at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 8

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

3. Detailed Operation
3.1
3.1.1
3.1.2
8
Signal Processing
AT42QT113B
Introduction
Drift Compensation Algorithm
Decreasing Sensitivity
In some cases the QT113B may be too sensitive, even on low gain. In this case gain can be
lowered further by decreasing Cs.
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.
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 (see
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.
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’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 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 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.
Threshold
Output
Drift Compensation
Signal
Figure 3-1 on page
R eference
H ysteresis
8) is performed by making the reference level track
9525B–AT42–09/10