数字电路的噪声和布局.docx
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数字电路的噪声和布局
南京师范大学电气与电子工程学院教学授课计划60
Chp.8DIGITALCIRCUITNOISEANDLAYOUT
（数字电路的噪声和布局）
本章学习要求：
了解如下主要概念、方法和内容。
1．时域与频域关系，模拟与数字电路关系等
2．数字逻辑器件的噪声（内部噪声源分析）
3．数字电路接地系统噪声（接地线电感的影响；减小电感的方法，实际数字电路
接地系统考虑等）
4．电源分配（电源解偶解偶电容类型与数值，电解解偶电容，解偶电容的配置）
5．噪声电压控制目标与测量
§8.1Introduction
1.Adigitalsystemisalsoaradio-frequency（RF）systemwithnoiseandinterferencepotential
2.SmallICdigitallogicgates（数字逻辑门电路）,whichdrawonlyafewmAcurrent,canalsobeaseriousnoisesourcefor
（1）highswitchingspeed
（2）combinedwithinductanceoftheconductor
Thenoisesourcewhencurrentchangethroughaninductor
VLdidt
Example:
“on”state,I=5mA,
“off”state,I=1mA
Soi4mA,howevert2ns!
!
Alsoifapowersupplywiringhasaninductanceof500mH,L=500mH
南京师范大学电气与电子工程学院教学授课计划61
Thenoisevoltageacrossthepowersupplywiringwhengatechangestate
di
VNL
dt
500mH
=1V!
4mA
2ns
Realizingthepowersupplyvoltageofthissystemisonly5V,therefore
VN%1V%20%
V5V
3.Chp.8andChp.9willdiscusstechniquestominimize
（1）internalnoisegeneration
（2）radiatedemission
§8.2FrequencyVersusTimeDomain频（域与时域关系）
1.Thoughdigitalcircuitworksintimedomain,becauselegalrequirementontheemissionarespecifiedinfrequency-domain,so“f”versus“t”shouldbeknown
2.Bandwidthofadigitalsystem
Definition:
logicpulsebandwidthshouldbethepoint,wheretheenergycontentintheharmonic（consistasquarewave）isnegligiblewhenbeyondthispoint.OrthebreakpointwheretheFouriercoefficientstarttodecayat40dB/decade
BW
1
tr
tr:
pulserisetimee.g.tr=2ns,BW=159MHz
Typicalrise/falltimeandrelatedbandwidthforlogicdevice
§8.3AnalogVersusDigitalCircuit
1.Inananalogcircuit,smallnoisecoupledintocircuitmaycauseinterference,whichoftenoccursinlowsignallevel（mV,mA）orinhigh-gainamplifier;Incontrast,digitalcircuithavenoamplifierand
南京师范大学电气与电子工程学院教学授课计划62
operateatlargesignallevel
2.FornoisemarginLSTTL,V=400mV~600mV,CMOS,V
N
=1.5V（Vcc
N
=5V）,Sodigitalcircuithaveaninherentimmunitytolow-levelnoise
§8.4DigitalLogicNoise
1.Inanalogcircuit,externalnoiseisusuallytheprimaryconcern,however,indigitalcircuit,internalnoisesourceisthemajorconcern
2.Reasonforinternalnoise
（1）groundbusnoise
（2）powerbusnoise
（3）transmissionlinereflection
（4）crosstalk
3.Noisemeasurementrequirement
A）groundvoltagedifferencebetweenvariouspointsinthesystem
B）Vcc-to-groundvoltageonthepowersupplypinsofallIC’s
§8.5InternalNoiseSource
1.Fig.8-1Noisegenerationwhenoutputofgate“1”switchesfromhightolow（P277）
2.Sincegate“1”resistanceRisverysmallindischargepath（R,L,Cinseries）,high-Q
resonantmaycauseoutputtonegative
Q
1L
RC
Fig.8-2outputvoltagewaveform（p278）
（A）ringingduetostraycapacitanceandinductance（B）ringingdampedbyaddingoutputresistor
SoRQ
南京师范大学电气与电子工程学院教学授课计划63
§8.6DigitalCircuitGroundNoise
Groundnoiseismoreofaproblemthanpowersupplynoise
1.Reasonforgroundnoise:
transientpowersupplycurrent+signal-returncurrent
Usuallypowersupplytransientcurrentcanbecontrolledbydecouplingcapacitor,butsignal-returncurrentcan’t
2.Impedanceofground
UsuallyaPCBconductor（0.02inwide）:
12m/in2pF/in15mH/in
Impedanceofa1-inchPWB（15mH）versusfrequencyisrelatedtorise/falltime
Conclusion:
inductanceisthemostconcernwhenlayingoutadigitalPCB
3.Minimizinginductance
（1）lengthofconductor
（2）diameterorwidthofaconductor
4h
L0.005ln（）roundconductoraboveacurrent-returnpathd
L0.005ln（2h）flatconductoraboveacurrent-returnpathw
（3）providealternativepathforcurrentflow
4.Mutualinductance
5.Practicaldigitalcircuitgroundsystem
Apracticaldigitalcircuitgroundsystemmustprovidealow-impedance（lowinductance）connection
（1）Themostpracticalwayistoprovideasmanyalternative（parallel）pathaspossible,thereforeinfinitenumberofparallelpathresultinagroundplane
（2）Sincegroundplaneneedlargeareaandnotcost-effective,using
groundgridisbetterFig.8-3Agrid-typegroundsystem（p284）
6.Looparea
Iftwoconductorwithcurrentinoppositedirection（e.g.asignalloadanditsground-returnlead）
南京师范大学电气与电子工程学院教学授课计划64
LtL1L22M
IfL1=L2,Lt=2（L-M）Therefore,MLt
Conclusion:
Placingforwardandreturncurrentpathclosetogetherisaaneffectivewayofreducinginductance.Sotwistedpairorcoaxialcableispreferable,thenLt<1mH/in
§8.7PowerDistribution
1.Power-supplydecoupling
Fig.8-4transientpower-supplycurrent（p287）
（A）withoutdecouplingcapacitor（B）withdecouplingcapacitor
2.Decouplingcapacitortypeandvalue
（1）diskceramiccapacitorormultiplayerceramiccapacitor
high-frequency15~150MHz;Lowinductance
（2）TheminimumvalueofcapacitorrequiredbyICwhenitswitchesis
dIdt
C
dV
Example:
anICrequiresatransientcurrentof50mAfor2nSandonewishtolimitpower-supplyvoltagetransientlessthen0.1V,then
50mA2nS
C
0.1V
=0.001uF
（3）Capacitancevaluesislimited
A）“C”valuecan’tbetoolarge,otherwiseathigh-F,inductanceL
Analysis:
acapacitorhasinductanceinseries,resonantfrequencyis
1
sowhenLisdefiniteand“C”
ff
effectofL
fr
2
LC
（becausewhenffrXL>Xc）
南京师范大学电气与电子工程学院教学授课计划65
B）“C”valuecan’tbetoosmall,otherwisenotsufficientchargestoragetosupplythetransientcurrent
Experiment:
“C”valuecanbedeterminedbymeasuringnoisevoltageacrossICchip.Usually470-1000pF
3.Decouplingcapacitorplacement
BeingplacedascloseastotheICispossible
Fig.8-5poorandbetterplacementofdecouplingcapacitor（p290）
Rule:
keeptheloopareabetweenICanddecouplingcapacitorassmallaspossibletodecreasetheinductance
EquivalentcircuitfordecouplingcapacitorconnectedtoICInductanceconsistofthreecomponents:
①inductanceofcapacitoritself
②inductanceofPWBtrace
③inductanceofleadframewithinIC
§8.8NoiseVoltageObjective
1.Followingmeasurementshouldbemade
Groundnoisevoltage;power-supplynoisevoltage
2.Thebestmethodistomeasurepeak-differentialgroundnoisevoltagebetweenvariouspointsontheboard,andalsotheVcctogroundvoltageoneachIC
§8.9MeasuringNoiseVoltage
1.Considerationofmeasuringnoisevoltage
1）bandwidthofmeasuringinstrument（minimum100MHz,better200MHz）
2）athigh-F,CMMRoftheinstrument
3）leadsfromtheinstrumenttothecircuitundertestCUT
2.Equalleadlengthofinstrument,andperpendiculartothecircuitboard
南京师范大学电气与电子工程学院教学授课计划66
Fig.8-6（p294）A）incorrectsetupB）correctone
SUMMARY（小结）
Chp.9DIGITALCIRCUITRADIATION（数字电路辐射）
本章学习要求：
了解如下主要概念、方法和内容。
1.差模辐射（辐射计算公式，环路面积，环路电流，
Fourier
级数，差模辐射包
络等）
2.控制差模辐射
3.共模辐射
4.控制共模辐射（共模电压控制，电缆解偶与屏蔽，共模扼流圈，共模电流测量等）
Introduction
1.Emissioncontrolshouldbetreatedasadesignproblem,andthroughoutthedesignprocess
南京师范大学电气与电子工程学院教学授课计划67
2.Radiationparameterandmethodsforpracticalradiatedemissionareintroducedinthischp.
Radiationtypeindigitalelectronics:
3differential-mode（DM）radiation:
resultofcurrentflowingaroundloopformedbyconductor
Fig.9-1differential-moderadiationformPCB（p299）
smallloopantennaradiatingmagneticfield
mon-mode（CM）radiation:
resultofundesiredvoltagedropthatcausepartofcircuittobeatacommon-modepotentialabove“true”ground.Whenexternalcableconnectedtothis,it’sdrivenbythisCMpotentialandradiateelectricfield
Fig.9-2common-moderadiationfromsystemcable（p299）
smallcurrentsegment（straightwire）radiatingEfield
§9.1Differential-Mode（DM）Radiation
1.EfieldcalculationforDMradiation
Forlooparea“A”,carryingcurrent“I”,freespacedistance“r”
E131.610
16（f
2AI）
（1）sin
V/m
r
Note:
forsmallloop,thisisaccurate;forlargeloop,thisisapproximate
Ifacircuitonagroundplaneismeasured,reflectioneffectshouldbe
1621
considered.SoE26310（fAI）（）sin
andEf2,A,I
Conclusion:
differential-moderadiationcanbecontrolledby
1）reducingthemagnitudeofcurrent
2）reducingthefrequencyofharmoniccontentofcurrent
3）reducingthelooparea
南京师范大学电气与电子工程学院教学授课计划68
2.Looparea
Fordigitalcircuitdesign,Aplacingsignalleadsandtheirassociated
ground-returnleadsclosetogether.Especiallyforclockleads,backplanewireing,andinterconnectingcable.
Example:
byFCCClassBLimitmeasuredatdistance3m
IfI=25mA,f=30MHz,looparea“A”=100cm2,r=3mthen
1621
E131.610（fAI）（）
=197uV/m（aboutthetwicetheallowableemissionofFCCLimit）
Butwhenlooparea“A”5cm2
E=100uV/m（meetFCCrequirement）
3.Loopcurrent
Measuring“I“byusingaccurateprobetoclipthecircuitwire
Loopantennaconfiguration（p302）
（A）ashortloopantenna（B）acapacitor-terminalloop
Usuallyacapacitor-loadedloopwithzero-impedanceisfrequentlyassumed4.FourierSeries
Sincedigitalcircuitusesquarewave,theFourierseriesofthecurrentshouldbeknownas
In
2Idsin（n
d）sin（n
tr/T）
nd
n
tr/T
I:
peak-to-peakamplitudeofthewaved:
dutycycle
tr:
risetime
Fig.9-3exampleofFourierspectrumfroma50%dutycycle（d=0.5）
trapezoidwave（p303）
南京师范大学电气与电子工程学院教学授课计划69
Sodifferential-moderadiatedemissioniscalculatedbyStep1:
calculatethecurrentcontainedineachharmonic
Step2:
substitutethiscurrentandtherespec
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