Unused channels:
If a channel is not used, a dummy sense
capacitor (nominal value: 1nF) of any type must be
connected between the unused SNSnA / SNSnB pins ensure
correct operation.
Unused pins:
Unused device pins labeled NC should
remain unconnected.
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.
1.2 ELECTRODE DRIVE
These devices have completely independent sensing
channels. The internal ADC treats Cs on each channel as a
floating transfer capacitor; as a direct result, sense
electrodes can be connected to either SNSnA or SNSnB and
the sensitivity and basic function will be the same; however
there is an advantage in connecting electrodes to SNSnA
lines to reduce EMI susceptibility.
The PCB traces, wiring, and any components associated
with or in contact with SNSnA and SNSnB will become touch
sensitive and should be treated with caution to limit the touch
area to the desired location.
Multiple touch electrodes connected to SNSnA can be used,
for example to create control surfaces on both sides of an
object.
It is important to limit the amount of stray capacitance on the
SNSnA and SNSnB terminals, for example by minimizing
trace lengths and widths to allow for higher gains and lower
values of Cs.
1.3.2 K
EY
G
EOMETRY
, S
IZE
,
AND
L
OCATION
There is no restriction on the shape of the key electrode; in
most cases common sense and a little experimentation can
result in a good electrode design. The devices will operate
with long thin electrodes, round or square ones, or keys with
odd shapes. Electrodes can also be on 3-dimensional
surfaces. Sensitivity is related to the amount of electrode
surface area, overlying panel material and thickness, and the
ground return coupling quality of the circuit.
If a relatively large touch area is desired, and if tests show
that the electrode has more capacitance than the part can
tolerate, the electrode can be made into a sparse mesh
(Figure 1-3) having lower Cx than a solid plane.
Since the channels acquire their signals in time-sequence,
any of the electrodes can be placed in direct proximity to
each other if desired without cross-interference.
1.3.3 B
ACKLIGHTING
K
EYS
Touch pads can be back-illuminated quite readily using
electrodes with a sparse mesh (Figure 1-3) or a hole in the
middle (Figure 1-4). The holes can be as large as 4 cm in
diameter provided that the ring of metal is at least twice as
wide as the thickness of the overlying panel, and the panel is
greater than 1/8 as thick as the diameter of the hole. Thin
panels do not work well with this method as they do not
propagate fields laterally very well, and will have poor
sensitivity in the middle. Experimentation is required.
A good example of backlighting can be found in the E160
evaluation board.
1.3 KEY DESIGN
1.3.1 K
IRCHOFF
’
S
C
URRENT
L
AW
Like all capacitance sensors, these parts rely on Kirchoff’s
Current Law (Figure 1-2) 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 galvanic ground
connections; capacitive coupling to ground (Cx1) is always
1.3.4 V
IRTUAL
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 1-2), which is more
than two orders of magnitude greater than that required to
create a detection. The sensor’s PCB however may be
physically small, so there may be little ‘free space’ coupling
(Cx1 in Figure 1-2) between it and the environment to
Figure 1-2 Kirchoff's Current Law
C
X2
Figure 1-3 Mesh Electrode Geometry
S e nse E le ctro de
S EN SO R
C
X 1
C
X3
Su rro und in g e nv iro nm en t
lQ
3
QT140/150 1.01/1102
QUANTUM [ QUANTUM RESEARCH GROUP ]
