Analog Electronics for Measuring Systems

168 Pages·2017·2.31 MB·English

by  Davide Bucci

Analog Electronics for
Measuring Systems
Davide Bucci
First published 2017 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
ISTE Ltd John Wiley & Sons, Inc.
27-37 St George’s Road 111 River Street
London SW19 4EU Hoboken, NJ 07030
UK USA
www.iste.co.uk www.wiley.com
© ISTE Ltd 2017
Library of Congress Control Number: 2017930069

British Library Cataloguing-in-Publication Data
A CIP record for this book is available from the British Library
ISSN 2051-2481 (Print)
ISSN 2051-249X (Online)
ISBN 978-1-78630-148-2
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Chapter 1. Fundamentals of Sensing
and Signal Conditioning . . . . . . . . . . . . . . . . . . . . 1
1.1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2.Voltagegeneratingsensors . . . . . . . . . . . . . . . . . 1
1.2.1. Generaldescription . . . . . . . . . . . . . . . . . . 1
1.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 2
1.3.Currentgeneratingsensors . . . . . . . . . . . . . . . . . 9
1.3.1. Generaldescription . . . . . . . . . . . . . . . . . . 9
1.3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.3. Conditioningcircuits . . . . . . . . . . . . . . . . . 15
1.4.Chargegeneratingsensors . . . . . . . . . . . . . . . . . 19
1.4.1. Generaldescription . . . . . . . . . . . . . . . . . . 19
1.4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 20
1.4.3. Conditioning . . . . . . . . . . . . . . . . . . . . . 22
1.5.Resistivesensors . . . . . . . . . . . . . . . . . . . . . . 24
1.5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 25
1.5.2. Caveats. . . . . . . . . . . . . . . . . . . . . . . . . 29
1.5.3. Signalconditioning:measuringthetotalresistance 29
1.5.4. Measuringaresistancevariation:
theWheatstonebridge . . . . . . . . . . . . . . . . 31
1.6.Reactivesensors . . . . . . . . . . . . . . . . . . . . . . . 36
1.7.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Chapter 2. Amplification and Amplifiers . . . . . . . . . 39
2.1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.2.Introductiontooperationalamplifiers . . . . . . . . . . . 40
2.2.1. Theoperationalamplifierasadifferentialamplifier 40
2.2.2. Modelingidealoperationalamplifiers . . . . . . . 41
2.3.Limitationsofrealoperationalamplifiers. . . . . . . . . 42
2.3.1. Saturationandrail-to-railoperationalamplifiers . 42
2.3.2. Inputoffset . . . . . . . . . . . . . . . . . . . . . . 43
2.3.3. Commonmoderejectionratio . . . . . . . . . . . . 44
2.3.4. Biascurrents . . . . . . . . . . . . . . . . . . . . . 44
2.3.5. Stabilityandfrequencyresponse . . . . . . . . . . 45
2.3.6. Examples . . . . . . . . . . . . . . . . . . . . . . . 46
2.4.Instrumentationamplifiers . . . . . . . . . . . . . . . . . 47
2.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 47
2.4.2. Differentialamplifierwithoneoperationalamplifier 48
2.4.3. Differentialamplifierwithtwooperational
amplifiers . . . . . . . . . . . . . . . . . . . . . . . 51
2.4.4. Differentialamplifierwiththreeoperational
amplifiers . . . . . . . . . . . . . . . . . . . . . . . 53
2.5.Isolationamplifiers . . . . . . . . . . . . . . . . . . . . . 57
2.6.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Chapter 3. Elements of Active Filter Synthesis . . . . . 61
3.1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.2.Low-passfilterapproximation . . . . . . . . . . . . . . . 64
3.2.1. Aliasinginsampledsystemsandanti-aliasingfilters 64
3.2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 65
3.2.3. All-polefilters:normalizationandfactorization . . 66
3.2.4. Butterworthapproximation . . . . . . . . . . . . . 69
3.2.5. Chebyshevapproximation . . . . . . . . . . . . . . 71
3.2.6. Bessel–Thompsonapproximation . . . . . . . . . . 73
3.2.7. Examples . . . . . . . . . . . . . . . . . . . . . . . 75
3.3.Activefiltersynthesisbymeansofstandardcells . . . . 76
3.3.1. Low-passSallen-Keycell:apairofcomplex
conjugatepoles . . . . . . . . . . . . . . . . . . . . 77
3.3.2. Low-passactiveRCcell:arealnegativepole . . . 79
3.3.3. Cellorder . . . . . . . . . . . . . . . . . . . . . . . 80
3.4.Frequencytransformtechniques . . . . . . . . . . . . . . 82
3.4.1. High-passfilters. . . . . . . . . . . . . . . . . . . . 83
3.4.2. Band-passfilters . . . . . . . . . . . . . . . . . . . 84
3.4.3. Band-reject(notch)filters . . . . . . . . . . . . . . 85
3.4.4. High-passandband-passcells . . . . . . . . . . . . 86
3.5.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Chapter 4. Analog to Digital Converters . . . . . . . . . 89
4.1.Digitaltoanalogconvertersandanalogtodigital
converters:anintroduction . . . . . . . . . . . . . . . . . 89
4.2.Notationsanddigitalcircuits. . . . . . . . . . . . . . . . 91
4.3.Sampleandholdcircuits . . . . . . . . . . . . . . . . . . 94
4.4.Converterstructures . . . . . . . . . . . . . . . . . . . . . 96
4.4.1. Generalfeatures. . . . . . . . . . . . . . . . . . . . 96
4.4.2. FlashADCs . . . . . . . . . . . . . . . . . . . . . . 98
4.4.3. AsimpleDAC:R2Rladder . . . . . . . . . . . . . 101
4.4.4. Half-flashandpipelineADCs . . . . . . . . . . . . 102
4.4.5. Successiveapproximationconverters . . . . . . . . 104
4.4.6. Single-anddouble-rampconverters . . . . . . . . 106
4.4.7. Sigma-deltaconverters . . . . . . . . . . . . . . . . 110
4.5.Nosilverbullet:choosingthebesttrade-off . . . . . . . 112
4.5.1. Conversionerrorsandartifacts . . . . . . . . . . . 112
4.5.2. Performancesoftypicalconverters . . . . . . . . . 114
4.6.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 118
Chapter 5. Introduction to Noise Analysis in Low
Frequency Circuits . . . . . . . . . . . . . . . . . . . . . . . 121
5.1.Whatisnoise? . . . . . . . . . . . . . . . . . . . . . . . . 121
5.2.Stochasticmodelingofanoise. . . . . . . . . . . . . . . 123
5.2.1. Somedefinitions . . . . . . . . . . . . . . . . . . . 123
5.2.2. MeasurementunitsforpB(b)andΓB(f) . . . . . . 127
5.2.3. Negativeandpositivefrequencies . . . . . . . . . . 128
5.3.Differentkindsofstochasticnoises . . . . . . . . . . . . 129
5.3.1. Thermalnoise(Johnson–Nyquist) . . . . . . . . . 130
5.3.2. Flickeror1/f noise . . . . . . . . . . . . . . . . . 131
5.3.3. Avalancheorbreakdownnoise . . . . . . . . . . . 132
5.3.4. Burstor“popcorn”orrandomtelegraph
signalnoise . . . . . . . . . . . . . . . . . . . . . . 132
5.3.5. ShotnoiseorPoissonnoise . . . . . . . . . . . . . 133
5.4.Limitsofmodeling . . . . . . . . . . . . . . . . . . . . . 134
5.5.Contributionsfromstochasticallyindependentnoise
sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.6.Noiseequivalentbandwidthandnoisefactor . . . . . . . 137
5.7.Amplifiersandnoise . . . . . . . . . . . . . . . . . . . . 139
5.7.1. Noisemodelsofoperationalamplifiers . . . . . . . 139
5.7.2. Example:noisefactorofanon-invertingamplifier 142
5.7.3. Noisemodelsofinstrumentationamplifiers . . . . 147
5.8.Noisefrom“outerspace”:electromagneticcompatibility 148
5.9.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Introduction
I.1. Purpose
A measuring system is a coordinated ensemble of different devices
allowingameasurementoperationthroughtheirinteraction.
Thanks to its intrinsic flexibility, electronics is a powerful tool
available to measurement science. This book is therefore dedicated to
the exploration of several recurrent problems in this context, for what
concerns the analog part of the measurement chain. We try to follow
theusualanalogsignalpaththroughageneralacquisitionchainandwe
describe the elements most frequently found there, with a level of
generality sufficient to be useful in different domains (physics,
biology...).
Figure I.1 shows the most traditional and general organization of a
complete acquisition system from the sensor to the data storage
system. Every measurement operation starts with a goal, which is the
determination of a quantity (temperature, gas pressure, electric signals
fromheartbeating,etc.).Thisquantityiscalledthemeasurand.
Thesensorhastheroleoftranslatingthemeasurandintoanelectrical
quantity. If needed, the latter is in turn transformed into a voltage by a
signal conditioning system. Chapter 1 describes the most used classes
ofsensorsalongwithsomeclassicconditioningstrategies.
FigureI.1.Functionalorganizationofaverygeneral
acquisitionsystem.Shadedelementsaretreatedinthisbook,
inthechaptersindicated
The output voltage is then amplified and filtered to obtain
amplitudes that are easy to manipulate and reduce as much as possible
the noise, which is inevitably present along with the useful signal.
Amplifiers (especially differential ones) and filters are, respectively,
described in Chapters 2 and 3. In fact, filtering the analog signal has a
paramount importance in those situations where a risk of signal
aliasing appears. Filters employed in this context usually have a
low-pass response and are called anti-aliasing filters. The overall
qualityofameasurementchaindepends(evencriticallyinsomecases)
onthequalityofsuchafilter.
Digital electronics offers a huge range of very advanced
signal-processingcapabilities.Itisveryeasy,today,toacquireasignal
with an analog to digital converter in order to further process it or for
storage purposes. The interface between analog and digital worlds is
assured by a sample and hold circuit, working in tandem with an
analog to digital converter. Those two devices can be shared among
differentseparateacquisitionchannelsbecauseofamultiplexer.Thisis
describedinChapter4.
Noise is the companion of every analog circuit and the main
performance limiting factor. Understanding its origins and behavior is,
therefore, a key factor to design high-performance systems. We briefly
introducenoiseanalysisinlow-frequencycircuitsinChapter5.
Finally,acontrolsystemmonitorseveryelementofthemeasurement
system, and usually a computer manipulates acquired data for storage
orvisualization.Wewillnotdiscusstheseelementsinthisbook.
In this book, we discuss the analog elements described above to a
certain degree of detail: sensors, amplifiers and filters, for
low-frequencyacquisitionsystems.Weinsistthattheoverallqualityof
measurements is determined individually by each element through its
interaction in the chain. For this reason, when possible, we present
someexamples,inspiredbyapplicationnotesandliterature.
I.2. Prerequisites
This book is addressed to readers with a background in electronic
circuits who want to begin to have an idea of the usual problems that
arisewhendesigninglow-frequencyanalogcircuitsthattreatthesignal
comingfromasensor.Tolimittheoverallsizeofthebook,wedecided
to concentrate on solutions based on discrete devices and integrated
circuits (i.e. the specific problems associated with the design of analog
integratedcircuitswillnotbeaddressed).Themainprerequisitesare:
–AC and DC analysis of circuits, transfer functions and basics of
operationalamplifiers;
–conceptsofpower,calculationandinterpretationoftherootmean
squarevalueofavoltageofacurrent;
–being able to subdivide a complex circuit in more elementary
blocks;
–knowthemostfrequentlyusedelectronicsdevicesandunderstand
datasheetsandtechnicalliteraturededicatedtorealdevices;
–basic concepts of signal processing (Fourier transform, sampling
Nyquist–Shannontheorem,filtering);
–basic probability and statistical tools (probability density
functions,expectedvalues,statisticalindependence,etc.).
Those prerequisites are addressed in undergraduate electronics
coursesinmostengineeringfacultiesaswellasbooks[MAL15].
I.3. Scope of the book
When writing a book about engineering, it is somewhat difficult to
findthegoodtrade-offbetweenabstractionandpracticalcraftsmanship
thattogetherconstitutethecoreofafieldsuchaselectronics.Wechoose
to employ maths when necessary (for example while discussing filter
synthesis in Chapter 3 or for the noise analysis in Chapter 5), yet we
tried to keep the mathematical developments close to the engineering
problemsandthereal-worldintuition.
On the other hand, when possible, we present extract from data
sheets and technical literature. It should be clear, however, that we do
notwanttoendorseaparticularproduceroraparticularmodel.Wejust
selected those components that, for a reason or another, may appear to
berathersignificativeofacertainclassofdevices.
The relation between electric circuits and measurement techniques
started very early in the 19th Century and still continues today. This
meansthat:
–anincrediblyhugenumberofsolutionsarealreadyknownforthe
mostdisparatemeasurementsituations;
–ready-madelow-costintegratedcircuitsandmodulesareavailable,
accomplishingwonderfullycomplexmeasurementtasks.
Having said that, reading a small book about electronic
measurement techniques may seem a futile exercise. Something has to
be considered though. First of all, knowing how things work helps
when a ready-made solution fails to accomplish its duty. In fact, a