. In order to do so:
DocID024847 Rev 2
11/26
25