© Quantum Research Group Ltd.
1 Overview
QMatrix devices are digital burst mode charge-transfer (QT)
sensors designed specifically for matrix geometry touch
controls; they include all signal processing functions
necessary to provide stable sensing under a wide variety of
changing conditions. Only a few external parts are required
for operation. The entire circuit can be built within 8 square
centimeters of PCB area.
Figure 1-2 Sample Electrode Geometries
PARALLEL LINES
SERPENTINE
SPIRAL
Figure 1-1 Field flow between X and Y elements
overly ing panel
X
elem e nt
Y
elem ent
edge transitions of the X drive pulse. The charge emitted by
the X electrode is partly received onto the corresponding Y
electrode which is then processed. The parts use 8 'X'
edge-driven rows and 8 'Y' sense columns to permit up to 64
keys. Keys are typically formed from interleaved conductive
traces on a substrate like a flex circuit or PCB (Figure 1-2).
The charge flows are absorbed by the touch of a human
finger (Figure 1-3) resulting in a decrease in coupling from X
to Y. Thus, received signals decrease or go negative with
respect to the reference level during a touch.
Water films cause the coupled fields to increase slightly,
making water films easy to distinguish from touch.
QMatrix devices include charge cancellation methods which
allow for a wide range of key sizes and shapes to be mixed
together in a single touch panel. These features permit the
construction of entirely new classes of keypads never before
contemplated, such as touch-sliders, back-illuminated keys,
and complex warped panel shapes, all at very low cost.
1.2 Circuit Model
An electrical circuit model is shown in Figure 1-5. The
coupling capacitance between X and Y electrodes is
represented by Cx. While the reset switch is open, a sample
switch is gated so that it transfers charge flows only from the
rising edge of X into a charge integrator. On the falling edge
The devices use an SPI interface running at up to 1.5MHz to
of X, the switch connects the Y line to ground to allow the
allow key data to be extracted and to permit individual key
charge across Cx to neutralize to zero. The voltage change
parameter setup. The interface protocol uses simple single
on the output of the charge integrator after each X edge is
byte commands and responds with single byte responses in
most cases. The command structure is designed to minimize quite small, on the order of a few tens of millivolts. Changes
due to touch are typically under 0.1% of total integrator
the amount of data traffic while maximizing the amount of
voltage. The X pulse can be
information conveyed.
repeated in a burst of up to 64
Figure 1-3 Field Flows When Touched
In addition to normal operating
pulses to increase the change in
and setup functions the device
integrator output voltage due to
can also report back actual
touch during an acquire (Section
signal strengths and error codes.
3.6) to increase gain.
QmBtn software for the PC can
be used to program the IC as
well as read back key status and
signal levels in real time.
QMatrix parts employ transverse
charge-transfer ('QT') sensing, a
new technology that senses
changes in the charge forced
across an electrode by a digital
edge.
The parts are electrically
identical with the exception of the
number of keys which may be
sensed.
ov e rly in g pan el
X
elem e nt
Y
elem ent
Figure 1-4 Fields With a Conductive Film
The charge detector is an opamp
configured as an integrator with a
reset switch; this creates a virtual
ground input, making the Y lines
appear low impedance when the
sample switch is closed. This
configuration effectively
eliminates cross-coupling among
Y lines while greatly lowering
susceptibility to EMI. The circuit
is also highly immune to
capacitive loading on the Y lines,
since stray C from Y to ground
appears merely as a small
parallel capacitance across a
virtual ground.
The circuit uses an 8-bit ADC,
with a subranging structure to
effectively deliver a 14-bit total
conversion 'space' (see Figure
1-6 and Section 3.3). In this way
the circuit can tolerate very large
1.1 Field Flows
Figure 1-1 shows how charge is
transferred across an electrode
set to permeate the overlying
panel material; this charge flow
exhibits a high dQ/dt during the
lQ
4
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QT60xx5B / R1.06
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