2.4.2
Increasing Sensitivity
In some cases it may be desirable to increase sensitivity; for example, when using the sensor
with very thick panels having a low dielectric constant, or when the device is used as a proximity
sensor. Sensitivity can often be increased by using a larger electrode or reducing panel
thickness. Increasing electrode size can have diminishing returns, as high values of Cx will
reduce sensor gain.
The value of Cs also has a dramatic effect on sensitivity, and this can be increased in value with
the trade-off of slower response time and more power. Increasing the electrode's surface area
will not substantially increase touch sensitivity if its diameter is already much larger in surface
area than the object being detected. Panel material can also be changed to one having a higher
dielectric constant, which will better help to propagate the field.
In the case of proximity detection, usually the object being detected is on an approaching hand,
so a larger surface area can be effective.
Ground planes around and under the electrode and its SNSK trace will cause high Cx loading
and destroy gain. The possible signal-to-noise ratio benefits of ground area are more than
negated by the decreased gain from the circuit, and so ground areas around electrodes are
discouraged. Metal areas near the electrode will reduce the field strength and increase Cx
loading and should be avoided, if possible. Keep ground away from the electrodes and traces.
2.4.3
2.4.4
Decreasing Sensitivity
In some cases the QT1011 may be too sensitive. In this case gain can be easily lowered further
by decreasing Cs.
Proximity Sensing
By increasing the sensitivity, the QT1011 can be used as a very effective proximity sensor,
allowing the presence of a nearby object (typically a hand) to be detected.
In this scenario, as the object being sensed is typically a hand, very large electrode sizes can be
used, which is extremely effective in increasing the sensitivity of the detector. In this case, the
value of Cs will also need to be increased to ensure improved sensitivity, as mentioned in
Section 2.4.2. Note that, although this affects the responsiveness of the sensor, it is less of an
issue in proximity sensing applications; in such applications it is necessary to detect simply the
presence of a large object, rather than a small, precise touch.
3. Operation Specifics
3.1
Run Modes
3.1.1
Introduction
The QT1011 has three running modes which depend on the state of the SYNC pin (high or low).
3.1.2
Fast Mode
The QT1011 runs in Fast mode if the SYNC pin is permanently high. In this mode the QT1011
runs at maximum speed at the expense of increased current consumption. Fast mode is useful
when speed of response is the prime design requirement. The delay between bursts in Fast
mode is approximately 1 ms, as shown in Figure 3-1.
4
AT42QT1011
9542G–AT42–03/10
ATMEL [ ATMEL ]
