qt100a Quantum Research Group, qt100a Datasheet - Page 5

• Manufacturer Part Number: qt100a
• Description: Charge-transfer Qtouch? Ic
• Manufacturer: Quantum Research Group

Max On-duration feature and is set to ~80s (at 3V). This will
vary slightly with Cs and if Sync mode is used. As the
internal timebase for Max On-duration is determined by the
burst rate, the use of Sync can cause dramatic changes in
this parameter depending on the Sync pulse spacing.
2.4 Detect Integrator
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish
this, the QT100A incorporates a ‘detect integration’ (DI)
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 QT100A, the required count is
four. In LP mode the device will switch to Fast mode
temporarily in order to resolve the detection more quickly;
after a touch is either confirmed or denied the device will
revert back to normal LP mode operation automatically.
The DI can also be viewed as a 'consensus' filter, that
requires four successive detections to create an output.
2.5 Forced Sensor Recalibration
The QT100A has no recalibration pin; a forced recalibration
is accomplished when the device is powered up or after the
recalibration timeout. However, supply drain is low so it is a
simple matter to treat the entire IC as a controllable load;
driving the QT100A's Vdd pin directly from another logic gate
or a microcontroller port will serve as both power and 'forced
recal'. The source resistance of most CMOS gates and
microcontrollers are low enough to provide direct power
without problem.
2.6 Drift Compensation
Signal drift can occur because of changes in Cx and Cs over
time. It is crucial that drift be compensated for, otherwise
false detections, nondetections, and sensitivity shifts will
follow.
Drift compensation (Figure 2.5). 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 QT100A drift compensates using a
slew-rate limited change to the reference level; the threshold
and hysteresis values are slaved to this reference.

Getting HeartBeat pulses with a pull-up resistor
HeartBeat™ Pulses
Figure 2.6
Ro
1
OUT
VDD
VDD
VSS
2
5
SNSK
SYNC
SNS
3
4
6
5
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 QT100A'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 approaching the sense electrode.
However, an obstruction over the sense pad, for which the
sensor has already made full allowance, 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.
With large values of Cs and small values of Cx, drift
compensation will appear to operate more slowly than with
the converse. Note that the positive and negative drift
compensation rates are different.
2.7 Response Time
The QT100A's response time is highly dependent on run
mode and burst length, which in turn is dependent on Cs and
Cx. With increasing Cs, response time slows, while
increasing levels of Cx reduce response time. The response
time will also be a lot slower in LP or Sync mode due to a
longer time between burst measurements.
Microcontroller
Using a micro to obtain HeartBeat pulses in either output state
Threshold
Output
Figure 2.5 Drift Compensation
Port_M.x
Port_M.y
S ignal
Figure 2.7
R0
R eference
 
C opyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
1
OUT
H ysteresis
SYNC
SNSK
SNS
3
4
6