at42qt100a ATMEL Corporation, at42qt100a Datasheet - Page 5

• Manufacturer Part Number: at42qt100a
• Description: Qtouch? Charge Transfer Ic
• Manufacturer: ATMEL Corporation

2.4
2.4.1
2.4.2
2.4.3
3. Operation Specifics
3.1
3.1.1
3.1.2
9531A–AT42–02/09
Sensitivity
Run Modes
Introduction
Increasing Sensitivity
Decreasing Sensitivity
Introduction
Fast Mode (SYNC = 1)
The sensitivity on the QT100A is a function of things like the value of Cs, electrode size and
capacitance, electrode shape and orientation, the composition and aspect of the object to be
sensed, the thickness and composition of any overlaying panel material, and the degree of
ground coupling of both sensor and object.
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. 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.
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.
In some cases the QT100A may be too sensitive. In this case gain can be easily lowered by
decreasing Cs.
The QT100A has three running modes which depend on the logic level applied to the SYNC pin.
The QT100A runs in Fast mode if the SYNC pin is permanently high. In this mode the QT100A
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.
AT42QT100A
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