1.3 ELECTRODE DESIGN
1.3.1 ELECTRODE GEOMETRY AND SIZE
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
QT118HA will operate equally well with
long, thin electrodes as with round or
square ones; even random shapes are
acceptable. The electrode can also be
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.
1.3.2 KIRCHOFF’S CURRENT LAW
Like all capacitance sensors, the
QT118HA 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'. 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.
1.3.3 VIRTUAL CAPACITIVE 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 1-4), which is more than
two orders of magnitude greater than that required to create
a return path to the QT118HA via earth. The QT118HA's PCB
however can be physically quite small, so there may be little
‘free space’ coupling (Cx1 in Figure 1-4) between it and the
environment to complete the return path. If the QT118HA
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
QT118HA’s own circuit ground to:
- A nearby piece of metal or metallized housing;
- A floating conductive ground plane;
- Another electronic device (to which its might be
connected already).
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.
‘Ground’ as applied to capacitive fields can also mean power
wiring or signal lines. The capacitive sensor, being an AC
device, needs only an AC ground return.
1.3.5 SENSITIVITY ADJUSTMENT
1.3.5.1 Gain Pin
The QT118HA can be set for one of 3 gain levels using
option pin 5 (Table 1-1). 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 values of Cs and Cx, electrode size, 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.
The Gain input should never be connected to a pullup or
pulldown resistor or tied to anything other than SNS1 or
SNS2, or left unconnected (for high gain setting).
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Figure 1-3 Internal Switching & Timing
C
s
C
x
SNS2
SNS1
ELECTRODE
Single -Slo pe 14-bit
Switched Cap acitor ADC
Charg e
Am p
Burst C ontroller
Result
Do ne
Start
Figure 1-4 Kirchoff's Current Law
Sense E lectrode
C
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Su rro und ing e n viro nm en t
C
X3
SENSO R
C
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