Datasheet

6
AT42QT1040 [DATASHEET]
9524D–AT42–05/2013
2.2.5 Auto 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, non-detections, and sensitivity shifts will follow.
Drift compensation 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.
Once an object is sensed and a key is in detect, the drift compensation mechanism ceases, since the signal is
legitimately high and should not therefore cause the reference level to change.
The QT1040 drift compensation is asymmetric, that is, the reference level drift-compensates in one direction faster
than it does in the other. Specifically, it compensates faster for decreasing (towards touch) signals than for
increasing (away from touch) signals. The reason for this difference in compensation rates is that increasing signals
should not be compensated for quickly, since a nearby finger could be compensated for partially or entirely before
even approaching the sense electrode. However, decreasing signals need to be compensated for more quickly. For
example, 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.
Negative drift (that is, towards touch) occurs at a rate of ~3 seconds, while positive drift occurs at a rate of
~1 second.
Drifting only occurs when no keys are in detect state.
2.2.6 Response Time
The QT1040 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 slower in slow mode due to a longer time between burst measurements. This mode offers an increased
detection latency in favor of reduced average current consumption.
2.2.7 Spread Spectrum
The QT1040 modulates its internal oscillator by ±7.5% during the measurement burst. This spreads the generated
noise over a wider band reducing emission levels. This also reduces susceptibility since there is no longer a single
fundamental burst frequency.
2.2.8 Max On-duration
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 known as the Max On-duration feature which monitors
detections. If a detection exceeds the timer setting, the sensor performs an automatic recalibration. Max On-duration
is set to ~30s.