Data Sheet

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% 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 HeartBeat Output

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.

AT42QT1011

Datasheet

DS40001947A-page 13