2114953_a_w - Fluke Corporation
Contents
1. Migrating from dc voltage dividers to modern reference multimeters Introduction Until the late 1980 s electrical calibration systems used to compare primary and secondary voltages and resistance standards consisted of several different components Systems like the Fluke 7105A and the Datron 4900 were the backbone of the majority of electrical calibration laboratories the world over These systems were specifically combined to provide a traceable source according to a set of measurement parameters For example the Fluke 7105A system comprised the following instruments e Fluke 720A Kelvin Varley Divider e Fluke 750A Reference Divider e Fluke 335A DC Voltage standard e Fluke 721A Lead Compensator e 845AR High Impedance Null Detectors Similarly a comparable system from Datron later acquired by Fluke in January 2000 was also available Much like the Fluke 7105A the Datron 4900 system included e 4901 Calibration Bridge Lead Compensator e 4902 DC Voltage Divider e 4903 DC Calibration Unit e 4904 Standard Cell Buffer However as new innovative technology and techniques were introduced both the 7105A and 4900 calibration systems were soon replaced So what caused their extinction eal EEN h E lt S f oN N SN SN gt Ag ne an i tr D Ur KEN oper re LIS DN 23 D f aS gt Fig 1 Fluke 7105A calibration system Applicati2. Switch the PWM technique operates on the basis of dimensionless ratio That is there are no absolute quantities involved that are subject to change over time offering very repeatable linearity dependent only upon an extremely reliable digital clocking waveform To further Fig 4 A representation of the output voltage prior maintain high confidence to filtering linearity is subsequently verified The development of Traditionally these comparisons during artifact calibration This artifact calibration were performed using an approach compares the same assortment of ratio measuring two fixed voltages Vi and V2 Artifact calibration is a process equipment to achieve this on different ranges Figure 5 where calibrators automatically However over the last twenty illustrates this comparison If perform internal ratiometric years instruments with artifact the PWM is perfectly linear comparisons and store the calibration capability have all oie empress extemal aa P eliminated many of thes eee ds labor intensive measurement tasks Many traditional manual operations to establish voltage DAC Setting resistance or current ratios can now be accomplished auto matically within the instrument consequently providing consis tent and efficient calibrations as well as significantly reducing the costs previously associated with higher labor intensity and a larger equipment inventory Fundamentally an instrument with artifac
3. andard Typical noise of the reference multimeter is less than 50 nV pk pk 71 2 Normal amp 81 2 Fast ADC modes with the differen tial linearity in 81 2 digit mode being better than O 1 ppm of range over a value ranging from 10 V to 1 V halve the typical linearity spec for values spanning the entire DMM scale from O to 19 99999 V Note the above procedure assumes that the 10 V dc reference standard being used is calibrated and has an assigned value The assigned value is keyed into the 8508A math memory subsequently correcting any residual gain error on the multimeters 20 V range Typical procedure sequence 1 Select DCV 20 V range 2 Connect low thermal cables to the 8508A front and rear terminals 8508A LEAD 3 Short the Front A and Rear B inputs at the cable ends of the 8508A and perform zero range function 4 Remove shorts and connect calibrator to 8508A front input 5 Connect 10 V reference standard e g Fluke 732B to 8508A rear input Configure 8508A rear input rolling average 16 samples 8 5 digits and record reference reading after initiating a two minute delay Select math mode deselect rolling average enter the reference standard value as recorded in step 6 into the 8508A m variable 8 Select scan F R and allow two minutes for stabilization 9 Select Math m 10 The 8508A will now perform the ratio reading multiply it by the traceable reference standard va
4. en PRT Platinum Resistance Thermometer calibrators This ultimately means faster calibrations reduced support costs greater throughput and minimal manual operations HIGH ORDER SWITCH MAIN Calibrators evolve with pulse width modulation PWM Pulse width modulation topology can be found in a variety of applications including various telecommunications applications power generation and signal processing Because of its exceptional linearity benefits most calibrators today now include this technique in their own internal ratio divider Such a circuit is typically made up from a two stage switching FET design with synchronous control clock This circuit passes the dc reference voltage through the switching FET array and then filters their summed outputs to provide an average output voltage that is determined by the resulting waveform s duty cycle see figures 3 and 4 FILTER LOW ORDER SWITCH SECONDARY FILTER Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters The output is then passed through a multi stage low pass filter network capable of elimi nating all ripple and noise content and thus providing a highly stable and linear output voltage The output voltage can be expressed using the formula Vo Vw x X n The ratio divider criterion in a calibrator is consequently set by the frequency of the control clock driving the two FETs As with any ratio divider
5. equently the need to compensate for these errors Having connected the 10 V reference to the Fluke 8508A s second input channel rear input the 100 V dc from the UUT being standardized can then be applied to the 8508A s front input channel This is typically done as shown in figure 7 The 8508A Reference Multimeter has two input channels that can be automatically switched to perform a ratio measurement The 10 V reference would be connected to the 8508A s rear input channel B with the UUT s 100 V dc voltage connected to the 8508A s front input channel A In Ratio mode the 8508A displays the ratio of the inputs in the form F R front minus rear or F R front as a percentage of the rear or F R R the difference as a percentage of the rear In the example given the F R i e the front as a percentage of the rear mode would be used In this mode with the 10 V dc reference connected to the rear channel and 100 V connected to the front channel the display would show 10 000 OOO This is the ratio of the unknown 100 V to the known 10 V reference Note that the reference multimeter is measuring the whole voltage for each channel and is configured to a single dc voltage range 200 V Consequently the only significant error contributions to this measurement are the uncertainty of the 10 V reference standard the noise and differential linearity of the reference multimeter and the noise of the UUT 100 V st
6. f calibrations However while being an acceptable method of calibration it does come at a price Therefore artifact technology is normally found on calibrators at the premium end of the range While lower performance less accurate calibrators forego artifact design adopting more traditional direct function to function range to range verification Even with artifact calibration it has been generally recom mended by all manufacturers to fully verify each range using external methods at least twice in its first year and then subsequently every two years It is this reason that many laboratories would and in some cases still do resort to more traditional calibration systems like the Fluke 7105A or Datron 4900 to accomplish this Today most laboratories with a large installed base of calibrators carry out the process of their verifications using high resolution digital multimeters like the Fluke 8508A Reference multimeter These meters can be used to perform all of the functions previously associated with the older calibration systems with little or no degradation of uncertainties Another real benefit with using these meters comes from the considerable reduction with inter connection leads and the immense time saved to re configure the setup for a different measurement CALIBRATED DIVIDER Fig 6 An example of a Fluke 7105A or datron 4900 voltage calibration system 4 Fluke Corporation Migrating from dc vol
7. libration Kit Other precision instruments in the range 5720A Multifunction 525A Temperature Pressure Calibrator Calibrator t i OUT TFE T w mia pe A The lowest uncertainties of any Superior accuracy and multifunction calibrator functionality in an economical benchtop package 9500B Oscilloscope Calibrator High accuracy calibration of analog and digital storage oscilloscopes up to 3 2 and 6 GHz Fluke 8508A Reference Multimeter Reference standard accuracy and stability in one functionally versatile easy to use solution Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters Fluke Keeping your world up and running Fluke Corporation PO Box 9090 Everett WA USA 98206 Fluke Europe B V PO Box 1186 5602 BD Eindhoven The Netherlands For more information call In the U S A 800 443 5853 or Fax 425 446 5116 In Europe M East Africa 31 40 2 675 200 or Fax 31 40 2 675 222 Canada 800 36 FLUKE or Fax 905 890 6866 From other countries 1 425 446 5500 or Fax 1 425 446 5116 Web 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
8. lue and display the UUT normalized value on the front display 11 It is recommended that after approximately 10 averaged readings have been recorded refer back to step 4 to regain a new reference point 6 7 6 Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters The uncertainty associated with this measurement is similar to that which might be obtained by a skilled metrologist with a newly calibrated voltage divider and a null detector In addition the reference multi meter can make this measure ment for prolonged periods as its linearity does not change significantly over time Uncertainty components 1 DMM short term stability when measuring 100 V 2 DMM short term stability when measuring 10 V 3 DMM linearity The 8508A specifications detail the uncertainty for the DMM as 10 V measured on the 200 V range to eliminate range switching using 20 minute transfer specification 0 4 ppm of Reading O 1 ppm of range RSS above result with 100 V measured on the 200 V range using similar transfer specification and the total measurement uncertainty is approximately 2 5 ppm of Ratio The 0 4 ppm of reading accounts for noise and O 1 ppm of range accounts for a conservative linearity specification Therefore as two individual measurements are being performed during ratio mode the transfer specification for each measurement is RSS d together yield a measure
9. m but moreover allowing full automation of virtually all measurement tasks This in turn liberated the senior metrologist from this task and allowed him her to concentrate on other important laboratory responsibilities Reference multimeter with reference standard accuracy and stability High resolution precision DMMs have been available for almost thirteen years but since their launch in the late 1980s the products have remained comparable in both performance and application Since Fluke s acquisition of precision instrument manufacturer Wavetek Datron in 2000 design teams in the US and UK have worked together and pooled their expertise to produce the best in precision and long scale DMM design The Fluke 8508A Reference Multimeter The 8508A has taken many of the leading Fluke and Wavetek Datron patented multimeter designs and then improved them further using the latest state of the art technology and new electronic measurement design techniques For the example given in Figure 6 the Fluke 8508A eliminates every instrument other than the 10 V reference In essence the function of both the ratio divider network and null detector has now been replicated into the 8508A Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters 5 Furthermore all inter connecting leads that existed between these two instruments have also been eradicated removing the probability of lead errors and cons
10. ment uncertainty of 2 5 ppm of Ratio reading Some additional uncertainty maybe considered for the source 0 040 WV J v 0 010 0 020 O N O Q amp oe 15 o S fan O a t 5 58 m Input Voltage Volts Fig 8 A typical example of type testing linearity on the Fluke 8508A A conservative linearity specification is assigned to multimeters during development as part of type testing The specification supports a worst case linearity measurement which is often at the two extremes of a range In practice this spec is often very conservative an example would be to compare a known traceable 10 V with unknown 10 V standard on a fixed range what is the linearity spec In this example there is negligible contribution to linearity as each measurement is being made at the same point in a given range Linearity measurements could be performed on every multimeter to yield better specs However who should determine how many points are measured to gain confidence This exercise is time consuming and requires extremely low measurement uncertainty often only achievable using JJ Array standards Figure 8 shows an example of type testing linearity on the 8508A multimeter The graph indicates deviation from ideal in ppm of range at various points on the multimeters 20 V range A published spec of 0 2 ppm of range is assigned yet type testing results in bette
11. on Note Fig 2 Datron 4900 calibration system From the Fluke Digital Library www fluke com library 2 Evolution caused these more mature calibration systems to begin the road to obsolescence There have been several contributing factors to the demise of the old 7105A and 4900 calibration systems First the development of artifact calibration has not only consolidated the system into a single device but has also fully automated the process second the design of modern calibrators incorporates pulse width modulation PWM techniques to maintain a right by design philosophy that provides extremely repeatable source linearity Furthermore zener reference technology improved and when incorporated within calibration equipment subsequently improved stability reducing uncertainties Finally high resolution DMMs like the Wavetek 1281 then managed to combine these features into a highly accurate electrical measurement instrument Fig 3 Two stage PWM circuit More recently the introduction of the Fluke 8508A Reference Multimeter has taken all of these philosophies a step further to improve accuracy linearity and stability and has combined them into a functionally versatile easy to use solution This has enabled metrologists to perform highly accurate and automated measurement tasks within a single instrument replacing the need for Kelvin Varley dividers null detectors resistance bridges and ev
12. r than 0 035 ppm of range as indicated by the two extremes of deviation from the ideal lin earity Summary In summary a comparison of measurement uncertainty between divider system and 85084A will often favor the traditional divider systems However the improvements in measurement uncertainty will be at a cost Uncertainty may be compromised using modern instrumentation but further consideration must be given to the intended application and how it may have changed Precision calibrators once required divider type measurement uncertainty now implement artifact calibration Where linearity verification is involved divider technology is now internal to the calibrator design This is not true of all calibrators where linearity measurement using multimeter performance remains adequate for lower priced less accurate calibrators Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters 7 Ordering information Model 8508A 8 5 digit Reference Multimeter Certificate of Calibration and User Manual 8508A 01 8 5 digit Reference Multimeter with front amp rear input binding posts Certificate of Calibration and User Manual Accessories NVLAP NVLAP Accredited Calibration UKAS UKAS Accredited Calibration 8508A SPRT Standard Platinum Resistance Thermometer 8508A PRT 100 Q PRT 8508A LEAD Comprehensive Measurement Lead Kit Y8508 Rack Mount Kit Y85088S Rack Mount Kit Slides 8508 7000K Ca
13. t calibration capabilities will first transfer 20 Volt Range and then reference to a set of external artifact voltage and resistance standards Having been transferred this internal voltage reference can then be configured to appear as if it had been applied to an internal array of comparable instruments Output Voltage like a Kelvin Varley divider a null detector or even a decade divider though in practice this Fig 5 Converter linearity verification is not really the case 10 Volt Range V2 Fluke Corporation Migrating from dc voltage dividers to modern reference multimeters 3 Technology advances have shrunk the null detector onto a miniature hybrid integrated circuit the ratio system is now a single PWM printed circuit board and improved lower cost thin film resistor networks have replaced bulky wire wound resistor ratios This kind of advance in technology now means that designers can produce highly comprehensive instruments with artifact calibration capability So now having eliminated these extra external devices and mimicked the same capability from within the instrument we can now transfer the accuracy of the artifact to the various ranges of the instrument with minimal uncertainty with greater accuracy and stability and with complete traceability The dawning of the high resolution DMMs As discussed earlier artifact calibration is a particularly efficient and easy method of carrying out a multitude o
14. tage dividers to modern reference multimeters Add to this the ability to fully automate the calibration process and the reference multimeter becomes very easy to justify over most of the traditional systems 10 V STANDARD Calibrating the calibrator before the days of long scale DMMs The advantages of using a single high resolution DMM over the traditional multi instrument calibration systems are probably best demonstrated by firstly describing how a typical calibration would have been carried out Figure 6 illustrates the source calibrator UUT an external divider null detector and a 10 V dc reference in a conventional voltage calibration setup Here the 100 V dc source UUT is being verified against a 10 V reference using a ratio divider of 10 1 In reality the ratio divider would have several taps calibrated to a set of definitive ratios i e 100 1 10 1 1 1 V and 0 1 1 V Before any verification of voltage could take place the individual ratios would have first been calibrated separately so that the given ratios exactly represented the source instrument s output voltage at each voltage step The possibility of drift with time due to the temperature Ratio Measurement 100 V 10V as s0 P100000716u T gt 2 a Rear 10 V Fluke 7001 Voltage Reference F 100 000 000 V R 10 000 000 0 V F R 10 000 000 F R R 0 100 000 O coefficient of
15. the ratio resistors meant that this process would have required a skilled metrologist who knew how to perform this operation both competently and promptly Having calibrated the divider the UUT calibrator could now be connected as described in figure 6 With a null detector between the ratio divider and 10 V dc reference the source calibrator would now be adjusted until the null detector indicator displayed zero Any residual error would contribute to the expanded uncertainty From this brief description you can probably begin to comprehend how complex this particular measurement process is In addition the lack of any kind of remote capability the time consuming makeup of the procedure and above all the overall cost of the system only serves to further compound the situation Nonetheless advances in technology coupled with the need to make the methodology simpler faster cheaper and more efficient help set a new precedent within the industry Fluke 5520A Multi Product Calibrator UUT Fig Z An example of the reference multimeter being used to accurately verify the output of the artifact calibrator to a known voltage reference Early precision high resolution DMMs like the Fluke 8505 8506 consolidated the methods used by all of the test devices illustrated in figure 6 into a single instrument so eliminating most interconnecting lead errors greatly reducing the overall cost of the calibration syste
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