Fault Sensitivity Analysis and Reliability Enhancement of An(3)

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

x(n 6)DFF

x(n 7)

(Z)

x(n 1)

x(n)

DFF(Z)

1 1

x(n 5)DFF(Z)

x(n 2)DFF(Z)

1 1

x(n 4)DFF(Z)

DFF

x(n 3)DFF(Z)

1 1

Digital Decimation

Modulator

(Z)

1

Filter

h(0)h(1)h(2)h(3)

Fig.5. -ΣADC

y(n)

Fig.7.ADigitalDecimationFilter

aroundthemainlobe,andtherefore,onlyfourcoef cients(h(0)

toh(3))arerequired.

III.FAULTSENSITIVITYANALYSIS

Therearedifferentapproachestoinvestigatetheeffectsoftransientfaults.Hardwareprototypinghasbeenused[11]butistootimeconsumingandexpensive.Simulationbasedap-proachesincludeexhaustiveandMonte-Carlomethods.Ex-haustivesimulationsareaccuratebutbecomeintractableforlargedesigns.Monte-Carlomethods,thoughtractableforlargedesigns,arenotasaccurate.SincetheADCswhichhavebeenanalyzedinthisworkarerelativelysmall,wepreferredthemoreaccurateexhaustivesimulationapproachandusedHspice[12]forthispurpose.Thefollowingsectiondiscussesthetheo-reticalbasisforthefaultsensitivityanalysismethodologyusedforthiswork.Wepresentthetransientfaultmodelusedfortheanalysisandthetheoreticalbasisforthefaultsensitivityanal-ysismethodologyused.Wealsoillustratethevariouskindsofanalysisthatcanbeperformedwiththedataobtainedfromafaultsimulationrun.A.TransientFaultModel

Severaltransientfaultmodelshavebeenproposedin[13],[14].Sincethisworkconcentratesonα-particleinducedtran-sients,thedoubleexponentialα-particletransientmodelfortheinjectioncurrentproposedin[13]isused.Theinjectioncurrentduetoanα-particlestrikeonanode,denotedbyIinj,isgivenby

Iinj(t)=I0(e t/τ1 e t/τ2)(1)whereI0isthemaximumcurrent,τ1isthecollectiontimecon-stantforajunctionandτ2istheiontrackestablishmenttime

constant.Thetimeconstantsdependonseveralprocessrelatedfactors,andinthiswork,thetimeconstantsgivenin[15]areused:τ1=1.63×10 10secandτ2=0.5×10 10sec.I0canbecalculatedby

Qinj

I0=(2)

τ1 τ2whereQinjisthechargeinjectionlevelinCoulombs.Chargeinjectionlevelisafunctionoftheangleatwhichtheα-particlehits.I0canbepositiveornegativedependingonwhethertheα-particlehitsanNMOSdrainoraPMOSdrain[15].FigureFig.6. -ΣModulator

(Cj=2·Cj 1,j=2,···,n 1andC1=C0).Inthesam-plingmodethebottom-plateisgrounded(inCLinFigure4) 1toVrefaccordingtoabinarysearchalgorithm,suchthatthetopplateeventuallybecomes0.TheobjectiveduringtheconversionistodrivetozerothedifferencebetweentheDAC(convertlatch)outputandthesam-pledinput.Onebitisconvertedineachcycle,startingwiththemostsigni cantbit.Aprecisecapacitormatchingisrequiredforthisconversion.Currentfabricationtechnologiescatertothisrequirementquiteeffectively.D. -ΣADC

The -ΣADCfallsunderthecategoryofoversamplingconverterswhichhavebecomepopularforhigh-resolution,medium-to-lowspeedapplicationssuchashigh-qualitydigitalaudio.Figure5showstheblockdiagramofa -ΣADC.The -Σmodulatorisananalogcomponentandthedigitaldecimation lterisadigitalcomponent.Themostcommonimplementationofthe -Σmodulator(showninFigure6)pro-videsanoversampledserialoutputwhichisadigitalrepresen-tationoftheinputsignal.Thisserialoutputthusobtainedhashighfrequencynoiseinadditiontothesignalinformation.Thedigitaldecimation lterstage,followingthemodulator, ltersoutthisnoiseandprovidesahighresolutionoutput.Adigitallowpass lterrealizationinvolvesamultiplicationoftheserialbitpatternwithcoef cientswhichrepresentthesincfunction.Sinceanidealsincfunctionwouldneedanin nitenumberofcoef cients,practicalcasesimplementawindowedsincfunc-tion.Figure7showsablockdiagramofan8-pointdigital-decimation lter.DFFisadelayelementandx(i)istheserialinputattheithtimeinstance.Thecoef cientsaresymmetrical