Fault Sensitivity Analysis and Reliability Enhancement of Analog-to-Digital Converters(6)

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

TABLEII

POF

1

0.1

OVERALLPOFCOMPARISONFORREPRESENTATIVEVSCOMPLETE

INPUTSFORTHE4-BITFLASHADC

0.01

0.001

TABLEIII

FLASHADC

BLOCKSENSITIVITY,

0.0001

Fig.14.

Blocksensitivity(logscale)variationwithinjectionlevels(FI),p=3,r=4

0.1

SHAFA1

q=8,r=4

POF

0.01

1

0.1

0.001

012345678

Charge Injected (in pC)

POF

0.01

Fig.15.Blocksensitivities(logscale)variationwithinjections(FI),p=3,r=4

0.001

1Maximum Relative Error

0.0001

0.8

Fig.13.Blocksensitivity(logscale)variationwithinputsfortheFIADC,q=14,r=4

0.6

0.4

havebeenperformedforonerepresentativeinputinthelower,middleandupperinputranges.

1)FlashADC:TheAREmetric(8)wasusedforevaluatingthesensitivityoftheblocksina ashADC.TableIIIshowsthattheanalogblockofthe ashADCwhichcomprisesofcompara-torsismoresensitivethanthedigitalblock.Itisalsoseenthattheanalogblockremainsmoresensitivethanthedigitalblockforalltheinputsubrangesconsidered.Thus,forthe ashADCtheanalogblockisidenti edasthecriticalblock.

2)FoldingandInterpolatingADC:ThevariationsofthesensitivityofthisADCtochangesininputvoltageandlevelofinjectionwereinvestigated.ThePOFmetric(6)wasusedforevaluatingthesensitivityoftheindividualblocks.Theana-logblockinthisADCwasfurtherpartitionedasitcomprisedofmorecomponentsasopposedtothe ashADCwhereintheanalogblockcomprisedofcomparatorsonly.Figure13showsthesensitivitiesoftheblocksforthethreeinputlevels.This g-ureshowsthatthesensitivitiesofsomeblockstoα-particlehitsvaryfromoneinputvaluetoanother(C2andC3inFigure13).Thecomparators(C1throughC4)inFIwerefoundtobemoresusceptiblewhentheiroutputisalogic0.ThiscorrespondstoanADCinputintherangeof1.42-1.52vforthecompara-torC2.Hence,itcanbeconcludedthatC2isrelativelymoresensitiveintheseinputranges(asisshowninthebargraphcor-respondingtoaninputof1.5vinFigure13).Figure14showsthatthesensitivitiesofblocks

donotvaryconsiderablyifthe

0.2

Fig.16.Maximumrelativeerror(FI),p=3,q=14,r=4

injectionlevelismorethan6pico-Coulomb(pC).Thus,thereisnoneedtorepeatthesensitivityanalysisforinjectionslevelsbeyond6pC.

Figure15showsthatforlowerinjectionlevelstheorderingofcriticalblocksmightchange(SHAismoresensitivethanFA1from0pCto0.25pC).This gurealsoshowsthatbeyondacertaininjectionlevelthereisnofurtherincreaseinblocksensi-tivity.Figure16showsthemaximumrelativeerrorduetoeachblock.Theresultsagainshowthataswegettoblocksclosertotheinputthemaximumrelativeerrorincreases,reachingapeakforthesampleandholdampli ers(SHAinFigure16).Thus,theSHAandFAblockshavebeenidenti edascriticalblocksfortheFIADC.

3)SuccessiveApproximationADC:ThePOFmetric(6)wasusedforevaluatingthesensitivityoftheblocksintheSuc-cessiveApproximation(SA)ADC.Figure17showsthesensi-tivitiesoftheblocksforthethreerepresentativeinputs.Itisev-identfromthis gurethatthesensitivitiesofsomeblockstoα-particlehitsvaryfromoneinputvaluetoanother(outputlatch,convertlatchinFigure17).Figure18showsthatforlowerinjec-

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