Migrating From Dc Voltage Dividers To Modern Reference .
Migrating from dc voltage dividers tomodern reference multimetersApplication NoteIntroductionUntil the late 1980’s electricalcalibration systems used tocompare primary and secondaryvoltages and resistance standardsconsisted of several differentcomponents. Systems like theFluke 7105A and the Datron4900 were the backbone of themajority of electrical calibrationlaboratories the world over.These systems were specificallycombined to provide a traceablesource, according to a set ofmeasurement parameters. Forexample, the Fluke 7105Asystem comprised the followinginstruments: Fluke 720A Kelvin VarleyDivider Fluke 750A Reference Divider Fluke 335A DC VoltageStandard Fluke 721A Lead Compensator 845AR High Impedance NullDetectorsSimilarly, a comparable systemfrom Datron (later acquired byFluke in January 2000) wasalso available. Much like theFluke 7105A, the Datron 4900system included: 4901 Calibration Bridge/LeadCompensator 4902 DC Voltage Divider 4903 DC Calibration Unit 4904 Standard Cell BufferHowever, as new innovativetechnology and techniqueswere introduced, both the7105A and 4900 calibrationsystems were soon replaced. Sowhat caused their extinction?Fig. 1 Fluke 7105A calibration systemFig. 2 Datron 4900 calibration systemFrom the Fluke Digital Library @ www.fluke.com/library
Evolution caused thesemore mature calibrationsystems to begin theroad to obsolescenceThere have been severalcontributing factors to thedemise of the old 7105A and4900 calibration systems. First,the development of artifactcalibration has not onlyconsolidated the system into asingle device, but has also fullyautomated the process.Second, the design of moderncalibrators incorporates pulsewidth modulation (PWM)techniques to maintain a ‘rightby design philosophy’ thatprovides extremely repeatablesource linearity. Furthermore,zener reference technologyimproved, and, whenincorporated within calibrationequipment, subsequentlyimproved stability reducinguncertainties. Finally, highresolution DMMs like theWavetek 1281 then managedto combine these features intoa highly accurate electricalmeasurement instrument.More recently, the introductionof the Fluke 8508A ReferenceMultimeter has taken all ofthese philosophies a stepfurther to improve accuracy,linearity and stability, and hascombined them into afunctionally versatile, easy touse solution. This has enabledmetrologists to perform highlyaccurate and automatedmeasurement tasks within asingle instrument, replacing theneed for Kelvin-Varley dividers,null detectors, resistance bridgesand even PRT (PlatinumResistance Thermometer)calibrators. This ultimatelymeans faster calibrations,reduced support costs, greaterthroughput and minimalmanual operations.Fig. 3 Two stage PWM circuit2Fluke Corporation Migrating from dc voltage dividers to modern reference multimetersCalibrators evolve withpulse width modulation(PWM)Pulse width modulation topologycan be found in a variety ofapplications, including varioustelecommunications applications,power generation and signalprocessing. Because of itsexceptional linearity benefits,most calibrators today nowinclude this technique in theirown internal ratio divider. Sucha circuit is typically made upfrom a two stage switching FETdesign with synchronouscontrol clock. This circuitpasses the dc reference voltagethrough the switching FETarray and then filters theirsummed outputs to provide anaverage output voltage that isdetermined by the resultingwaveform’s duty cycle. (seefigures 3 and 4).
The output is then passedthrough a multi-stage, low passfilter network, capable of eliminating all ripple and noisecontent, and thus providing ahighly stable and linear outputvoltage. The output voltagecan be expressed using theformula: VO VIN x X/nThe ratio divider criterion in acalibrator is consequently setby the frequency of the controlclock driving the two FETs.As with any ratio divider,the PWM technique operateson the basis of ‘dimensionless’ratio. That is, there are noabsolute quantities involvedthat are subject to change overtime offering very repeatablelinearity dependent only uponan extremely reliable digitalclocking waveform. To furthermaintain high confidence,linearity is subsequently verifiedduring artifact calibration. Thisapproach compares the sametwo fixed voltages, V1 and V2,on different ranges. Figure 5illustrates this comparison. Ifthe PWM is perfectly linear,then N4/N3 N2/N1.Fig. 5 Converter linearity verificationFig. 4 A representation of the output voltage priorto filteringThe development ofartifact calibrationArtifact calibration is a processwhere calibrators automaticallyperform internal ratiometriccomparisons and store thecorrection data relative to afew precise external artifactstandards.Traditionally these comparisonswere performed using anassortment of ratio measuringequipment to achieve this.However, over the last twentyyears, instruments with artifactcalibration capability have allbut eliminated many of theselabor-intensive measurementtasks. Many traditional manualoperations to establish voltage,resistance or current ratios cannow be accomplished automatically within the instrument,consequently providing consistent and efficient calibrations,as well as significantly reducingthe costs previously associatedwith higher labor intensity anda larger equipment inventory.Fundamentally, an instrumentwith artifact calibrationcapabilities will first transferand then reference to a set ofexternal artifact voltage andresistance standards. Havingbeen transferred, this internalvoltage reference can then beconfigured to appear as if ithad been applied to an internalarray of comparable instrumentslike a Kelvin-Varley divider, anull detector or even a decadedivider, though in practice thisis not really the case.Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters3
Technology advances haveshrunk the null detector onto aminiature hybrid integratedcircuit, the ratio system is nowa single PWM printed circuitboard, and improved lower costthin film resistor networks havereplaced bulky wire-woundresistor ratios. This kind ofadvance in technology nowmeans that designers canproduce highly comprehensiveinstruments with artifactcalibration capability. So now,having eliminated these extraexternal devices and mimickedthe same capability from withinthe instrument, we can nowtransfer the accuracy of theartifact to the various ranges ofthe instrument with minimaluncertainty, with greateraccuracy and stability, andwith complete traceability.The dawning of the highresolution DMMsAs discussed earlier, artifactcalibration is a particularlyefficient and easy method ofcarrying out a multitude ofcalibrations. However, whilebeing an acceptable method ofcalibration, it does come at aprice. Therefore, artifacttechnology is normally foundon calibrators at the premiumend of the range.While lower performance, lessaccurate calibrators foregoartifact design, adopting moretraditional direct function-tofunction, range-to-rangeverification.Even with artifact calibration,it has been generally recommended by all manufacturers tofully verify each range usingexternal methods at least twicein its first year, and thensubsequently every two years.It is this reason that manylaboratories would, and in somecases still do, resort to moretraditional calibration systems,like the Fluke 7105A or Datron4900, to accomplish this.Today, most laboratorieswith a large installed base ofcalibrators carry out theprocess of their verificationsusing high resolution digitalmultimeters like the Fluke8508A Reference multimeter.These meters can be used toperform all of the functionspreviously associated with theolder calibration systems withlittle or no degradation ofuncertainties. Another realbenefit with using these meterscomes from the considerablereduction with inter-connectionleads and the immense timesaved to re-configure the setupfor a different measurement.Fig. 6 An example of a Fluke 7105A or datron 4900 voltage calibration system4Fluke Corporation Migrating from dc voltage dividers to modern reference multimetersAdd to this the ability to fullyautomate the calibrationprocess, and the referencemultimeter becomes very easyto justify over most of thetraditional systems.
Calibrating the calibratorbefore the days of longscale DMMsThe advantages of using asingle high resolution DMMover the traditional multiinstrument calibration systemsare probably best demonstratedby firstly describing how atypical calibration would havebeen carried out. Figure 6illustrates the source calibrator(UUT), an external divider, nulldetector and a 10 V dc referencein a conventional voltagecalibration setup.Here the 100 V dc source(UUT) is being verified againsta 10 V reference, using a ratiodivider of 10:1. In reality theratio divider would haveseveral ‘taps’ calibrated to a setof definitive ratios i.e. 100:1,10:1, 1:1 V and 0.1:1 V. Beforeany verification of voltage couldtake place, the individual ratioswould have first been calibratedseparately so that the givenratios exactly represented thesource instrument’s outputvoltage at each voltage step.The possibility of drift withtime, due to the temperaturecoefficient of the ratio resistors,meant that this process wouldhave required a skilledmetrologist who knew how toperform this operation bothcompetently and promptly.Having calibrated the divider,the UUT calibrator could nowbe connected as described infigure 6. With a null detectorbetween the ratio divider and10 V dc reference, the sourcecalibrator would now beadjusted until the null detectorindicator displayed zero. (Anyresidual error would contributeto the expanded uncertainty.)From this brief description,you can probably begin tocomprehend how complex thisparticular measurement processis. In addition, the lack of anykind of remote capability, thetime consuming makeup of theprocedure and, above all, theoverall cost of the system, onlyserves to further compound thesituation. Nonetheless,advances in technology, coupledwith the need to make themethodology simpler, faster,cheaper and more efficient,help set a new precedentwithin the industry.Early precision high resolutionDMMs like the Fluke 8505/8506consolidated the methods usedby all of the test devicesillustrated in figure 6 into asingle instrument, so eliminatingmost interconnecting leaderrors, greatly reducing theoverall cost of the calibrationsystem, but moreover, allowingfull automation of virtually allmeasurement tasks. This inturn liberated the seniormetrologist from this task andallowed him/her to concentrateon other important laboratoryresponsibilities.Reference multimeterwith reference standardaccuracy and stabilityHigh resolution precision DMMshave been available for almostthirteen years, but since theirlaunch in the late 1980s theproducts have remainedcomparable in both performanceand application. Since Fluke’sacquisition of precisioninstrument manufacturerWavetek-Datron in 2000,design teams in the US and UKhave worked together andpooled their expertise toproduce the best in precisionand long-scale DMM design The Fluke 8508A ReferenceMultimeter. The 8508A hastaken many of the leadingFluke and Wavetek-Datronpatented multimeter designsand then improved themfurther, using the latest state ofthe art technology and newelectronic measurement designtechniques.For the example given inFigure 6, the Fluke 8508Aeliminates every instrumentother than the 10 V reference.In essence, the function of boththe ratio divider network andnull detector has now beenreplicated into the 8508A.Fig. 7 An example of the reference multimeter being used to accurately verifythe output of the artifact calibrator to a known voltage referenceFluke Corporation Migrating from dc voltage dividers to modern reference multimeters5
Furthermore, all interconnecting leads that existedbetween these two instrumentshave also been eradicated,removing the probability oflead errors and consequentlythe need to compensate forthese errors. Having connectedthe 10 V reference to the Fluke8508A’s second input channel(rear input), the 100 V dc fromthe UUT being standardizedcan then be applied to the8508A’s front input channel.This is typically done as shownin figure 7. The 8508AReference Multimeter has twoinput channels that can beautomatically switched toperform a ratio measurement.The 10 V reference would beconnected to the 8508A’s rearinput (channel B), with theUUT’s 100 V dc voltageconnected to the 8508A’s frontinput (channel A). In Ratiomode, the 8508A displays theratio of the inputs in the formF-R (front minus rear), or F/R(front as a percentage of therear), or (F-R)/R (the differenceas a percentage of the rear). Inthe example given, the F/R (i.e.the front as a percentage of therear) mode would be used. Inthis mode, with the 10 V dcreference connected to the rearchannel and 100 V connectedto the front channel, the displaywould show 10.000 000 %.This is the ratio of the unknown100 V to the known 10 Vreference. Note that the referencemultimeter is measuring thewhole voltage for each channeland is configured to a single dcvoltage range (200 V).Consequently, the only significanterror contributions to thismeasurement are the uncertaintyof the 10 V reference standard,the noise and differentiallinearity of the referencemultimeter and the noise of theUUT 100 V standard.6Typical noise of the referencemultimeter is less than 50 nVpk/pk (7½ Normal & 8½ FastADC modes) with the differential linearity in 8½ digit modebeing better than 0.1 ppm ofrange over a value rangingfrom 10 V to 1 V (halve thetypical linearity spec for valuesspanning the entire DMM scalefrom 0 to 19.99999 V).Note: the above procedureassumes that the 10 V dcreference standard being usedis calibrated and has anassigned value. The assignedvalue is keyed into the 8508Amath memory subsequentlycorrecting any residual gainerror on the multimeters 20 Vrange.Typical procedure sequence:1) Select DCV 20 V range2) Connect low thermal cablesto the 8508A front and rearterminals (8508A-LEAD).3) Short the Front A and RearB inputs at the cable endsof the 8508A and performzero range function.4) Remove shorts and connectcalibrator to 8508A front input.5) Connect 10 V referencestandard (e.g. Fluke 732B)to 8508A rear input.6) Configure 8508A, rear input,rolling average, 16 samples,8.5 digits, and record referencereading after initiating atwo-minute delay.7) Select math mode, deselectrolling average, enter thereference standard value(as recorded in step 6) intothe 8508A “m” variable.8) Select scan, F/R and allowtwo minutes for stabilization.9) Select Math *m.10) The 8508A will nowperform the ratio reading,multiply it by the traceablereference standard valueand display the UUTnormalized value on thefront display.11) It is recommended thatafter approximately 10averaged readings havebeen recorded, refer backto step #4 to regain a newreference point.Fluke Corporation Migrating from dc voltage dividers to modern reference multimetersThe uncertainty associatedwith this measurement issimilar to that which might beobtained by a skilled metrologistwith a newly calibrated voltagedivider and a null detector. Inaddition, the reference multimeter can make this measurement for prolonged periods, asits linearity does not changesignificantly over time.Uncertainty components.1) DMM short term stabilitywhen measuring 100 V.2) DMM short term stabilitywhen measuring 10 V.3) DMM linearity.The 8508A specifications detailthe uncertainty for the DMM as:10 V measured on the 200 Vrange to eliminate rangeswitching using 20 minutetransfer specification 0.4 ppmof Reading 0.1 ppm of rangeRSS above result with 100 Vmeasured on the 200 V rangeusing similar transfer specificationand the total measurementuncertainty is approximately2.5 ppm of Ratio.The 0.4 ppm of readingaccounts for noise and 0.1 ppmof range accounts for aconservative linearity specification.Therefore as two individualmeasurements are beingperformed during ratio modethe transfer specification foreach measurement is RSS’dtogether yield a measurementuncertainty of 2.5 ppm of Ratioreading. Some additionaluncertainty maybe consideredfor the source.
Fig. 8 A typical example of type testing linearity on the Fluke 8508AA conservative linearityspecification is assigned tomultimeters during developmentas part of type testing. Thespecification supports a worstcase linearity measurementwhich is often at the twoextremes of a range. In practicethis spec is often very conservative,an example would be tocompare a known (traceable)10 V with unknown 10 Vstandard on a fixed range what is the linearity spec? Inthis example there is negligiblecontribution to linearity as eachmeasurement is being made atthe same point in a givenrange. Linearity measurementscould be performed on everymultimeter to yield betterspecs. However, who shoulddetermine how many points aremeasured to gain confidence?This exercise is time consumingand requires extremely lowmeasurement uncertainty oftenonly achievable using JJ Arraystandards.Figure 8 shows an example oftype testing linearity on the8508A multimeter. The graphindicates deviation from idealin ppm of range at variouspoints on the multimeters 20 Vrange. A published spec of 0.2ppm of range is assigned, yettype testing results in betterthan 0.035 ppm of range asindicated by the two extremesof deviation from the ideal linearity.Precision calibrators oncerequired divider typemeasurement uncertainty nowimplement artifact calibration.Where linearity verification isinvolved divider technology isnow internal to the calibratordesign. This is not true of allcalibrators where linearitymeasurement using multimeterperformance remains adequatefor lower priced, less accuratecalibrators.SummaryIn summary, a comparison ofmeasurement uncertaintybetween divider system and8508A will often favor thetraditional divider systems.However the improvements inmeasurement uncertainty willbe at a cost!Uncertainty may becompromised using moderninstrumentation, but furtherconsideration must be given tothe intended application andhow it may have changed.Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters 7
Ordering informationModel8508A8508A/018.5 digit Reference Multimeter, Certificate of Calibration and User Manual8.5 digit Reference Multimeter with front & rear input binding posts,Certificate of Calibration and User -LEADY8508Y8508S8508-7000KNVLAP Accredited CalibrationUKAS Accredited CalibrationStandard Platinum Resistance Thermometer100 Ω PRTComprehensive Measurement Lead KitRack Mount KitRack Mount Kit SlidesCalibration KitOther precision instrumentsin the range5720A MultifunctionCalibrator525A Temperature/PressureCalibratorThe lowest uncertainties of anymultifunction calibratorSuperior accuracy andfunctionality in an economicalbenchtop package9500B OscilloscopeCalibratorHigh accuracy calibration ofanalog and digital-storageoscilloscopes up to 3.2 and6 GHzFluke. Keeping your worldup and running.Fluke CorporationPO Box 9090, Everett, WA USA 98206Fluke 8508A Reference MultimeterReference standard accuracy and stability, in onefunctionally versatile, easy to use solutionFluke CorporationMigrating from dc voltage dividers to modern reference multimetersFluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The NetherlandsFor more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222Canada (800)-36-FLUKE orFax (905) 890-6866From other countries 1 (425) 446-5500 orFax 1 (425) 446-5116Web access: http://www.fluke.com/ 2003 Fluke Corporation. All rights reserved.Trademarks are the property of their respective owners.Printed in UK 10/2003 2114953 D-ENG-N Rev A, DS271
example, the Fluke 7105A system comprised the following instruments: Fluke 720A Kelvin Varley Divider Fluke 750A Reference Divider Fluke 335A DC Voltage Standard Fluke 721A Lead Compensator 845AR High Impedance Null Detectors Similarly, a comparable system from Datron (later acquired by
7-4: Series Voltage Divider with Parallel Load Current Voltage dividers are often used to tap off part of the applied voltage for a load that needs less than the total voltage. Fig. 7-6: Effect of a parallel load in part of a series voltage divider . (a) R1 and
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Design And Performance Analysis Of 1:6 Power Divider For L Band Frequency 17 From the figure, it shows that 1:6 power dividers is formed from Wilkinson 1:2 power divider that is commonly used in several power dividers which is combined with 2 of 1:3 power dividers that are developed from [2]. FR4 substrate is used with nine
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Situations abound in integrated circuits design where a voltage level different from the power supply voltage is needed. A traditional way to obtain a lower voltage is to use a resistor-in-series voltage divider. This design provides reference voltage values with desirable accuracy and is also robust to temperature change and manufacturing .
Amplifier Configurations . Voltage Amplifier: Voltage input and Voltage output The controlled source is a Voltage -controlled-Voltage Source A. v Open Circuit Voltage Gain can be found by applying a voltage source with R. s 0, and measuring the open circuit output voltage(no load or R. L infinity) Source Amplifier Load . Amplifier Input .
IEC 60060-2. Divider scale factor matrix The different scale factors of different voltage dividers in a test field (DC, AC, SI, and LI, according to IEC 60060-2) can be entered into a predefined matrix and will be automatically applied by the software. Up to 10 different dividers with th
in advanced mathematics used in US universities are also popular in Australian universities for students studying engineering and some areas of applied sciences. However, the advanced mathematics .
