4-1 to 4-3). The value of Cs also has a dramatic effect on

sensitivity, and this can be increased in value (up to a limit).

Also, increasing the electrode's surface area will not

substantially increase touch sensitivity if its diameter is

already much larger in surface area than the object being

detected. The panel or other intervening material can be

made thinner, but again there are diminishing rewards for

doing so. Panel material can also be changed to one having

a higher dielectric constant, which will help propagate the

field through to the front. Locally adding some conductive

material to the panel (conductive materials essentially have

an infinite dielectric constant) will also help; for example,

adding carbon or metal fibers to a plastic panel will greatly

increase frontal field strength, even if the fiber density is too

low to make the plastic bulk-conductive.

1.3.5.2 Decreasing Sensitivity

In some cases the QT113 may be too sensitive, even on low

gain. In this case gain can be lowered further by a number of

strategies:

making

the

electrode smaller,

making

the

electrode

into

a

sparse

mesh

using

a

high

space-to-conductor ratio (Figure 1-3), or by decreasing Cs.

2 - QT113 SPECIFICS

2.1 SIGNAL PROCESSING

The QT113 processes all signals using 16 bit

math, using a number of algorithms pioneered by

Quantum.

The

algorithms

are

specifically

designed to provide for high 'survivability' in the

face

of

numerous

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.

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 QT113 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

QT113'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 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.

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.

Unlike the QT110 device, the internal threshold level is fixed

at one of two setting as determined by Table 1-1. These

setting are fixed with respect to the internal reference level,

which in turn can move in accordance with the drift

compensation mechanism..

The QT113 employs a hysteresis dropout

below the

threshold level of 17% of the delta between the reference and

threshold levels.

If an object or material obstructs the sense pad the signal

may rise enough to create a detection, preventing further

- 4 -

Figure 1-5

Shielding Against Fringe Fields

Sen se

wire

Sense

wire

U ns hielded

E lectrode

Shielded

E lec trode

Figure 2-1 Drift Compensation

Thr es ho ld

S ignal

H yste resis

Re fe re nce

Ou tpu t

Vss (Gnd)

Low - 12 counts

Vdd

High - 6 counts

Tie Pin 5 to:

Gain

Table 1-1 Gain Setting Strap Options