Fundamentals of Designing with Sensors

The World Leader in High Performance Signal Processing Solutions

FUNDAMENTALS OF DESIGN

Class 1

Introduction

Presented by David Kress

The Goal

Capture what is going on in the real world

Convert into a useful electronic format

Analyze, Manipulate, Store, and Send

Return to the real world

The real world is NOT digital

Analog to Electronic signal processing

Sensor

(INPUT)Digital ProcessorAmp Converter

Actuator

(OUTPUT)Amp Converter

The Sensor

Sensor

(INPUT)Digital ProcessorAmp Converter

Actuator

(OUTPUT)Amp Converter

Analog, but

NOT

electronic

Analog

AND

electronic

Popular sensors

Sensor Type Output

Thermocouple Voltage

Photodiode Current

Strain Gauge Resistance

Microphone Capacitance

Touch Button Charge Output

Antenna Inductance

Thermocouple

Very low level (µV/ºC)

Non-linear

Difficult to handle

Wires need insulation

Susceptible to noise

Fragile

Sensor Signal Conditioning

Sensor Amp

Analog,

electronic,

but “dirty”

Analog,

electronic,

and “clean”

•Amplify the signal to a noise-resistant level

•Lower the source impedance

•Linearize (sometimes but not always)

•Filter

•Protect

Types of Temperature Sensors

THERMOCOUPLE RTD THERMISTOR SEMICONDUCTOR

Widest Range:

–184ºC to +2300ºC

Range:

–200ºC to +850ºC

Range:

0ºC to +100ºC

Range:

–55ºC to +150ºC

High Accuracy and

Repeatability

Fair Linearity Poor Linearity Linearity: 1ºC

Accuracy: 1ºC

Needs Cold Junction

Compensation

Requires

Excitation

Requires

Excitation

Requires Excitation

Low-Voltage Output Low Cost High Sensitivity 10mV/K, 20mV/K,

or 1µA/K Typical

Output

Common Thermocouples

JUNCTION MATERIALS

TYPICAL

USEFUL

RANGE (ºC)

NOMINAL

SENSITIVITY

(µV/ºC)

ANSI

DESIGNATION

Platinum (6%)/ Rhodium-

Platinum (30%)/Rhodium

38 to 1800 7.7 B

Tungsten (5%)/Rhenium -

Tungsten (26%)/Rhenium

0 to 2300 16 C

Chromel - Constantan 0 to 982 76 E

Iron - Constantan 0 to 760 55 J

Chromel - Alumel –184 to 1260 39 K

Platinum (13%)/Rhodium-

Platinum

0 to 1593 11.7 R

Platinum (10%)/Rhodium-

Platinum

0 to 1538 10.4 S

Copper-Constantan –184 to 400 45 T

Thermocouple Output Voltages

for Type J, K and S Thermocouples

-250 0 250 500 750 1000 1250 1500 1750

-10

0

10

20

30

40

50

60T

HE

RM

OC

OU

PL

E O

UT

PU

T V

OL

TA

GE

(m

V)

TEMPERATURE (°C)

TYPE J

TYPE K

TYPE S

-250 0 250 500 750 1000 1250 1500 1750

-10

0

10

20

30

40

50

60T

HE

RM

OC

OU

PL

E O

UT

PU

T V

OL

TA

GE

(m

V)

TEMPERATURE (°C)

TYPE J

TYPE K

TYPE S

Thermocouple Seebeck Coefficient vs.

Temperature

-250 0 250 500 750 1000 1250 1500 1750

0

10

20

30

40

50

60

70S

EE

BE

CK

CO

EF

FIC

IEN

T -

µV

/ °C

TEMPERATURE (°C)

TYPE J

TYPE K

TYPE S

-250 0 250 500 750 1000 1250 1500 1750

0

10

20

30

40

50

60

70S

EE

BE

CK

CO

EF

FIC

IEN

T -

µV

/ °C

TEMPERATURE (°C)

TYPE J

TYPE K

TYPE S

Thermocouple Basics

T1

Metal A

Metal B

Thermoelectric

EMF

RMetal A Metal A

R = Total Circuit Resistance

I = (V1 – V2) / R

V1 T1 V2T2

V1 – V2

Metal B

Metal A Metal A

V1

V1

T1

T1

T2

T2

V2

V2

V

Metal AMetal A

Copper Copper

Metal BMetal B

T3 T4

V = V1 – V2, If T3 = T4

A. THERMOELECTRIC VOLTAGE

B. THERMOCOUPLE

C. THERMOCOUPLE MEASUREMENT

D. THERMOCOUPLE MEASUREMENT

I

V1 T1

Metal A

Metal B

Thermoelectric

EMF

RMetal A Metal A

R = Total Circuit Resistance

I = (V1 – V2) / R

V1 T1 V2T2

V1 – V2

Metal B

Metal A Metal A

V1

V1

T1

T1

T2

T2

V2

V2

V

Metal AMetal A

Copper Copper

Metal BMetal B

T3 T4

V = V1 – V2, If T3 = T4

A. THERMOELECTRIC VOLTAGE

B. THERMOCOUPLE

C. THERMOCOUPLE MEASUREMENT

D. THERMOCOUPLE MEASUREMENT

I

V1

Using a Temperature Sensor for Cold-

Junction Compensations

TEMPERATURE

COMPENSATION

CIRCUIT

TEMP

SENSORT2V(T2)T1 V(T1)

V(OUT)

V(COMP)

SAME

TEMP

METAL A

METAL B

METAL A

COPPERCOPPER

ISOTHERMAL BLOCKV(COMP) = f(T2)

V(OUT) = V(T1) – V(T2) + V(COMP)

IF V(COMP) = V(T2) – V(0°C), THEN

V(OUT) = V(T1) – V(0°C)

TEMPERATURE

COMPENSATION

CIRCUIT

TEMP

SENSORT2V(T2)T1 V(T1)

V(OUT)

V(COMP)

SAME

TEMP

METAL A

METAL B

METAL A

COPPERCOPPER

ISOTHERMAL BLOCKV(COMP) = f(T2)

V(OUT) = V(T1) – V(T2) + V(COMP)

IF V(COMP) = V(T2) – V(0°C), THEN

V(OUT) = V(T1) – V(0°C)

AD594/AD595 Monolithic Thermocouple

Amplifier with Cold-Junction Compensation

ICE

POINT

COMP

+

OVERLOAD

DETECT

VOUT10mV/°C

+5V

BROKEN

THERMOCOUPLE

ALARM

4.7k

G

+

–TC––

+TC+

+ATHERMOCOUPLE

G

AD594/AD595

TYPE J: AD594

TYPE K: AD595

0.1µF

ICE

POINT

COMP

+

OVERLOAD

DETECT

VOUT10mV/°C

+5V

BROKEN

THERMOCOUPLE

ALARM

4.7k

G

+

–TC––

+TC+

+ATHERMOCOUPLE

G

AD594/AD595

TYPE J: AD594

TYPE K: AD595

0.1µF

Basic Relationships For Semiconductor

Temperature Sensors

IC IC

VBE VN

VBE VBE VNkT

qN ln( )

VBEkT

q

ICIS

ln VN

kT

q

ICN IS

ln

INDEPENDENT OF IC, IS

ONE TRANSISTORN TRANSISTORS

Classic Bandgap Temperature Sensor

"BROKAW CELL"R R

+I2 @ I1

Q2

NA

Q1

A

R2

R1

VN VBE(Q1)

VBANDGAP = 1.205V

+VIN

VPTAT = 2R1

R2

kTq

ln(N)

VBE VBE VNkT

qN ln( )

Analog Temperature Sensors

Product Accuracy

(Max)

Max Accuracy

Range

Operating

Temp

Range

Supply

Range

Max

Current

Interface Package

AD590± 0.5°C

± 1.0°C

25°C

-25°C to 105°C

-55°C to

150°C4 to 30V 298uA Current Out

TO-52,2-ld FP,

SOIC, Die

AD592± 0.5°C

± 1.0°C

25°C

-55°C to 150°C

-25°C to

105°C4 to 30V

298uACurrent Out TO-92

TMP35 ± 2.0°C0°C to 85°C

-25°C to 100°C

-55°C to

150°C2.7 to 5.5V 50uA Voltage Out

TO-92, SOT23,

SOIC

TMP36 ± 3.0°C-40°C to 125°C -55°C to

150°C2.7V to 5.5V

50uAVoltage Out

TO-92, SOT23,

SOIC

AD22100± 2.0°C -50°C to 150°C -50°C to

150°C4 to 6.5V 650uA Voltage Out TO-92, SOIC, Die

AD22103± 2.5°C 0°C to 100°C 0°C to 100°C

2.7 to 3.6V 600uA Voltage Out TO-92, SOIC

Digital Temperature Sensors

Comprehensive Portfolio of Accuracy Options

21

Product Accuracy (Max) Max Accuracy

Range

Interface Package

ADT7420/7320± 0.2°C

± 0.25°C

-10°C to 85°C

-20°C to 105°CI2C/SPI LFCSP

ADT7410/7310 ± 0.5°C -40°C to 105°C I2C/SPI SOIC

ADT75± 1°C (B grade)

± 2°C (A grade)

0°C to 85°C

-25°C to 100°CI2C MSOP, SOIC

ADT7301± 1°C 0°C to 70°C

SPI SOT23, MSOP

TMP05/6± 1°C 0°C to 70°C

PWM SC70, SOT23

AD7414/5± 1.5°C -40°C to 70°C

I2C SOT23,MSOP

ADT7302 ± 2°C 0°C to 70°C SPI SOT23,MSOP

TMP03/4± 4°C

-20°C to 100°C PWM TO-92,SOIC,TSSOP

Position and Motion Sensors

Linear Position: Linear Variable Differential Transformers

(LVDT)

Hall Effect Sensors

Proximity Detectors

Linear Output (Magnetic Field Strength)

Rotational Position:

Optical Rotational Encoders

Synchros and Resolvers

Inductosyns (Linear and Rotational Position)

Motor Control Applications

Acceleration and Tilt: Accelerometers

Gyroscopes

LVDT – Linear Variable Differential

Transformer

~AC

SOURCE

VOUT = VA – VB

+

_

VOUT

POSITION+_

VOUT

POSITION+_

VA

VB

1.75"

THREADED

CORE

SCHAEVITZ

E100

A

B

AD698 LVDT Signal Conditioner

(Simplified)

AMP ~

+

_

FILTER AMP

VB

VOUT

AD698

EXCITATION

4-WIRE LVDT

OSCILLATORA

B

VA

REFERENCE

A, B = ABSOLUTE VALUE + FILTER

Hall Effect Sensors

I I

T

B

VH

CONDUCTOR

OR

SEMICONDUCTOR

I = CURRENT

B = MAGNETIC FIELD

T = THICKNESS

VH = HALL VOLTAGE

AD22151 Linear Output Magnetic

Field Sensor

_

+

CHOPPER

AMP

VCC / 2

R1

R2

R3

OUTPUT

AMP

VCC = +5V

VCC / 2

TEMP

REF

+

_

VOUT = 1 + R3

R20.4mV Gauss NONLINEARITY = 0.1% FS

AD22151 VOUT

Accelerometer Applications

Tilt or Inclination

Car Alarms

Patient Monitors

Cell phones

Video games

Inertial Forces

Laptop Computer Disc Drive Protection

Airbag Crash Sensors

Car Navigation systems

Elevator Controls

Shock or Vibration

Machine Monitoring

Control of Shaker Tables

ADI Accelerometer Fullscale g-Range: ± 2g to ± 100g

ADI Accelerometer Frequency Range: DC to 10kHz

ADXL-family Micro-machined

Accelerometers

FIXED

OUTER

PLATES

CS1 CS1< CS2= CS2

DENOTES ANCHOR

BEAM

TETHER

CS1 CS2

CENTER

PLATE

AT REST APPLIED ACCELERATION

Using an Accelerometer to Measure Tilt

X

0°

+90°

1g

Acceleration

X

–90°

–1g

0°

+1g

+90°

Acceleration = 1g × sin

0g

–90°

X

0°

+90°

1g

Acceleration

X

–90°

–1g

0°

+1g

+90°

Acceleration = 1g × sin

0g

–90°

Gyro Axes of Rotational Sensitivity

Coriolis acceleration example.

Displacement due to the Coriolis Effect

Photograph of mechanical sensor.

High Impedance Sensors

Photodiodes

Piezoelectric Sensors

Accelerometers

Hydrophones

Humidity Monitors

pH Monitors

Chemical Sensors

Smoke Detectors

Charge Coupled Devices and

Contact Image Sensors for Imaging

Photodiode Equivalent Circuit

PHOTO

CURRENTIDEAL

DIODE

INCIDENT

LIGHT

RSH(T)

100k -

100G

CJ

NOTE: RSH HALVES EVERY 10 C TEMPERATURE RISE

Current-to-voltage Converter (Simplified)

ISC = 30pA

(0.001 fc)

+

_

R = 1000M

VOUT = 30mV

Sensitivity: 1mV / pA

Preamplifier DC Offset Errors

~

VOS

IB

IB

R1

R21000M

+

_

IB DOUBLES EVERY 10 C TEMPERATURE RISE

R1 = 1000M @ 25 C (DIODE SHUNT RESISTANCE)

R1 HALVES EVERY 10 C TEMPERATURE RISE

DC NOISE GAIN = 1 + R2

R1

OFFSET

RTO

R3

R3 CANCELLATION RESISTOR NOT EFFECTIVE

Sensor Resistances Used In Bridge

Circuits Span A Wide Dynamic Range

Strain Gages 120, 350, 3500

Weigh-Scale Load Cells 350 - 3500

Pressure Sensors 350 - 3500

Relative Humidity 100k - 10M

Resistance Temperature Devices (RTDs) 100 , 1000

Thermistors 100 - 10M

Wheatstone Bridge Produces An Output Null

When The Ratios Of Sidearm Resistances Match

THE WHEATSTONE BRIDGE:

VO

R4

R1

R3

R2

VB

+

+

R3R2

R2

R4R1

R1VV BO

AT BALANCE,

VO R3

R2

R4

R1if0=

Output Voltage Sensitivity And Linearity Of Constant Current Drive

Bridge Configurations Differs According To The Number Of Active

Elements

R R

R R+R

R+R

R+R R+R R+R

RR R+R RRR R

R RR

VOVO VO

VO

IB IB IB IB

VO:

Linearity

Error:0.25%/% 0 0 0

IBR

4

R

R

4R +

IB2

R IB RIB2

R

(A) Single-Element

Varying

(B) Two-Element

Varying (1)(C) Two-Element

Varying (2)

(D) All-Element

Varying

R

R R

R

+

IN AMP

REF VOUT

RG

+VS

-VS*R+R

* SEE TEXT REGARDING

SINGLE-SUPPLY OPERATION

OPTIONAL RATIOMETRIC OUTPUTVB

VREF

= VB

VB4

R

R

2R +

VOUT = GAIN

A Generally Preferred Method Of Bridge Amplification Employs

An Instrumentation Amplifier For Stable Gain And High CMR

Upcoming webcasts

Converter Simulation: Beyond the Eval Board January 19th at 3:00 p.m. (ET)

RF Detectors February 16th at Noon (ET)

Challenges in Embedded Design for real-time systems

March 16th at Noon (ET)

www.analog.com/webcast

Fundamentals Webcasts 2011

January Introduction and Fundamentals of Sensors

February The Op Amp

March Beyond the Op Amp

April Converters, Part 1, Understanding Sampled Data Systems

May Converters, Part 2, Digital-to-Analog Converters

June Converters, Part 3, Analog-to-Digital Converters

July Powering your circuit

August RF: Making your circuit mobile

September Fundamentals of DSP/Embedded System design

October Challenges in Industrial Design

November Tips and Tricks for laying out your PC board

December Final Exam, Ask Analog Devices

www.analog.com/webcast