Analog to Digital Conversion.docx

Analog to Digital Conversion.docx

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AnalogtoDigitalConversion

AnalogtoDigitalConversion

SamerEl-Haj-Mahmoud

ElectronicsEngineeringTechnologyProgram

TexasA&MUniversity

Instructor’sPortion

Summary

ThepurposeofthislabistoprovidethestudentswithhandonexperienceinusingtheanaloginputsofaNationalInstrumentsdataacquisitioncard.ThelabassumesthatthestudentshavepriorexposuretoLabVIEW,andthattheyknowthebasicsofwritinganddebuggingLabVIEWVIs（VirtualInstruments）.TheexperimentwillteachthemhowtoapplytheirLabVIEWknowledgetoacquireananalogsignalfromthefunctiongenerator.TheywilldisplayitinLabVIEWandcalculateitsRMSandaveragevalues.ThestudentsshouldhavestudiedAnalogtoDigitalconversiontheoryinclassbeforecomingtothelab.

TheobjectiveforthisexperimentisforthestudentstowritetheneededLabVIEWVItosampleananalogsignalanddisplayitonawaveformchartinLabVIEW.Theinstructionsprovidedinthestudent’ssectiondescribeastep-by-stepapproachtowritethebasicstructureoftheVI.Therestoftheimplementationislefttothestudents.TheVIprovidedisasampleimplementationandisintendedfortheinstructor’sreferenceuseandnotforthestudents.ThediagramcanbepasswordprotectedincasethestudentswanttolookatthefrontpanelandseehowtheVItheyarewritingshouldbehave.

Uses

Thisexperimentappliestogeneralinstrumentationandelectronicssystemsinterfacingcoursesinelectricalengineeringorengineeringtechnologyprograms.TheexperimentcanalsobeusefulinbasicLabVIEWcourses,teachingtheconceptsofanaloginputthroughdataacquisitioncards,inadditiontootherLabVIEWtechniquessuchastheuseofWaveformChartsandtheuseofstatisticalfunctionssuchasRMSandMean.

EquipmentList

∙PCrunningMSWindows（visit forrequirementsspecifictoyouroperatingsystem）

∙LabVIEWFullDevelopmentSystem

∙PCI1200DAQfromNationalInstruments（partnumber777386-01）

∙CB-50LPI/OConnectorBlockfromNationalInstruments（partnumber777101-01）

∙NBIRibbonCablefromNationalInstruments（partnumber180624-10）

∙TektronixTAS475FourChannelOscilloscope,oranygenerallabscopewith2ormorechannels.

∙HP3312AFunctionGenerator,oranygeneralfunctiongeneratorwithsinusoidalandsqaureoutputsignalsofvariablefrequency.

Setup

Followthestepslistedbelowtopreparetheworkstationforthisexperiment.Theinstructionsassumeyouareusingtheequipmentlistshownpreviously.

Note:

MostofthemanualsthatarereferredtoshipwithNationalInstrumentshardwareandsoftware.Ifyoucan’tfindyourhardcopyofthemanuals,youcangetthemonlineatIf youencounterproblemsduringsetup,contacttechnicalsupportat

BeforetheDayoftheLab

1.InstallLabVIEW（seetheLabVIEWReleaseNotesforyourversionofLabVIEW）.

2.InstallyourPCI-1200board（seethePCI1200UserManual,oronlineat:

3.ConnecttheribboncabletothePCI1200cardandtotheI/Oconnectorblock.

OntheDayoftheLab

4.Powerupthecomputer.

5.StartLabVIEW.

6.Powerupthefunctiongenerator.

References

∙NI’swebsite:

∙Schweber,William.ElectronicCommunicationSystems.UpperSaddleRiver:

Prentice-Hall,1999;p292-3.

∙Burns,MarkandRoberts.AnIntroductiontoMixed-SignalICTestandMeasurement.TexasInstruments,2000;p.174.

Student’sPortion

Introduction

Inthisexperiment,youwillwriteaLabVIEWVItoacquireananalogsignalfromtheanaloginputpinsofthePCI1200DataAcquisitionCard（DAQ）,displayitinLabVIEW,andcalculateitsRMSandaveragevalues.Youwillalsoshowtherelationshipofamplitudetonumberofaverages.ThislabrequiresbackgroundinanalogtodigitalconversionandtheconceptsofsamplingandNyquisttheorem.TheseconceptsaresummarizedintheTheorysectionofthisexperiment.

Objective

TolearnhowtouseLabVIEWforanaloginputthroughaDAQcard.

TolearnaboutAnalogtoDigitalconversionandhowtoacquireasignalanddisplayitinLabVIEW.

Tostudytheeffectofthesamplingfrequencyinanalogtodigitalconversion.

Theory

In1928,H.Nyquistshowedthatananalogsignalcouldbeperfectlyreconstructedsolelyfromitssamplevalues.Thiscouldbedonewithoutanylossofitsoriginalinformationifthesamplingrateisatleasttwicethebandwidthofthesignal,thusthefamousrule:

Nyquistsamplingrate=2xsignalbandwidth

Forexample,asignalwithabandwidthof1000Hzmustbesampledat2000ormoresamples/secondwithoutanylossofinformation.

Figure1.Sineandcosinewavessampledattwicethesignalfrequency

（Source:

Burns,MarkandRoberts.AnIntroductiontoMixed-SignalICTestandMeasurement）

Samplingatexactlytwicethesignalfrequencyisnotenoughinmostpracticalsituations.Figure1illustratesthisproblem:

“Asisevident,allthesamplesfromthesinewavearezerowhereasthosefromthecosinesignalarenot.Clearly,anyinformationcontainedinthesinewavesuchasitsamplitudewouldbelostandunobtainablefromthesamples.WecanthereforeconcludethatoneshouldnotattempttosampleatexactlytwicetheNyquistrate”

（Source:

Burns,MarkandRoberts.AnIntroductiontoMixed-SignalICTestandMeasurement,P.174）

Thisiswhyasamplingrateasmuchas10timesthesignalbandwidthisusedwhendoingananologtodigitalconversion.

QuantizationErrors

Thetermquantizationerror（orquantizationnoise）isusedtodescribethefactthatadigitalvaluecancorrespondtoadistinctspanofanalogsignals.AnyanalogsignalwithinthezoneofoneLSBwillhavethesamedigitalvalueandthereforelookidenticaltoallothersaftertheanalog-to-digitalprocess.Itisthissmalldegreeoferrorinmisrepresentationthatisknownasquantizationerror.Inordertocloselyreconstructtheoriginalanalogsignal,theanalogtodigitalconversionprocessneedstominimizethequantizationerrorasmuchaspossible.Toreducethequantizationerror,moredivisions（higherresolution）shouldbeusedtorepresenttheanalogvalue.Forthisexperiment,thistypeoferrorisminimizedbythefactthatthePCI1200boardsuseadevicewithrelativelygoodresolution（12-bits）.

Figure2.QuantizationError

（Source:

Schweber,William.ElectronicCommunicationSystems.p287）

Figure2,presentstheideathattheoriginalanalogsignal（a）canberepresentedinvaryingdegreesofresolutiontherebydeterminingtheamountofquantizationerrorincluded.Atwo-bitrepresentationasin（b）hasmorequantizationerrorthanthethree-bitrepresentationin（c）.Infact,witheveryincreasingbitofresolution,quantizationerroriscutinhalf.

Pre-LabPreparation

Readthisexperimentbeforecomingtothelab.YoushouldunderstandtheconceptsofsamplingandquantizationerrorsandstudytheNyquisttheorem.

Bringthefollowingtolabwithyou:

∙Thisexperiment.

∙Yourlabnotebookandpencil.

∙2virus-freeformatted3.5-inchfloppydisks（alwaysmakeabackupcopyofyourcodeontheseconddisk）.

Answerthefollowingquestionsinthedatasheetprovidedattheendofthisexperimentbeforecomingtothelab.Remembertoincludetheseanswersinyourlabreporttoo.

1.WhatarethepinnumbersforanalogoutputandanaloginputonthePCI1200DAQcard?

Howmanychannelsarethereforanaloginputandanalogoutput?

2.WhichVIisusedtoreadtheanalogsignalfromtheanaloginputpin?

3.WhichVIscalculatetheRMSandmeanvaluesofasignal?

WorkstationDetails

Yourworkstationshouldhavethefollowingitems:

∙ComputerwithNationalInstrumentsLabVIEWsoftware

∙NationalInstrumentsDAQboard（insidethecomputer）

∙NationalInstrumentsDAQboardterminalblock

∙TektronixOscilloscope.

∙HP3312AFunctionGenerator.

LabProcedure

1.StartanewVIinLabVIEW.Youshouldthenaddthefollowingcontrolsandindicatorstothefrontpanel:

a.Start/StopSwitch:

ThisisthemainOn/OffswitchfortheVI.

b.NumberofReadingsDigitalControl:

ThisvaluewillbeusedtocontrolthemainloopoftheVI.

c.NumberofSamplesperReadingDigitalControl:

ThisVIwilldeterminethenumberofbufferedsamplesforeachreadingfromtheanaloginput.

d.SampleRateDigitalControl:

ThisvaluewillconfigurethesamplingrateoftheanaloginputontheDAQ.

e.WaveformoftheAcquiredSignal:

Thischartwilldisplaytheacquiredanalogsignal.Itshouldmatchthesignalobtainedontheoscilloscopewhenmeasuredfromthefunctiongenerator.

f.RMSvalueDigitalIndicator:

YouwilldisplaytheRMSvalueofeachbufferedinputoftheacquiredsignal.

g.MeanvalueDigitalIndicator:

Youwilldisplaythemeanvalueofeachbufferedinputfromtheacquiredsignal.

h.AverageDCValueDigitalIndicator:

YouwilldisplaytherunningaverageDCvalue（averageofthemeanvalues）fortheentireacquiredsignal.

2.TheVIissupposedtoworkasfollows:

itreadsananalogsignalbyacquiringanumberof“samples”atacertain“samplingrate”（bufferedinput）.Theacquiredsignal,alongwiththemeanandRMSvaluesofthosesamplesaredisplayedonthefrontpanel.Then,theprocessrepeats.Eachiterationoftheloopiscalledone“reading”.ItisalsorequiredtodisplaytheaverageDCvalue（averageofmeansforallreadings）.

3.Afteraddingthosecontrols,startwiringtheVIdiagramswiththecontrolstoimplementthefunctionalityofyourprogram.YoumightfindthefollowingVIsusefulinyourcode:

a.Amainloop,bestimplementedasaForLoopstructure,whichwillbecontrolledbytheStart/Stopswitchandthenumberofreadingsdigitalcontrol.

b.AnAnalogInp