ANALOG DEVICES AN-830 Service Manual

AN-830

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: [phone] • Fax: [phone] • www.analog.com

Factors Affecting Sensor Response

by Susan Pratt

INTRODUCTION

Capacitance sensing has the potential to replace current user
input mechanisms in consumer devices. Products as diverse as
cell phones, digital cameras, MP3 players, and other portable
media players are all suitable for implementing capacitance
sensing. Capacitance sensing gives the user an interface with
greater sensitivity and control than standard mechanical input
technologies.

Analog Devices’ capacitance sensing solution has three
components: the AD7142 capacitive-to-digital converter IC,
sensors on the PCB, and software to communicate with the
AD7142. The solution consists of an excitation source
connected to a transmitter, which generates a capacitive field to
a receiver. The capacitive field lines measured at the receiver are
translated into the digital domain by a Σ-Δ analog-to-digital
converter. The total capacitance measured at the receiver
decreases when a grounded object, such as a finger, comes close
to the induced capacitive field. The excitation source and Σ-Δ
CDC are implemented on the AD7142, while the transmitter
and receiver are constructed on the sensor PCB.

The sensor PCB is glued to the underside of the case or
covering of the finished product. The capacitive field lines
extend above the sensor PCB for about 4 mm. The field also
extends above any covering material over the sensor PCB. One
advantage of this sensor arrangement is that the user is never in
contact with the sensor PCB itself, so there is no wear on the
sensor.

The case or covering material housing consumer products such
as MP3 players, digital still cameras, and handsets, is made from
a variety of materials. Materials such as plastic or glass are
suitable covering materials for use with capacitance sensing;
metal cannot be used.

The response of the capacitance sensor depends on three
factors:

• The size and type of the sensor element

• The size of the object touching the sensor

• The thickness and type of the covering material

Each of these factors affects the magnitude of change measured
by the CDC when the sensor is touched. If the change in CDC
output is very small, then it becomes difficult to differentiate
between the sensor-touched and the sensor-not-touched
conditions. This application note details how each of these
factors affects the sensor response and can be used as a
guideline when deciding the size and form of the sensor

RX

16-BIT

DATA

Σ-Δ

ADC

Figure 1. Capacitance Sensing

TX

EXCITATION
SIGNAL
240kHz

05849-001

configuration, as well as the covering plastic specification.

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AN-830

TABLE OF CONTENTS

Introduction ...................................................................................... 1

Table of Contents.............................................................................. 2

Revision History ........................................................................... 2

Factors Affecting Sensor Response ................................................ 3

Sensor Element ............................................................................. 3

Object Touching the Sensor........................................................ 3

Covering Material......................................................................... 3

Button Sensor.................................................................................... 4

Button Sensor Response .............................................................. 4

Slider Sensor...................................................................................... 5

Slider Sensor Response ................................................................ 5

Recommendations............................................................................ 6

REVISION HISTORY

12/05—Revision 0: Initial Version

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AN-830

FACTORS AFFECTING SENSOR RESPONSE

SENSOR ELEMENT

The size of the sensor element determines the size of the
capacitive field induced between the transmitter and receiver.
A smaller sensor element has a smaller field to interfere with
than a larger sensor element. If the sensor element is too small,
there is not a sufficient change in capacitance measured by the
CDC when the sensor is touched.

The type of sensor element is also important. For a button
sensor, only on/off or touch/no touch information is required.
A button can tolerate some loss of sensor response, as long as it
is possible to determine if the button is touched or not. A slider
sensor, however, must output position data relating to the
length of the slider. A reduction in sensor response for a slider
reduces the number of CDC codes that are used to describe a
full traverse of the slider, thus affecting the resolution and
accuracy of the slider sensor position data.

OBJECT TOUCHING THE SENSOR

For all applications, the object touching the sensor is a finger or
hand, which is naturally grounded. However, the size of the
object touching the sensor is not constant; finger size can vary
from person to person, or indeed the same person can use
different fingers at different times to activate the sensors.
Consumer devices need to be designed for a range of finger
sizes, from small to large, to ensure that everyone can operate
the device successfully.

Any grounded object can activate Analog Devices’ sensors. For
this application note, a grounded metal probe was used to
simulate a finger during the data gathering experiments. Three
probes of different sizes were used to simulate different finger
sizes: 5 mm, 10 mm, and 15 mm diameter probes.

COVERING MATERIAL

The properties of the material covering the sensor must be
looked at closely. The capacitive field extends about 4 mm to 5
mm above the sensor PCB. This field must extend above any

covering material in order for the sensor to work. The material
must not absorb too much of the capacitive field. Some types of
plastic are more conductive than others, and so more of the
capacitive field gets through.
factor for a variety of plastic polymers. The dissipation factor is
a measure of how lossy the material is. The lower the dissipation
factor, the more of the capacitive field passes through the
material.

Table 1.

Polymer Material

LDPE 0.15 0.08
HDPE 0.24 .20
PP 0.4 0.5
PVC-plasticized 80 120
PS 0.1 to 0.4 0.05 to 0.4
ABS 3 to 8 2 to15
PMMA 40 to 60 4 to 40
POM 5 5
PTFE 0.2 0.2
PCTFE 1 20
PC 0.7 10
PET 2 20
PI 2 5
PUR-linear 120 70
PUR-thermoset 50 50
PUR-thermoplas 30 60
CAB 6 21
Silicone 5 to 13 7

Glass is also a suitable covering material. However, metal
cannot be used as a covering material

For this application note, the sensor PCBs were covered in ABS
that ranged in thickness from 0.5 mm to 4 mm.

Tabl e 1 shows the dissipation

Dissipation Factor (x 10-3)

@50 Hz @1 MHz

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