AN957 Datasheet, PDF (17/25 Pages) List of Unclassifed Manufacturers – Measuring HEXFETCharacteristics

AN957: TouchXpress™ Configuration and Profiling Guide
Configuration Tips
3.2 Touch Delta and Threshold Configuration
A sensor’s touch delta is the sensor’s output value relative to a baseline when the sensor is active or touched by a user. The touch
delta seen by a sensor is a function of the following factors:
• the size of the sensor
• the thickness of the overlay
• whether the sensor is surrounded by ground pour
• the drive strength setting of the sensor
For PCB layout recommendations, see Application Note AN447: Printed Circuit Design Notes for Capacitive Sensing with the CS0 Mod-
ule. Application notes can be found on the Silicon Labs website (www.silabs.com/interface-appnotes) or in Simplicity Studio using the
[Application Notes] tile.
The optimal sensor size tends to be around the diameter of the surface area created by a finger pressed against a surface, which is
7-10 mm. The most commonly used overlay is an overlay of 1/16th inch in thickness. The drive strength setting for most sensors using
these common hardware specifications is 8x gain. All of these factors result in a touch delta of around 700 codes, which is the default
touch delta setting in [Xpress Configurator].
Care should be taken when defining the touch deltas for sensors because a touch delta setting that does not reflect the delta seen by a
sensor in a design can result in a device that reverts to a more conservative touch qualification process, which will limit responsiveness.
It is good practice to check settings in [Capacitive Sense Profiler] to make sure that the configured touch delta for a sensor matches
the real world output of the TouchXpress device.
Thresholds should be set with at least 10% difference between active and inactive thresholds to ensure sufficient hysteresis between
qualification states. Setting thresholds too tightly relative to each other can result in a touch being rapidly qualified and disqualified if the
output codes drift above and below the threshold grouping. The default thresholds in [Xpress Configurator] provide sufficient guard
against this type of behavior.
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