at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 7

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

2.4.5
9525B–AT42–09/10
Sensitivity
Figure 2-3.
If one side of the panel to which the electrode is fixed has moving traffic near it, these objects
can cause inadvertent detections. This is called walk-by and is caused by the fact that the fields
radiate from either surface of the electrode equally well. Shielding in the form of a metal sheet or
foil connected to circuit ground will prevent walk-by; putting a small air gap between the
grounded shield and the electrode will keep the value of Cx lower to reduce loading and keep
gain high.
The QT113B can be set for one of two gain levels using option pin 5 (see
This sensitivity change is made by altering the internal numerical threshold level required for a
detection. Note that sensitivity is also a function of other 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.
Increasing Sensitivity
In some cases it may be desirable to increase sensitivity further, for example when using the
sensor with very thick panels having a low dielectric constant.
Sensitivity can often be increased by using a bigger electrode, reducing panel thickness, or
altering panel composition. Increasing electrode size can have diminishing returns, as high
values of Cx will reduce sensor gain (see the figures in
also has a dramatic effect on sensitivity, and this can be increased in value with the tradeoff of
reduced response time. 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 help propagate the field. Metal areas near the electrode will reduce the field strength and
increase Cx loading.
Ground planes around and under the electrode and its SNS 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.
Keep ground away from the electrodes and traces.
Shielding Against Fringe Fields
Sense
wire
U nshielded
Electrode
Sense
wire
E lectrode
Shielded
Appendix A on page
AT42QT113B
Table 3-1 on page
20). The value of Cs
9).
7