solid plane. Sensitivity may even remain the same, as the
sensor will be operating in a lower region of the gain curves.
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.
1.3.2 KIRCHOFF
’
S
C
URRENT
L
AW
Like all capacitance sensors, the QT113 relies on Kirchoff’s
Current Law (Figure 1-3) to detect the change in capacitance
of the electrode. This law as applied to capacitive sensing
1.3.5 SENSITIVITY
requires that the sensor’s field current must complete a loop, The QT113 can be set for one of 2 gain levels using option
returning back to its source in order for capacitance to be pin 5 (Table 1-1). This sensitivity change is made by altering
sensed. Although most designers relate to Kirchoff’s law with the internal numerical threshold level required for a detection.
regard to hardwired circuits, it applies equally to capacitive
field flows. By implication it requires that the signal ground
and the target object must both be coupled together in some
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
manner for a capacitive sensor to operate properly. Note that be sensed, the thickness and composition of any overlaying
panel material, and the degree of ground coupling of both
sensor and object.
there is no need to provide actual hardwired ground
connections; capacitive coupling to ground (Cx1) is always
sufficient, even if the coupling might seem very tenuous. For
example, powering the sensor via an isolated transformer will
provide ample ground coupling, since there is capacitance
between the windings and/or the transformer core, and from
the power wiring itself directly to 'local earth'. Even when
battery powered, just the physical size of the PCB and the
object into which the electronics is embedded will generally
be enough to couple a few picofarads back to local earth.
1.3.5.1 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 (Figures
4-1 to 4-3). The value of Cs 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.
1.3.3 VIRTUAL
C
APACITIVE
G
ROUNDS
When detecting human contact (e.g. a fingertip), grounding
of the person is never required. The human body naturally
has several hundred picofarads of ‘free space’ capacitance to
the local environment (Cx3 in Figure 1-3), which is more than
two orders of magnitude greater than that required to create
a return path to the QT113 via earth. The QT113's PCB
however can be physically quite small, so there may be little
‘free space’ coupling (Cx1 in Figure 1-3) between it and the
environment to complete the return path. If the QT113 circuit
ground cannot be earth grounded by wire, for example via
the supply connections, then a ‘virtual capacitive ground’ may
be required to increase return coupling.
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.
A ‘virtual capacitive ground’ can be created by connecting the
QT113’s own circuit ground to:
- A nearby piece of metal or metallized housing;
- A floating conductive ground plane;
- Another electronic device (to which its output might be
connected anyway).
1.3.5.2 Decreasing Sensitivity
In some cases the QT113 may be too sensitive, even on low
gain. In this case gain can be lowered further by decreasing
Cs.
Free-floating ground planes such as metal foils should
maximize exposed surface area in a flat plane if possible. A
square of metal foil will have little effect if it is rolled up or
crumpled into a ball. Virtual ground planes are more effective
and can be made smaller if they are physically bonded to
other surfaces, for example a wall or floor.
Figure 1-3 Kirchoff's Current Law
C
X2
1.3.4 FIELD
S
HAPING
The electrode can be prevented from sensing in undesired
directions with the assistance of metal shielding connected to
circuit ground (Figure 1-4). For example, on flat surfaces, the
field can spread laterally and create a larger touch area than
desired. To stop field spreading, it is only necessary to
surround the touch electrode on all sides with a ring of metal
connected to circuit ground; the ring can be on the same or
opposite side from the electrode. The ring will kill field
spreading from that point outwards.
Sense E lectrode
SENSOR
C
X1
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
C
X3
Surrounding environm ent
lQ
3
R1.05/0405
