2 - QT110 SPECIFICS

2.1 SIGNAL PROCESSING

The QT110 processes all signals using a number of algorithms

pioneered by Quantum. The algorithms are specifically

designed to provide for high 'survivability' in the face of all kinds

of adverse environmental changes.

Signal drift can occur because of changes in Cx and Cs over

time. It is crucial that drift be compensated for, otherwise false

detections, non-detections, and sensitivity shifts will follow. Cs

drift has almost no effect on gain since the threshold method

used is ratiometric. However Cs drift can still cause false

detections if the drift occurs rapidly.

Drift compensation (Figure 2-1) 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 QT110 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 QT110'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

touching the sense pad. However, an obstruction over the

sense pad, for which the sensor has already made full

allowance for, 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.

Sensitivity is dependent on the threshold level as well as ADC

gain; threshold in turn is based on the internal signal reference

level plus a small differential value. The threshold value is

established as a percentage of the absolute signal level. Thus,

sensitivity remains constant even if Cs is altered dramatically,

so long as electrode coupling to the user remains constant.

Furthermore, as Cx and Cs drift, the threshold level is

automatically recomputed in real time so that it is never in error.

The QT110 employs a hysteresis dropout below the threshold

level of 50% of the delta between the reference and threshold

levels.

The threshold setting is determined by option jumper; see

Section 1.3.4.

If an object or material obstructs the sense pad the

signal may rise enough to create a detection,

preventing further operation. To prevent this, the

sensor includes a timer which monitors detections.

If a detection exceeds the timer setting, the timer

causes the sensor to perform a full recalibration.

This is known as the Max On-Duration feature.

After the Max On-Duration interval, the sensor will

once again function normally, even if partially or

fully obstructed, to the best of its ability given

electrode conditions. There are two nominal

timeout durations available via strap option: 10 and

60 seconds. The accuracy of these timeouts is

approximate.

It is desirable to suppress detections generated by electrical

noise or from quick brushes with an object. To accomplish this,

the QT110 incorporates a detect integration counter that

increments with each detection until a limit is reached, after

which the output is activated. If no detection is sensed prior to

the final count, the counter is reset immediately to zero. In the

QT110, the required count is 4.

The Detection Integrator can also be viewed as a 'consensus'

filter, that requires four detections in four successive bursts to

create an output. As the basic burst spacing is 75ms, if this

spacing was maintained throughout all 4 counts the sensor

would react very slowly. In the QT110, after an initial detection

is sensed, the remaining three bursts are spaced about 20ms

apart, so that the slowest reaction time possible is

75+20+20+20 or 135ms and the fastest possible is 60ms,

depending on where in the initial burst interval the contact first

occurred. The response time will thus average about 95ms.

The QT110 has no recalibration pin; a forced recalibration is

accomplished only when the device is powered up. However,

the supply drain is so low it is a simple matter to treat the entire

IC as a controllable load; simply driving the QT110's Vdd pin

directly from another logic gate or a microprocessor port

(Figure 2-2) will serve as both power and 'forced recal'. The

source resistance of most CMOS gates and microprocessors is

low enough to provide direct power without any problems.

Almost any CMOS logic gate can directly power the QT110.

A 0.01uF minimum bypass capacitor close to the device is

essential; without it the device can break into high frequency

oscillation.

Option strap configurations are read by the QT110 only on

powerup. Configurations can only be changed by powering the

QT110 down and back up again; again, a microcontroller can

directly alter most of the configurations and cycle power to put

them in effect.

2.2 OUTPUT FEATURES

The devices are designed for maximum flexibility and can

accommodate most popular sensing requirements. These are

selectable using strap options on pins OPT1 and OPT2. All

options are shown in Table 2-1.

OPT1 and OPT2 should never be left floating. If they are

floated, the device will draw excess power and the options will

not be properly read on powerup. Intentionally, there are no

pullup resistors on these lines, since pullup resistors add to

power drain if the pin(s) are tied low.

The output of the device can respond in a DC mode, where the

output is active-low upon detection. The output will remain

active for the duration of the detection, or until the Max

4

QT110 R1.04/0405

Figure 2-1 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output