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9542G–AT42–03/10

AT42QT1011

The QT1011'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.

3.7

Response Time

The QT1011'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.

3.8

Spread Spectrum

The QT1011 modulates its internal oscillator by ±7.5 percent 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.

3.9

Output Features

3.9.1

Output

The output of the QT1011 is active-high upon detection. The output will remain active-high for

the duration of the detection.

3.9.2

The QT1011 output has a HeartBeat “health” indicator superimposed on it in all modes. This

operates by taking the output pin into a three-state mode for 15 µs, once before every QT burst.

This output state can be used to determine that the sensor is operating properly, using one of

several simple methods, or it can be ignored.

The HeartBeat indicator can be sampled by using a pull-up resistor on the OUT pin (Figure 3-6),

and feeding the resulting positive-going pulse into a counter, flip flop, one-shot, or other circuit.

The pulses will only be visible when the chip is not detecting a touch.