a slow rate, but only while there is no detection in effect. The
rate of adjustment must be performed slowly, otherwise
legitimate detections could be ignored. The QT118H drift
compensates using a slew-rate limited change to the
reference level; the threshold and hysteresis values are
slaved to this reference.
Table 1-1 Gain Strap Options
Gain
High
Tie Pin 5 to:
Leave open
Pin 6
Medium
Low
Pin 7
Once an object is sensed, the drift compensation mechanism
ceases since the signal is legitimately high, and therefore
should not cause the reference level to change.
1.3.5.2 Changing Cs, Cx
The values of Cs and Cx have a dramatic effect on
sensitivity, and Cs can be easily increased in value to
improve gain. Sensitivity is directly proportional to Cs and
inversely proportional to Cx:
The QT118H's drift compensation is 'asymmetric': the
reference level drift-compensates in one direction faster than
it does in the other. Specifically, it compensates faster for
decreasing signals than for increasing signals. Increasing
signals should not be compensated for quickly, since an
approaching finger could be compensated for partially or
entirely before even touching the sense pad. However, an
obstruction over the sense pad, for which the sensor has
already made full allowance for, could suddenly be removed
leaving the sensor with an artificially elevated reference level
and thus become insensitive to touch. In this latter case, the
sensor will compensate for the object's removal very quickly,
usually in only a few seconds.
k$CS
S =
Where ‘k’ depends on a variety of factors including the gain
pin setting (see prior section), Vdd, etc.
CX
Sensitivity plots are shown in Figures 4-1 and 4-2, page 10.
1.3.5.3 Electrode / Panel Adjustments
Sensitivity can often be increased by using a bigger
electrode, or reducing overlying panel thickness. Increasing
electrode size can have a diminishing effect on gain, as the
attendant higher values of Cx will start to reduce sensor gain.
2.1.2 THRESHOLD AND
H
YSTERESIS
Also, increasing the electrode's surface area will not
substantially increase touch sensitivity if its diameter is
already much larger in surface area than the object being
detected.
The internal signal threshold level can be set to one of three
settings (Table 1-1). These are fixed with respect to the
internal reference level, which in turn moves in accordance
with the drift compensation mechanism.
The panel or other intervening material can be made thinner,
but again there are diminishing rewards for doing so. Panel
material can also be changed to one having a higher
dielectric constant, which will help propagate the field through
to the front. Locally adding some conductive material to the
panel (conductive materials essentially have an infinite
dielectric constant) will also help; for example, adding carbon
or metal fibers to a plastic panel will greatly increase frontal
field strength, even if the fiber density is too low to make the
plastic bulk-conductive.
The QT118H employs a hysteresis dropout below the
threshold level of 17% of the delta between the reference and
threshold levels.
2.1.3 MAX
ON-DURATION
If an object or material obstructs the sense pad the signal
may rise enough to create a detection, preventing further
operation. To prevent this, the sensor includes a timer which
monitors detections. If a detection exceeds the timer setting,
the timer causes the sensor to perform a full recalibration.
This is known as the Max On-Duration feature.
1.3.5.3 Ground Planes
After the Max On-Duration interval, the sensor will once again
function normally, even if partially or fully obstructed, to the
best of its ability given electrode conditions. There are two
timeout durations available via strap option: 10 and 60
seconds.
Grounds around and under the electrode and its SNS trace
will cause high Cx loading and destroy gain. The possible
signal-to-noise ratio benefits of ground area are more than
negated by the decreased gain from the circuit, and so
ground areas around electrodes are discouraged. Keep
ground, power, and other signals traces away from the
electrodes and SNS wiring
2.1.4 DETECTION
INTEGRATOR
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish
this, the QT118H incorporates a detect integration counter
2 - QT118H SPECIFICS
2.1 SIGNAL PROCESSING
Figure 2-1 Drift Compensation
The QT118H digitally processes all signals using
a number of algorithms pioneered by Quantum.
The algorithms are specifically designed to
provide for high survivability in the face of all
kinds of adverse environmental changes.
Signal
Hysteresis
Threshold
2.1.1 DRIFT
COMPENSATION
ALGORITHM
Reference
Signal drift can occur because of changes in Cx
and Cs over time. It is crucial that drift be
compensated for, otherwise false detections,
non-detections, and sensitivity shifts will follow.
Drift compensation (Figure 2-1) is performed by
making the reference level track the raw signal at
Output
lq
4
QT118H R1.08 / 0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
