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;

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 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.

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 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.

2.8 Spread Spectrum

The QT100 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.

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.

5

QT100_3R0.09_0707

Figure 2.5 Drift Compensation

Threshold

S ignal

H ysteresis

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

VDD

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