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QT100 Datasheet(PDF) 4 Page - Quantum Research Group |
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QT100 Datasheet(HTML) 4 Page - Quantum Research Group |
4 / 10 page 2.5 Forced Sensor Recalibration The QT100 has no recalibration pin; a forced recalibration is accomplished when the device is powered up or after the recalibration timeout . However, supply drain is low so it is a simple matter to treat the entire IC as a controllable load; driving the QT100's VDD pin directly from another logic gate or a microcontroller port will serve as both power and 'forced recal'. The source resistance of most CMOS gates and microcontrollers are low enough to provide direct power without problem. 2.6 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, nondetections, and sensitivity shifts will follow. Drift compensation (Figure 2.5) 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 QT100 drift compensates using a slew-rate limited change to the reference level; the threshold and hysteresis values are slaved to this reference . 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 QT100's drift compensation is 'asymmetric'; the reference level drift-compensates in one direction faster than it does in the other. Specifically, it compensate s 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. 2.7 Response Time The QT100'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 Cs 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. 2.8 Spread Spectrum The QT100 modulates its internal oscillator by ±7.5% during the measurement burst. This spreads the ge nerated noise over a wider band reducing emission levels. This also reduces susceptibility since there is no longer a single fundamental burst frequency. 2.9 Output Features 2.9.1 Output The output of the QT100 is active-high upon detection. The output will remain active-high for the duration of the detection, or until the Max On-duration expires, whichever occurs first. If a Max On-duration timeout occurs first, the sensor performs a full recalibration and the output becomes inactive (low) until the next detection. lQ 4 QT100-ISG R3.06/0606 Figure 2.5 Drift Compensation Threshold Signal Hysteresis Reference Output Figure 2.7 Using a micro to obtain HeartBeat pulses in either output state Figure 2.6 Getting HeartBeat pulses with a pull-up resistor 6 4 5 2 ~ 5V 3 1 OUT SNS SYNC/MODE SNSK VSS VDD 2 Ro HeartBeat™ Pulses Microcontroller PORT _M .x PORT _M .y 6 4 3 1 OU T SN S SY N C /M OD E SN SK Ro |
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