Fault Sensitivity Analysis and Reliability Enhancement of An(11)

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

(ARE)(ARE)SENSITIVITY,

p=3,q=8,r=4

TABLEXI

FLASHADC(NFTVSFT)

0.16

Fig.30.FaultTolerant4-bitFlashADC

0.140.12

"2pC""4pC""6pC""8pC""10pC"

storesacopyofthevalueintheintegrator.Whentheinputisbeingsampled(T1ishighandT2islow),theintegratoroutput(markedbyXinFigure29)shouldnotchange.Intheeventthatafaultcausesittochange,theerrordetectionblock agsanerrorwhichactivatestheredundantblock(whenT2goeshigh).TableXshowstheresultofthesensitivityanalysisrunontheNon-FaultTolerant(NFT)andtheFaultTolerant(FT)versionsofthe -ΣModulator.Theresultsshowa65.8%im-provementinsensitivityand25%improvementinMREwithapproximately75%areaoverhead.C.PatternDetection

InsomeADCs(e.g.,FlashandFI)thesignallinesattheboundarybetweentheanalogandthedigitalblocksexhibitaspeci cpattern.Iftheexpectedpatternisnotdetected,eithera agcanbeassertedor,ifpossible,acorrectioncanbeat-tempted.Thistechniquehasbeenusedforimprovingthereli-abilityofa4-bit ashADC.Theoutputofthecomparatorsinthe ashADCexhibitathermometercodepattern.Therefore,a0detectedwithinastringof1sorvice-versa,indicatesanerror.Thiserrorcanbecorrectedbyselectingthemajorityvaluefromwithinaneighborhoodofxbitsoneithersideofthebittobecorrected,wherex≥1.Forourimplementation,xwastakenas1.Figure30showsthemodi edblockdiagramofthefaulttolerant4-bit ashADC.TheErrorCorrectionblockcontainsthreetypesoferrorcorrectingsubblocks,twofortheboundarysignals(A15andA0)andonefortherestofthesignals.ThefollowingBooleanexpressionsillustratethelogicusedforthecorrection.

A(11)15=A15·(A13+A14)

0=A0+(A1·A2)A

j=Aj·Aj 1+Aj·Aj+1+Aj+1·Aj 1A

(12)(13)

0.1

POF

0.080.060.040.0201

2

3

4

5

6

Sizing Factor

Fig.31.Sensitivityvariationwithsizingandinjectionlevels,q=14,r=4

D.TransistorSizing

Anα-particleinjectionresultsinacurrentspikeatthefaulty

node.Thiscurrenttranslatestoavoltage uctuationwhosemagnitudedependsonthedrivingstrengthofthetransistor,thecapacitanceatthenodeandtheinjectioncurrent[24].Oneoftheprimaryfactorsin uencingthemagnitudeofthe uctuationistheresistanceposedbythetransistorsconnectedtothatnode.Therefore,onewouldexpectanimprovementinthereliabilitybysizingupthetransistorandthusreducingtheresistance.Ontheotherhand,sizingupatransistoralsoincreasesitsfault-sensitivearea.Thebene tsoftransistorsizingwereanalyzedforbothdigitalandanalogcircuitsin[17]andarefurtherdis-cussedinthefollowingsubsections.

1)DigitalCircuits:Thistechniquewasimplementedina2-bitcounter(with73circuitnodes)usedinthedigitaldecima-tion lterinthe -ΣADC,andthevariationofthesensitivitywithsizingandbyboundingthemaximuminjectionlevelwasanalyzed.Figure31showsthatthesensitivityincreasesandthendecreaseswithsizingforboundedinjections.Forinjectionlevelsboundedby1pCanimprovementinreliabilityof33%isobservedwhensizingthecircuitbytwiceitsoriginalsize.Fur-thermore,themaximumvalueofthePOFforahigherinjectionboundoccursatahighersizingratio(1for1pCand2for4pCinFigure31).Fortheabovesimulationsthewholecircuitwassized.Thebene tsofselectivenodesizingonthesensitivityofthe2-bitcounterwerealsostudiedandarediscussednext.Thenodeswhichwillresultinmaximumsensitivitygainsshouldbechosenascandidatesforsizing.Oneoftheschemesthatcanbefollowedsortsthenodesindecreasingorderoftheircontri-butiontotheoverallsensitivityoftheblock.Outofthissortedlistthe rstnnodescanbeselectedascandidatesforresizing.Figure32showsthatforlowerinjectionlevelbounds(1pC)sensitivityimprovementofasmuchas60%isobservedwithanTableXIshowstheresultsofthesensitivityanalysisonthe ashADC.Itwasobservedearlier(SubsectionIII-C.1)thattheanalogportionoftheADCwhichisprimarilycomprisedofcomparatorswasmoresensitive(TableIII)thanthedigitalpart.Sincethistechniqueaddressestheerrorsduetofaultsin-jectedintheanalogblock,itwillprovideconsiderableoverallsensitivityimprovement.Animprovementofaround67.8%insensitivityatthecostof55%areaoverheadwasobserved.