2 - QT110 SPECIFICS
2.1 SIGNAL PROCESSING
The QT110 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.
2.1.1 D
RIFT
C
OMPENSATION
A
LGORITHM
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. Cs
drift has almost no effect on gain since the threshold method
used is ratiometric. However Cs drift can still cause false
detections if the drift occurs rapidly.
Drift compensation (Figure 2-1) is performed by making the
reference level track the raw signal at 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 QT110 drift compensates using a
slew-rate limited change to the reference level; the threshold
and hysteresis values are slaved to this reference.
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.
The QT110'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.
2.1.2 T
HRESHOLD
C
ALCULATION
Sensitivity is dependent on the threshold level as well as ADC
gain; threshold in turn is based on the internal signal reference
level plus a small differential value. The threshold value is
established as a percentage of the absolute signal level. Thus,
sensitivity remains constant even if Cs is altered dramatically,
so long as electrode coupling to the user remains constant.
Furthermore, as Cx and Cs drift, the threshold level is
automatically recomputed in real time so that it is never in error.
The QT110 employs a hysteresis dropout below the threshold
level of 50% of the delta between the reference and threshold
levels.
The threshold setting is determined by option jumper; see
Section 1.3.4.
2.1.3 M
AX
O
N
-D
URATION
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.
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 nominal
timeout durations available via strap option: 10 and
60 seconds. The accuracy of these timeouts is
approximate.
2.1.4 D
ETECTION
I
NTEGRATOR
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish this,
the QT110 incorporates a detect integration counter that
increments with each detection until a limit is reached, after
which the output is activated. If no detection is sensed prior to
the final count, the counter is reset immediately to zero. In the
QT110, the required count is 4.
The Detection Integrator can also be viewed as a 'consensus'
filter, that requires four detections in four successive bursts to
create an output. As the basic burst spacing is 75ms, if this
spacing was maintained throughout all 4 counts the sensor
would react very slowly. In the QT110, after an initial detection
is sensed, the remaining three bursts are spaced about 20ms
apart, so that the slowest reaction time possible is
75+20+20+20 or 135ms and the fastest possible is 60ms,
depending on where in the initial burst interval the contact first
occurred. The response time will thus average about 95ms.
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
The QT110 has no recalibration pin; a forced recalibration is
accomplished only when the device is powered up. However,
the supply drain is so low it is a simple matter to treat the entire
IC as a controllable load; simply driving the QT110's Vdd pin
directly from another logic gate or a microprocessor port
(Figure 2-2) will serve as both power and 'forced recal'. The
source resistance of most CMOS gates and microprocessors is
low enough to provide direct power without any problems.
Almost any CMOS logic gate can directly power the QT110.
A 0.01uF minimum bypass capacitor close to the device is
essential; without it the device can break into high frequency
oscillation.
Option strap configurations are read by the QT110 only on
powerup. Configurations can only be changed by powering the
QT110 down and back up again; again, a microcontroller can
directly alter most of the configurations and cycle power to put
them in effect.
2.2 OUTPUT FEATURES
The devices are designed for maximum flexibility and can
accommodate most popular sensing requirements. These are
selectable using strap options on pins OPT1 and OPT2. All
options are shown in Table 2-1.
OPT1 and OPT2 should never be left floating. If they are
floated, the device will draw excess power and the options will
not be properly read on powerup. Intentionally, there are no
pullup resistors on these lines, since pullup resistors add to
power drain if the pin(s) are tied low.
2.2.1 DC M
ODE
O
UTPUT
The output of the device can respond in a DC mode, where the
output is active-low upon detection. The output will remain
active for the duration of the detection, or until the Max
LQ
4 QT110 R1.04/0405
Figure 2-1 Drift Compensation
Threshold
Signal
Hysteresis
Reference
Output
