object, however it is impossible for the
sensor to distinguish between the two
touch areas.
Figure 1-3 Internal Switching & Timing
ELECTRODE
Result
SNS2
1.3 ELECTRODE DESIGN
1.3.1 ELECTRODE
GEOMETRY AND
S
IZE
There is no restriction on the shape of
the electrode; in most cases common
sense and a little experimentation can
result in a good electrode design. The
QT118H will operate equally well with
long, thin electrodes as with round or
square ones; even random shapes are
acceptable. The electrode can also be
Cs
Start
Cx
Done
SNS1
a
3-dimensional surface or object.
Sensitivity is related to electrode
surface area, orientation with respect
to the object being sensed, object
composition, and the ground coupling
quality of both the sensor circuit and
the sensed object.
C harge
Amp
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.
If a relatively large electrode surface is desired, and if tests
show that the electrode has more capacitance than the
QT118H can tolerate, the electrode can be made into a
1.3.3 VIRTUAL
C
APACITIVE
GROUNDS
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 1-5), which is more
than two orders of magnitude greater than that required to
create a return path to the QT118H via earth. The QT118H's
PCB however can be physically quite small, so there may be
little ‘free space’ coupling (Cx1 in Figure 1-5) between it and
the environment to complete the return path. If the QT118H
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.
A ‘virtual capacitive ground’ can be created by connecting
the QT118H’s own circuit ground to:
sparse mesh (Figure 1-4) having lower Cx than a solid plane.
Sensitivity may even remain the same, as the sensor will be
operating in a lower region of the gain curves.
(1) A nearby piece of metal or metallized housing;
Figure 1-5 Kirchoff's Current Law
1.3.2 KIRCHOFF
’S
CURRENT
LAW
Like all capacitance sensors, the QT118H relies on Kirchoff’s
Current Law (Figure 1-5) to detect the change in capacitance
of the electrode. This law as applied to capacitive sensing
requires that the sensor’s field current must complete a loop,
returning back to its source in order for capacitance to be
sensed. Although most designers relate to Kirchoff’s law with
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
manner for a capacitive sensor to operate properly. Note that
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'.
C
X2
Sense E lectrode
SENSOR
C
X1
C
X3
Surro und ing e nviro nm ent
lq
2
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