The Circuit Designer's Companion, Third Edition

445 Pages·2012·16.16 MB·English

by  Peter Wilson

The Circuit Designer’s
Companion
Third Edition
Peter Wilson
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Introduction
WhenIwasfirstapproachedtoproduceathirdeditionofTheCircuitDesigner’sCompanion,Iwasatfirst
reluctantto“messwithit”.Itisraretohaveacompanionbookthatisnotjustatextbook,orahandbook,
butisseeninmanyrespectstocontainalltheessentialinformationthata“real”circuitdesignerneedsto
notonlyproduceaworkingcircuit,buttoenablethatdesignertounderstandalltherelatedtopicsthat
makethedesignrobust,toleranttonoiseandtemperature,andabletooperateinthesystemthatitwas
designedfor.Thisbookisarareexampleofjustthat,andthereisnoothercomparabletextthatprovides
suchabroadrangeofdesignskillstobepassedontothenextgenerationofcircuitdesigners.
It isinteresting to note that twenty-oneyears on from the original edition of thisbook there isno
diminutionofdemandforanalogandmixedsignaldesignskills,however,mostuniversitiesandcolleges
stillteachasyllabusinelectronicsthatisdominatedbydigitaldesigntechniques.Thecommentmadeby
Timintheintroductiontothefirsteditionthatanalogelectronicswere“hard”andtherewasareluctance
toembarkonanalogelectronicscouldhavebeenwrittenthisyear,ratherthantwodecadesago!During
the revisionofthisbook,itwas alsointeresting tonote that muchofthe contentwas still completely
valid in today’s electronic systems, albeit some of the individual technologyelements have of course
movedon,withmanyofthefundamentalconceptsbeingessentiallythesame.
PeterWilson
INTRODUCTION TO THE FIRST EDITION (TIM WILLIAMS, 1990)
Electroniccircuitdesigncanbedividedintotwoareas:thefirstconsistsindesigningacircuitthatwill
fulfilitsspecifiedfunction,sometimes,underlaboratoryconditions;thesecondconsistsindesigningthe
samecircuitsothateveryproductionmodelofitwillfulfilitsspecifiedfunction,andnootherundesired
andunspecifiedfunction,always,inthefield,reliablyoveritslifetime.Whenrelatedtocircuitdesign
skills,thesetwoareascoincideremarkablywellwithwhatengineersaretaughtatcollege–basiccircuit
theory,Ohm’slaw,The´venin,Kirchhoff,Norton,Maxwellandsoon–andwhattheylearnonthejob–
thatthereisnosuchthingastheidealcomponent,thatprintedcircuitsaremorethanjustacollectionof
tracks,andthatelectronshaveanunfortunatehabitofneverdoingexactlywhatthey’retold.
Thisbookhasbeenwrittenwiththeintentionofbringingtogetherandtyingupsomeoftheloose
ends of analog and digital circuit design, those parts that are never mentioned in the textbooks and
rarely admittedelsewhere. In other words, itrelatesto the second of the above areas.
Itsgenesiscamewiththegrowingfrustrationexperiencedasaseniordesignengineer,attemptingto
recruitpeopleforjuniorengineerpositionsincompanieswhosefoundationsrestedonanalogdesign
excellence. Increasingly, it became clear that the people I and my colleagues were interviewing had
only the sketchiest of training in electronic circuitdesign, despite offering apparently sound degree-
level academic qualifications. Many of them were more than capable of hooking together a micro-
processorandafewlarge-scalefunctionalblockperipherals,butwereflooredbysimplequestionssuch
asthenatureofthep–njunctionorhowtogoaboutresistortolerancing.Itseemsthatthisexperienceis
by no means uncommon inother parts of the industry.
Thecollegesanduniversitiescanhardlybeblamedforputtingtheemphasisintheircoursesonthe
skillsneededtocopewithdigitalelectronics,whichisafterallbecomingmoreandmorepervasive.If
xiii
xiv Introduction
they are failing industry, then surely it is industry’s job to tell them and to help put matters right.
Unfortunatelyitisnotsoeasy.A1989reportfromImperialCollege,London,foundthatfewstudents
wereattractedtoanalogdesign,citinginadequateteachingandtextbooksaswellasthesubjectbeing
found “more difficult”. Also, teaching institutions are under continuous pressure to broaden their
curriculum,toproducemore“well-rounded”engineers,andthishastobeattheexpenseofgreaterin-
depth coverage ofthe fundamentaldisciplines.
Nevertheless,therealworldisobstinatelyanalogandwillremainso.Thereisadisturbingtendency
to treat analog and digital design as two entirely separate disciplines, which does not result in good
training for either. Digital circuits are in reality only over-drivenanalog ones, and anybodywho has
agoodunderstandingofanalogprinciplesiswellplacedtoanalyzethemoreobscurebehavioroflogic
devices. Even apparently simple digital circuits need some grasp of their analog interactions to be
designed properly, as Chapter 6 of this book shows. But also, any product which interacts with the
outsideworldviatypicaltransducersmustcontainatleastsomeanalogcircuitsforsignalconditioning
and the supply of power. Indeed, some products are still best realized as all-analog circuits. Jim
Williams, awell-known American linear circuitdesigner (who bears no relation tothe author of the
first two editions of this book), put it succinctly when he said “wonderful things aregoing on in the
forgotten land betweenONEand ZERO. This isReal Electronics.”
Becauseanalogdesignappearstobegettinglesspopular,thosepeoplewhodohavesuchskillswill
becomemoresought-afterintheyearsahead.Thisbookismeanttobeatoolforanyaspiringdesigner
whowishestodeveloptheseskills.Itassumesatleastabackgroundinelectronicsdesign;youwillnot
find in here more than a minimum of basic circuit theory. Neither will you find recipes for standard
circuits, as there are many other excellent books which cover those areas. Instead, there is a serious
treatment of those topics which are “more difficult” than building-block electronics: grounding,
temperatureeffects,EMC,componentsourcingandcharacteristics,theimperfectionsofdevices,and
howtodesign sothat someone else can makethe product.
I hope the book will be as useful to the experienced designer who wishes to broaden his or her
background as it will to the neophyte fresh from collegewho faces a first jobin industry with trepi-
dationandexcitement.Thetraditionalwayofgainingexperienceistolearnon-the-jobthroughpeer
contact, and this book ismeant to enhance rather than supplant that route. It is offered to thosewho
want their circuits to stand a greater chance of working first time every time, and a lesser chance of
being completely redesigned after six months. It does not claim to be conclusive or complete. Elec-
tronicdesign,analogordigital,remainsapersonalart,andalldesignershavetheirownfavoritetricks
and their own dislikes. Rather, it aims to stimulate and encourage the quest for excellence in circuit
design.
Imusthereacknowledgeadebttothemanycolleaguesovertheyearswhohavehelpedmetowards
an understanding of circuit design, and who have contributed towards this book, some without
knowingit:inparticularTimPrice,BrucePiggottandTrevorForrest.AlsotoJoyce,whohaspatiently
endured the manybrainstorms thatthewriting ofit produced inher partner.
INTRODUCTION TO THE SECOND EDITION (TIM WILLIAMS, 2004)
Thefirsteditionwaswrittenin1990andeventually,afteragoodlongrun,wentoutofprint.Butthe
demandforithasremained.Therefollowedaperiodoffalsestartsandmuchpestering,andfinallythe
Introduction xv
author was persuaded to pass through the book once more to produce this second edition. The aim
remainsthesamebuttechnologyhasprogressedintheinterveningfourteenyears,andsoanumberof
anachronismshavebeencorrectedandsomesectionshavebeenexpanded.Iamgratefultothosewho
have made suggestions for this updating, especially John Knapp and Martin O’Hara, and I hope it
continues togivethe same levelofhelp that the first editionevidently achieved.
INTRODUCTION TO THE THIRD EDITION (PETER WILSON, 2012)
Thethirdeditionofthebookhasreallybeenanexerciseofrevisionratherthanrevolution,andIhave
triedtokeepthephilosophythesameastheoriginalauthorintended.Aswiththesecondedition,the
aimhasbeentoupdatethetechnologicalaspectsinthebook,expandsomesectionsandofferaslightly
different personal perspective to hopefully further enhance the book. I am very grateful to
Tim Williams for allowing me to make these revisions, and for his discussions about the book’s
previous editions. I also acknowledge the advice, teaching and knowledge of many friends and
colleagues over the past three decades which have provided much insight into the art of analog
electronics,includingmyfather,TomWilson,FrankFisherwhileatFerranti,ProfessorAlanMantooth
at the University of Arkansas, and Dr Neil Ross and Dr Reuben Wilcock, at the University of
Southampton.Imustalsothankmywife,Caroline,whohastoleratedmyfascinationwithelectronics
formanyyears.Ihopethatfurthergenerationsofelectronicdesignerswillfindthiseditionusefuland
that the book will continue to provide the assistance and help to circuit designers that the previous
editionshavedone over the last two decades.
CHAPTER
1
Grounding and wiring
CHAPTER OUTLINE
1.1 Grounding.........................................................................................................................................2
Whentoconsidergrounding..............................................................................................3
1.1.1 Groundingwithinoneunit..............................................................................................4
1.1.2 Chassisground..............................................................................................................4
1.1.3 Theconductivityofaluminum........................................................................................5
Othermaterials.................................................................................................................6
1.1.4 Groundloops.................................................................................................................6
1.1.5 Powersupplyreturns.....................................................................................................8
Varyingloads....................................................................................................................9
Powerrailfeed................................................................................................................10
Conductorimpedance.....................................................................................................11
1.1.6 Inputsignalground.....................................................................................................11
Connectionto0VelsewhereonthePCB.........................................................................11
Connectionto0Vwithintheunit.....................................................................................11
Externalgroundconnection.............................................................................................11
1.1.7 Outputsignalground...................................................................................................13
Avoidingthecommonimpedance....................................................................................13
1.1.8 Inter-boardinterfacesignals.........................................................................................14
Partitioningthesignalreturn............................................................................................15
1.1.9 Star-pointgrounding....................................................................................................16
1.1.10 Groundconnectionsbetweenunits..............................................................................17
Breakingthegroundlink...............................................................................................18
1.1.11 Shielding..................................................................................................................19
WhichendtogroundforLFshielding............................................................................19
Electrostaticscreening..................................................................................................20
Surfacetransferimpedance..........................................................................................21
1.1.12 Thesafetyearth........................................................................................................21
1.2 Wiringandcables..........................................................................................................................22
1.2.1 Wiretypes...................................................................................................................22
Wireinductance..............................................................................................................22
Equipmentwire...............................................................................................................23
Wire-wrapwire................................................................................................................23
1.2.2 Cabletypes.................................................................................................................24
1.2.3 Powercables...............................................................................................................24
1
TheCircuitDesigner’sCompanion.DOI:10.1016/B978-0-08-097138-4.00001-X
Copyright(cid:1)2012,2005,1991ElsevierLtd.Allrightsreserved.
2 CHAPTER 1 Grounding and wiring
1.2.4 Dataandmulticorecables............................................................................................25
Datacommunicationcables............................................................................................26
Structureddatacable......................................................................................................26
Shieldingandmicrophony...............................................................................................27
1.2.5 RFcables...................................................................................................................28
1.2.6 Twistedpair................................................................................................................29
1.2.7 Crosstalk....................................................................................................................30
Digitalcrosstalk...............................................................................................................32
1.3 Transmissionlines.........................................................................................................................33
Transmissionlineeffects.................................................................................................34
Criticallengthsforpulses.................................................................................................34
1.3.1 Characteristicimpedance.............................................................................................34
1.3.2 Timedomain...............................................................................................................35
Forwardandreflectedwaves...........................................................................................35
Ringing...........................................................................................................................36
TheBergerondiagram....................................................................................................36
Theusesofmismatching................................................................................................38
1.3.3 Frequencydomain.......................................................................................................38
Standingwavedistributionversusfrequency....................................................................38
Impedancetransformation...............................................................................................39
Lossylines......................................................................................................................40
Understandingthetransmissionlineimpedancegraphically.............................................42
1.1 GROUNDING
Afundamentalpropertyofanyelectronicorelectricalcircuitisthatthevoltagespresentwithinitare
referencedtoacommonpoint,conventionallycalledtheground.Thistermisderivedfromelectrical
engineeringpractice,whenthereferencepointisoftentakentoacopperspikeliterallydrivenintothe
ground.Thispointmayalsobeaconnectionpointforthepowertothecircuit,anditisthencalledthe
0V(nought-volt)rail,andgroundand0Varefrequently(andconfusingly)synonymous.Then,when
wetalkaboutafive-voltsupplyoraminus-twelve-voltsupplyoratwo-and-a-half-voltreference,each
ofthese are referred tothe 0Vrail.
At the same time, ground is not the same as 0V. A ground wire connects equipment to earth for
safety reasons, and does not carry a current in normal operation. However, in this chapter the word
“grounding”willbeusedinitsusualsense,toincludebothsafetyearthsandsignalandpowerreturn
paths.
Perhapsthegreatestsinglecauseofproblemsinelectroniccircuitsisthat0Vandgroundaretaken
forgranted.Thefactisthatinaworkingcircuittherecanonlyeverbeonepointwhichistrulyat0V;
theconceptofa“0Vrail”isinfactacontradictioninterms.Thisisbecauseanypracticalconductor
hasafinitenon-zeroresistanceandinductance,andOhm’slawtellsusthatacurrentflowingthrough
anything other than a zero impedance will develop a voltage across it. Aworking circuit will have
currentflowingthroughthoseconductorsthataredesignatedasthe0Vrailandtherefore,ifanyone
1.1 Grounding 3
pointoftherailisactuallyat0V(say,thepowersupplyconnection)therestoftherailwillnotbeat
0V. This can beillustrated with the exampleinFigure1.1.
Now,aftersuch atrenchantintroduction, youmight betemptedtosaywell,there aremillionsof
electroniccircuitsinexistence,theymustallhave0Vrails,theyseemtoworkwellenough,sowhat’s
theproblem?Mostofthetimethereisnoproblem.Theimpedanceofthe0Vconductorisintheregion
ofmilliohms,thecurrentlevelsaremilliamps,andtheresultingfewhundredmicrovoltsdropdoesn’t
offend the circuit at all; 0Vplus 500mVisclose enough to 0Vfor nobody toworry.
Thedifficultywiththisansweristhatitistheneasytoforgetaboutthe0Vrailandassumethatitis
0Vunderallconditions,andsubsequentlybesurprisedwhenacircuitoscillatesorotherwisedoesn’t
work.Thoseconditionswheretrouble islikely toarise are:
(cid:1) where current flows are measuredin amps rather than milli-or microamps;
(cid:1) where the 0Vconductor impedance ismeasured inohms ratherthan milliohms;
(cid:1) wheretheresultantvoltagedrop,whateveritsvalue,isofamagnitudeorinsuchaconfigurationas
toaffect the circuit operation.
When to consider grounding
Oneoftheattributesofagoodcircuitdesigneristoknowwhentheseconditionsneedtobecarefully
considered and when they may be safely ignored. A frequent complication is that you as circuit
designermaynotberesponsibleforthecircuit’slayout,whichishandedovertoalayoutdraughtsman
(who may in turn delegate many routing decisions to a software package). Grounding is always
sensitivetolayout,whetherofdiscretewiringorofprintedcircuits,andthedesignermusthavesome
knowledge of andcontrol overthisif the design isnot tobe compromised.
Thetrick isalways tobesure thatyouknowwhereground returncurrents are flowing, andwhat
their consequences will be; or, if this is too complicated, to make sure that wherever they flow, the
FIGURE1.1 Voltagesalongthe0Vrail
Assumethe0Vconductorhasaresistanceof10mU/inchandthatpointsA,B,CandDareeachoneinchapart.
ThevoltagesatpointsA,BandCreferredtoDare:
V ¼ðI þI þI Þ(cid:3)10mU¼400mV
C 1 2 3
V ¼V þðI þI Þ(cid:3)10mU¼700mV
B C 1 2
V ¼V þðI Þ(cid:3)10mU ¼900mV
A B 1
4 CHAPTER 1 Grounding and wiring
consequences will be minimal. Although the above comments are aimed at 0Vand ground connec-
tions, because they are the ones most taken for granted, the nature of the problem is universal and
applies to any conductor through which current flows. The power supply rail (or rails) is another
special casewhereconductor impedance can create difficulties.
1.1.1 Grounding within one unit
In this context, “unit” can refer to a single circuit board or a group of boards and other wiring
connected together within an enclosure such that you can identify a “local” ground point, for
instance the point of entry of the mains earth. An example might be as shown in Figure 1.2. Let us
say that printed circuit board (PCB) 1 contains input signal conditioning circuitry, PCB2 contains
a microprocessor for signal processing and PCB3 contains high-current output drivers, such as for
relaysandforlamps.Youmaynotplaceallthesefunctionsonseparateboards,buttheprinciplesare
easier to outline and understand if they are considered separately. The power supply unit (PSU)
provides a low-voltage supply for the first two boards, and a higher-power supply for the output
board.ThisisafairlycommonsystemlayoutandFigure1.2willserveasastartingpointtoillustrate
good and bad practice.
1.1.2 Chassis ground
Firstofall,notethatconnectionsareonlymadetothemetalchassisorenclosureatonepoint.Allwires
thatneedtocometothechassisarebroughttothispoint,whichshouldbeametalstuddedicatedtothe
purpose.Suchconnectionsarethemainssafetyearth(aboutwhichmorelater),the0Vpowerrail,and
anypossiblescreeningandfilteringconnectionsthatmayberequiredinthepowersupplyitself,such
asanelectrostaticscreeninthetransformer.(Thetopicofpowersupplydesignisitselfdealtwithin
much greater detail inChapter7.)
FIGURE1.2 Typicalintra-unitwiringscheme