Multiple touch electrodes connected to any SNSnK can be

used, for example, to create control surfaces on both sides of

an object.

It is important to limit the amount of stray capacitance on the

SNS terminals, for example by minimizing trace lengths and

widths to allow for higher gain without requiring higher values

of Cs. Under heavy delta-Cx loading of one key, cross

coupling to another key’s trace can cause the other key to

trigger. Therefore, electrode traces from adjacent keys

should not be run close to each other over long runs in order

to minimize cross-coupling if large values of delta-Cx are

expected, for example when an electrode is directly touched.

This is not a problem when the electrodes are working

through a plastic panel with normal touch sensitivity.

1.4 Sensitivity

1.4.1 Introduction

Sensitivity can be altered to suit various applications and

situations on a channel-by-channel basis. The easiest and

most direct way to impact sensitivity is to alter the value of

each Cs; more Cs yields higher sensitivity. Each channel has

its own Cs value and can therefore be independently

adjusted.

1.4.2 Alternative Ways to Increase

Sensitivity

Sensitivity can also be increased by using bigger

electrode areas, reducing panel thickness, or using

a panel material with a higher dielectric constant .

1.4.3 Decreasing Sensitivity

In some cases the circuit may be too sensitive.

Gain can be lowered further by a number of

strategies: a) making the electrode smaller, b)

making the electrode into a sparse mesh using a

high space-to-conductor ratio, or c) by decreasing

the Cs capacitors.

1.4.4 Key Balance

A number of factors can cause sensitivity

imbalances. Notably, SNS wiring to electrodes can

have differing stray amounts of capacitance to

ground. Increasing load capacitance will cause a

decrease in gain. Key size differences, and

proximity to other metal surfaces can also impact

gain.

The four keys may thus require ‘balancing’ to

achieve similar sensitivity levels. This can be best

accomplished by trimming the values of the four

Cs capacitors to achieve equilibrium. The four Rs

resistors have no effect on sensitivity and should

not be altered. Load capacitances can also be added to

overly sensitive channels to ground, to reduce their gains.

These should be in the order of a few picofarads.

2 QT240 Specifics

2.1 Signal Processing

2.1.1 Introduction

These devices process all signals using 16 bit math , using a

number of algorithms pioneered by Quantum. These

algorithms are specifically designed to provide for high

survivability in the face of adverse environmental changes.

2.1.2 Drift Compensation

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

Vdd over time. If a low grade Cs capacitor is chosen, the

signal can drift greatly with temperature. If keys are subject

to extremes of temperature or humidity, the signal can also

drift. It is crucial that drift be compensated, else false

detections, nondetections, and sensitivity shifts will follow.

Drift compensation (Figure 2.1) is a method that makes the

reference level track the raw signal at a slow rate, only while

no detection is in effect. The rate of reference

adjustment must be performed slowly else

legitimate detections can also be ignored. The IC

drift compensates each channel independently

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.

4

QT240R R1.11/1006

Figure 1.2 Fast, Spread-spectrum Circuit

S1

S3

S2

VDD

OUT4

OUT1

OUT2

OUT3

10nF

CS1

RS1

2.2K

C1

22nF

10nF

CS3

10nF

CS4

RS3

2.2K

VDD

1M

R2

22K

RSNS1

360K

R5

22K

RSNS4

RS4

2.2K

OPT1

10nF

CS2

RS2

2.2K

1M

R1

R6

180K

VDD

62K

R4

1M

R3

10 second

timeout shown

22K

RSNS2

SPEED

OPT

OPT2

22K

RSNS3

QT240_ISS

Figure 2.1 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output