9
QT113B [DATASHEET]
9525D–AT42–05/2013
3. QT113B Specifics
3.1 Signal Processing
The QT113B processes all signals using 16-bit math, using a number of algorithms pioneered by Atmel. The
algorithms are specifically designed to provide for high 'survivability' in the face of numerous adverse environmental
changes.
3.1.1 Drift Compensation Algorithm
Signal drift can occur because of changes in C
X
and C
S
over time. It is crucial that drift be compensated for,
otherwise false detections, non-detections, and sensitivity shifts will follow.
Drift compensation (Figure 3-1) 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 QT113B drift compensates using a slew-rate limited change to the reference level;
the threshold and hysteresis values are slaved to this reference.
Figure 3-1. Drift Compensation
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 QT113B 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 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.
With large values of C
S
and small values of C
X
, drift compensation will appear to operate more slowly than with the
converse. Note that the positive and negative drift compensation rates are different.
3.1.2 Threshold Calculation
The internal threshold level is fixed at one of two setting as determined by Table 2-1 on page 7. These settings are
fixed with respect to the internal reference level, which in turn will move in accordance with the drift compensation
mechanism.
The QT113B employs a hysteresis dropout below the threshold level of 17% of the delta between the reference and
threshold levels.
Threshold
Signal
Hysteresis
Reference
Output
