©Quantum Research Group Ltd.
possible to promote equal field coupling through the overlying
panel material and to increase sensitivity.
Suggested design rules for interdigitated keys are shown in
Figure 1-8.
detection by a jumper option (Table 2-1); this option applies to all
keys.
Max On-duration has no interaction among keys; a timeout on
one key will have no effect on another key.
1.5 SIGNAL PROCESSING
The QT60040 calibrates and processes all signals using a
number of algorithms pioneered by Quantum. These algorithms
are specifically designed to survive most environmental
conditions.
1.5.5 D
ETECTION
I
NTEGRATOR
To suppress false detections caused by spurious events like
electrical noise, the QT60040 incorporates a detection integration
counter that increments with each detection sample until a limit is
reached, at which point a detection is confirmed. If no detection is
sensed on any of the samples prior to the final count, the counter
is reset immediately to zero, forcing the process to restart. The
required count is 3 samples per key.
1.5.1 S
ELF
-C
ALIBRATION
The QT60040 is fully self-calibrating. On powerup it scans the
matrix and sets appropriate calibration points for each. No special
operator or factory calibration or circuit tweak is required to bring
keys into operation. The self calibration procedure typically
requires 1 second to complete.
1.5.2 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.
Drift compensation (Figure 1-9) 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 is
performed slowly, otherwise legitimate detections might be
ignored. The QT60040 drift compensates using a slew-rate
limited change to the reference level; the threshold and
hysteresis values are slaved to this reference.
The QT60040's drift compensation is 'asymmetric': the
drift-compensation occurs in one direction faster than it does in
the other. Specifically, it compensates faster for decreasing
loads. Increasing loads (more contact with an object, which
results in a decreasing signal) should be compensated for slowly,
so that sensitivity to an approaching finger is not affected.
Removal of an object is compensated for at a faster rate to allow
the sensor to recover quickly to prepare for the next valid touch.
Figure 1-8 Key Design Rules
X1
18x18mm
key size
0.75mm
gaps
0.5mm
lines
X shield
tails
X2
1.5.3 T
HRESHOLD AND
H
YSTERESIS
C
ALCULATIONS
The threshold value is established as an offset to the reference
level. As Cx and Cs drift over time, the reference drift
compensates with the changes and the threshold level is
automatically recomputed in real time so that it is never in error.
Since key touches result in negative signal swings, the threshold
is set below the signal reference level.
The QT60040 employs a hysteresis of 25% of the delta between
the reference and threshold levels. The signal must rise by 25%
of the distance from threshold to reference before the detection
event drops out and the key registers as untouched.
Common
Y Line
1.5.4 M
AX
O
N
-D
URATION
If a foreign object contacts a key the signal may
change enough to create a detection lasting for the
duration of the contact. To overcome this, the part
includes individual key timers which monitor detection
duration. If a detection on a key exceeds the timer limit
setting, the sensor will perform a full recalibration. This
is known as the Max On-Duration feature.
After the Max On-Duration interval has expired and the
recalibration has taken place, the key will once again
function normally even if still in contact with the foreign
object, to the best of its ability. The Max On-Duration
can be set to either 10 or 60 seconds of continuous
Figure 1-9 Drift Compensation
Reference
Hysteresis
Threshold
Signal
Output
lQ
-4-
QT60040 / R1.04 / 0303
QUANTUM [ QUANTUM RESEARCH GROUP ]
