© 2009 Microchip Technology, Inc.

DS41393A-Page 25

8

BASICS OF RESISTIVE SENSORS

8.1

Terminology

ITO (Indium Tin Oxide) is the resistive coating that makes up the active area of the touch sensor. ITO is a

transparent semiconductor that is sputtered onto the touch sensor layers.

Flex or Film or Topsheet is the top sensor layer that a user touches. Flex refers to the fact that the top layer

physically flexes from the pressure of a touch.

Stable or Glass is the bottom sensor layer that interfaces against the display.

Spacer Adhesive is a frame of adhesive that connects the Flex and Stable layers together around the perimeter.

Spacer Dots maintain physical and electrical separation between the Flex and Stable layers. The dots are

typically printed onto the Stable layer.

Bus Bars or Silver Frit electrically connect to the ITO on the Flex and Stable layers to the sensor’s interface tail.

Bus bars are typically screen printed silver ink. They are usually much lower in resistivity than the ITO.

X-Axis is the left and right direction on the touch sensor.

Y-Axis is the top and bottom direction on the touch sensor.

Drive Lines supply a voltage gradient across the sensor.

8.2

General

Resistive 4, 5, and 8-wire touch sensors consist of two facing conductive layers, held in physical separation from

each other. The force of a touch causes the top layer to move and make electrical contact with the bottom layer.

Touch position measurements are typically made by applying a linear voltage gradient across a layer or axis of

the touch sensor. The touch position voltage for the axis can be measured using the opposing layer.

A comparison of typical sensor constructions is shown below.

Sensor

Comments

4 Wire

Less expensive than 5-wire or 8-wire

Lower power than 5-wire

More linear (without correction) than 5-wire

Touch inaccuracies occur from flex layer damage or resistance changes

5 Wire

Maintains touch accuracy with flex layer damage

Inherent non-linearity often requires touch data correction

Touch inaccuracies occur from resistance changes

8 Wire

More expensive than 4 wire

Lower power than 5 wire

More linear (without correction) than 5-wire

Touch inaccuracies occur from flex layer damaged

Maintains touch accuracy with resistance changes

Table 10 : Sensor Comparison