QTouch Schematic and Layout Checklist [APPLICATION NOTE]
Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014
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Figure 3-12. Larger Spatially Interpolated QTouch Wheel with an Additional Ring of Interleaving Teeth
3.2 QMatrix
This section consists of the checklist for reviewing the PCB layout for QMatrix designs.
3.2.1 Components
•
Sense components are close to the microcontroller
The sense components (Sampling Capacitor Cs, Series Resistor Rx/Ry) should be placed close to the
microcontroller sense pins. The Cs and Rx/Ry components form an RC filter and if they are placed too far from
the pin the filtering effect is diminished.
The Rsmp (sampling resistor) should be placed closer to Y sense pin. It is OK to place the Rsmp resistor further
from SMP pin since it is always driven.
3.2.2 Sensors
•
There are no ground planes directly under co-planar Buttons
These will load the sensor and decrease the sensitivity. But the effect is not as dramatic as in the case of QTouch
Sensors.
•
Y Electrodes are located on the top layer
The Y electrodes should be located on the layer closest to touch in order to maximize the sensitivity.
•
X Electrodes completely surround the Y Electrodes
In QMatrix sensors the field only exists in the gap between the X Electrode and Y Electrode. Surrounding the Y
Electrode with the X Electrode will keep the field contained and the touch sensitive area can be precisely defined.
•
The X-Y interdigitation is maximized
Designs that result in very small X-Y inter-digitation should be avoided as such designs result in poor sensitivity.