QT102 Просмотр технического описания (PDF) - Quantum Research Group

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QT102

QTOUCH™ TOGGLE-MODE CHARGE-TRANSFER IC WITH POWER MANAGEMENT FUNCTIONS

Quantum Research Group 

1.5 Forced Sensor Recalibration

The QT102 has no recalibration pin; a forced recalibration is

accomplished when the device is powered up, after the

recalibration timeout or when the auto-off override is released.

However, supply drain is low so it is a simple matter to treat

the entire IC as a controllable load; driving the QT102'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 problems.

1.6 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.

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.

1.7 Response Time

The QT102's response time is highly dependent on 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.

Drift compensation (Figure 1.2) 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 QT102 drift compensates using a

slew-rate limited change to the reference level; the threshold

and hysteresis values are slaved to this reference.

1.8 Spread Spectrum

The QT102 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.

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 QT102'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.

T hr e s h o ld

Output

Figure 1.2 Drift Compensation

S ig n a l

H ysteresis

R eference

lQ

4

QT102_2R3.04_0807

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