QT160 View Datasheet(PDF) - Quantum Research Group
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Description
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QT160 Datasheet PDF : 12 Pages
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R esul t
Figure 1-2 Internal Switching & Timing
SNS2
E LEC TRO DE
will generally be enough to couple a few
picofarads back to local earth.
Electrodes connected to the IC
themselves act as coupling plates back
to local ground, since when one
channel is sensing the other channels
are clamped to circuit ground.
Start
Do ne
SNS1
C harg e
Amp
do not propagate fields laterally very well, and will have poor
sensitivity in the middle. Experimentation is required.
Since the channels acquire their signals in time-sequence,
any of the 6 electrodes can be placed in direct proximity to
each other if desired without cross-interference.
A good example of backlighting can be found in the E160
eval board for the QT160.
1.3.3 KIRCHOFF’S CURRENT LAW
Like all capacitance sensors, these parts rely 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
Cs
1.3.4 VIRTUAL CAPACITIVE GROUNDS
When detecting human contact (e.g. a
Cx
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 IC via earth.
The 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 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 IC's own circuit ground to:
(1) A nearby piece of metal or metallized housing;
(2) A floating conductive ground plane;
(3) A nail driven into a wall;
(4) A larger electronic device (to which its output might be
connected anyway).
Figure 1-3 Mesh Key Geometry
lQ
Figure 1-4 Open Electrode for Back-Illumination
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.
1.3.5 FIELD SHAPING
The electrode can be prevented from sensing in undesired
directions with the assistance of metal shielding connected
3
QT160/161 1.07/0904
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