Abstract — Reliability of systems used in space, avionic and biomedical applications is highly critical. Such systems consist of an analog front-end to collect data, an Analog-to-Digital Converter (ADC) to convert the collected data to digital form and a

1

FaultSensitivityAnalysisandReliabilityEnhancementofAnalog-to-DigitalConverters

MandeepSingh,Member,IEEE,andIsraelKoren,Fellow,IEEE

Abstract—Reliabilityofsystemsusedinspace,avionicandbiomedicalapplicationsishighlycritical.Suchsystemsconsistofananalogfront-endtocollectdata,anAnalog-to-DigitalCon-verter(ADC)toconvertthecollecteddatatodigitalformandadigitalunittoprocessit.Thoughconsiderableamountofresearchhasbeenperformedtoincreasethereliabilityofdigitalblocks,thesamecannotbeclaimedformixedsignalblocks.Thereliabilityenhancementwhichweemploystartswithfaultsensitivityanalysisfollowedbyredesign.Thedataobtainedfromthesensitivityanal-ysisisusedtogradeblocksbasedontheirsensitivitytofaults.Thehighlysensitiveblockscanthenbereplacedbymorereliablealter-natives.Theimprovementgainedbyoptingformorerobustim-plementationsmightbelimitedduetothenumberofpossibleim-plementations.Inthesecasesalternativereliabilityenhancementtechniquessuchasaddingredundancymayprovidefurtherim-provements.ThestepsinvolvedinthereliabilityenhancementofADCsareillustratedinthispaperby rstproposingasensitivityanalysismethodologyforα-particleinducedtransientsandthensuggestingredesigntechniquestoimprovethereliabilityoftheADC.Anovelconceptofnodeweightsspeci ctoα-particletran-sientsisintroducedwhichimprovestheaccuracyofthesensitivityanalysis.Thefaultsimulationsshowthat,usingtechniquessuchasalternativerobustimplementations,addingredundancy,patterndetectionandtransistorsizing,considerableimprovementsinre-liabilitycanbeattained.

IndexTerms—FaultTolerance,FaultSensitivity,Analog-to-DigitalConverters,Alphaparticles,Reliability,TransientFaults

I.INTRODUCTION

Criticalsystemsusedinspace,avionicsandbiomedicalap-plicationshavetobehighlyreliablesincetheeffectofafaultinthesesystemscanbecatastrophic.Thereliabilityofthesesys-temscanbeincreasedbyredesigningthemforimprovedfaulttolerance.Thesystemunderredesignundergoesafaultsensi-tivityanalysisbeforeandaftertheredesigntogaugethereliabil-ityimprovement.Faultsensitivityanalysisinvolvesinjectionoffaultseitherintheactualhardwareorinsoftwarethroughsimu-lation.Thelattermethodispreferablesincetheformerrequiresaprototypewhichisexpensive.Thelatteralsoenablesanearlyanalysisinthedesignphasethuseliminatingcostlyredesign.Twotypesoffaultshavebeenknowntoaffecttheproperworkingofacircuit:permanentandtransient.Whereasper-manentfaultscanbeintroducedduringthefabricationstageandinthe eld,transientfaultsarecausedinthe eldduetoElectroMagneticInterference(EMI)suchaspowertransients,

M.SinghiswithAdvancedMicroDevicesintheComputationProductsGroup,Austin,TX78704USA.

I.KoreniswiththeDepartmentofElectricalandComputerEngineering,Uni-versityofMassachusetts,Amherst,MA01003,USA.

ThisworkhasbeensupportedinpartbyNSFundercontractMIP-9710130andbyJPLundercontract961294.

crosstalkandvariousparticlehitsinradiationintenseenviron-mentslikespace.Theeffectoftransientfaultsistotemporarilychangethebehaviorofthecircuitoftenresultinginerroneousoutputs.Thistypeoffaultshasbeenknowntoaccountfor85%ormorefailuresindigitalsystems[1],[2].Sincethismightbecatastrophicincriticalapplications,thesecircuitsusuallyincor-poratesomemeasurestoincreasetheirfaulttolerance.

Thereliabilityofasystemisdeterminedbythefaulttoler-anceofitsconstituentblocks.Systemsinspace,biomedicalandavionicsapplicationsconsistofananalogfront-endtocol-lectdataforcontrolandobservationpurposesandadigitalunitwhichprocessesthecollecteddata.Digitalcircuitshavebeenstudiedextensivelyfortheirsensitivitytotransientfaults[3],[4]andmanytechniqueshavebeensuggestedtoimprovetheirfaulttolerance[4],[5].Incontrast,verylittlehasbeendonetoaddresstheissueoffaulttoleranceinanalogcircuitsandADCswhichareintegralpartsofalmostallmixed-signalcir-cuits.Hence,itisnecessarytoexploretechniquestoincreasethefaulttoleranceofADCs.Theprocessofincreasingthetol-eranceofacircuittotransientfaultscanbedividedintotwosteps:

(i)Gradingblocksofthecircuitbasedontheirsensitivities

totransientfaultsandidentifyingcritical(i.e.,mostsensi-tive)blocks.

(ii)Increasingthefaulttoleranceoftheidenti edcritical

blocks.Thisworkaddressesbothofthesestepsby rstproposingamethodologytoanalyzethesensitivityofanADCandthenbysuggestingtechniquestoincreasethereliabilityoftheADC.Thefaultinjectionexperiments,forgaugingthesensitivityofthedesignsaddressedinthiswork,wereperformedforα-particleinducedtransients.Thisisbecauseα-particleshavebeenidenti edasoneoftheenergeticnuclearparticlesthatcancauseatransientfault.However,thetechniquesdevelopedforthesefaultscanbeextendedtotransientfaultscausedbyothersources.α-particlesarefoundinspace[6]andintraceamountsinICsonthegroundduetodecayofradioactiveele-mentspresentinthepackagingmaterialorsolder[7].Thus,theapplicabilityofthisworkisnotrestrictedtosystemsinouterspacebutalsotoothergroundbasedcriticalsystems.

Thispaperisorganizedasfollows,SectionIIpresentsatax-onomyforADCsandprovidesabrieffunctionaldescriptionoftheADCsaddressedinthiswork.SectionIIIdescribesthesen-sitivityanalysismethodologyused.InSectionIVtheprocessofincreasingreliabilitybyoptingforrobustimplementationsandbyintroducingredundancyisdiscussed.SectionVsummarizesthe ndingsofthisworkandSectionVIdiscussesfuturework.