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Bill Gaviria Regional Product Manager, Electrical, RF and Software Fluke Calibration

Welcome to this Seminar from Fluke Calibration…. Improved Measurement Techniques In DC & Low Frequency AC Metrology. Bill Gaviria Regional Product Manager, Electrical, RF and Software Fluke Calibration Office : +1.321.574.0728 Direct: +1.425.446.6031 Cell: +1.321.626.7845 

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Bill Gaviria Regional Product Manager, Electrical, RF and Software Fluke Calibration

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  1. Welcome to this Seminar from Fluke Calibration….. Improved Measurement Techniques In DC & Low Frequency AC Metrology Bill Gaviria Regional Product Manager, Electrical, RF and Software Fluke Calibration Office: +1.321.574.0728 Direct: +1.425.446.6031Cell: +1.321.626.7845  Email:  Bill.gaviria@flukecal.com Web:  www.flukecal.com (UTC/GMT-5)

  2. Session Topic • Improved Metrology Measurements Using Precision Digital Multimeters (or Long Scale DMMs) • Describe the long scale DMM & its metrology application • Introduce new capabilities of a new class of Multimeter, the Fluke 8508A Reference Multimeter • Provide Technical Overviews of Improved Measurement Capabilities • Eliminating Common Measurement Errors

  3. Long Scale DMMs are Versatile • 8 1/2 Digit DMM can replace the following ... • Standard Cell Comparators • Null Detectors • Nanovoltmeters • Kelvin Varley Dividers • Resistance Bridges • AC/DC Transfer Standards • Multifunction Transfer Standards

  4. What is a long Scale DMM? • Typically 8 1/2 digits of Measurement Resolution • ±1.00000000 • A High Resolution Analog To Digital Converter with from 120 Million to 200 Million counts • Maximum Measurements from 120% or 200% of the range (examples – 1.19999999 or 1.99999999) • Very good Long and Short term stability: • 3-6 ppm 1 year • 0.5-1 ppm 24 hours • DCV, ACV, Ohms, DCI, ACI functionality • Frequency, math, ratio, IEEE • High input impedance

  5. Critical DC Measurement Specifications • Fast, Multi-Slope, Multi-Cycle Analog to Digital Converter • Proven Linearity <0.05ppm of Full Scale • High Sensitivity/Low Noise • Short term stability of 0.12 ppm • 1 nV resolution • High Input Impedance >1010  • Low Bias Current - Typically <10pA • Wide Dynamic Range 2x108 Counts • Stable Reference <2ppm/year

  6. New Class of Multimeter, the8508A Reference Multimeter • Combines the Long Scale DMM with technology from other functions used in Metrology • Includes features from • Electrometers, • Pico-ammeters, • External ac/dc current shunts • Micro-ohmmeters, • Precision thermometers • External shunts

  7. Some Practical DC Applications • Direct Measurements with up to .01 ppm resolution and .12ppm short term stability • Using the Multimeter as a Nanovoltmeter • Voltage Reference Intercomparison • Automated, Long-Scale “Null Detector”

  8. Comparing Voltage Standards Difference Measurement 10V-10V Front Input

  9. Using the Long Scale DMM as a Null Detector • Key Attributes for Intercomparing Voltage Standards • Short Term Stability (0.12 ppm in 1V) • High Input Impedance (>1010Ohm up to 20V) • Low Noise (<50nV) • Good Resolution (1nV) • Excellent CMRR (140dB at DC)

  10. Ratio Mode and Rear Inputs Key Feature for DC/LF Metrology • Selectable Rear + Front Inputs • Automatic Channel Switching • Ratio:- A-B, A/B, (A-B)/B + Math... • High Relative Accuracy • Voltage Ratio Calibration Rear Panel

  11. V Using Rear Inputs to Compare Voltage Standards Rear Inputs Ratio Measurement 10V:1.018V Front Inputs Front 10V B = +10.000 000 0 V A = + 1.018 165 2 V A/B(%)= + 10.181 652 % A/B(%)/Z= + 1.018 165 2 (%)

  12. Long Scale DMM and Reference Multimeters can Replace the Kelvin Varley Divider • Voltage Ratio Measurements • Key Attributes • Linearity (0.1ppm to 20V) • Scale Length (±1.999 999 99) • High Input Impedance (>1010Ohm up to 20V) • Ratio Switch • Fully floating input • 8508A will replace Kelvin Varley and Reference Dividers • Fluke 720A • Datron 4900 series

  13. Using the Multimeter as a AC/DC Transfer Standard • AC Measurement by precision DMMs now often replace measurements formerly done with AC/DC Transfer Standards • Precision AC measurement is simpler because of the DMM’s measurement technique, as compared to the more complex AC/DC transfer technique

  14. Outperforms Traditional AC/DC Transfer Standards • Traditional AC/DC Transfer • Multiple Measurements made with AC values compared relative to a DC value • Accuracy commonly limited to 100 ppm • Best performance limited to specific voltage/frequency combinations points • Not easy to use and can be very slow • Modern DMMs • Needs only a single measurement • Accuracy commonly made to 60 ppm accuracies • Performance improves with spot frequency capability • Useable at any volt/frequency combination within its amplitude and bandwidth limits • Faster, more reliable and simpler measurement techniques

  15. Ammeter Applications • Low level, high frequency current measurements are subject to large errors caused by leakage impedance and instrument burden • Pico-ammeters and electrometers use high gain amplifier with negative feedback for the input stage (a virtual ground input technique) • 8508A uses this feedback technique • Lowers burden voltage • Higher bandwidth available because fewer errors • Plus, internal 20A current shunt

  16. Multimeter . . . . . . . x1 Buffer Shunt . Current Source . . Reference Multimeter Multimeter . . . . . . . . x1 . Op -Amp Shunt . Current Source . . . Current measurement techniques • Shunt Ammeter • More susceptible to leakage currents • But, more practical for higher currents • 8508A has internal 20A shunt • Feedback Ammeter • Pico-ammeter topology • Minimal burden current • Easier to guard (virtual ground) • Higher bandwidth (100 kHz) • 8508A has 10 pA resolution

  17. Resistance Applications • Wide Measurement Ranges (2to 20G in the 8508A) • Standard Resistor Comparisons • Ohms to Ohms Ratio Measurement Technique • Voltage RatioTechnique Micro-ohmmeter applications • High Voltage ohms measurements

  18. Resistance Features of the Reference Multimeter • All of DC Advantages + • Static and Dynamic Offset Rejection via True Bipolar Ohms Current Switching • High or Low (selectable) Test Current (up to 100 mA) • High voltage (200V) stimulus selectable • Insensitive to Lead Resistance (100 Ohms) • Active Guard Eliminates Leakage • Ratio Mode allows Automation

  19. 8508A Reference Multimeter has features of Micro-ohmmeters • 2 ohm range (100 mA stimulus) • 10 nano ohm resolution • Bipolar True Ohms to eliminate measurement errors caused by thermal emfs • Selectable source current levels (down to 200 mV max compliance)

  20. DC Voltage Pre-Amp Input Hi Sense Hi Rx Sense Lo Input Lo Lo Follower Constant Current Sink Ohms Range Control Resistance Measurement Topology • Current source sinks current from Input to Lo • Low Follower maintains Sense Lo at 0V • Resulting potential difference measured via Sense Hi by dc Voltage sub-system

  21. (Rx) V Front Active (Rs) V Rear Rs = Standard Resistor Rx = Unknown Resistor Traditional DMM Resistance Ratio Measurement Techniques • Two input channels • front & rear terminals • Typical application: • Comparing resistance standards • Stimulus current & potential difference measurement scanned between inputs • each resistor connected separately to measurement circuits • But… resistor power dissipation modulated at scan rate • can lead to errors due to resistor temperature changes

  22. INPUT Hi SENSE Hi Front Input SENSE Lo INPUT Lo Potential Difference Measurement • Stimulus Current Source (Reversing) INPUT Hi SENSE Hi Rear Input SENSE Lo INPUT Lo 8508 Resistance Ratio Measurement Technique • Stimulus current passes continuously though both resistors in series • Potential difference measurement scanned between the two (front & rear) channels

  23. Using the 8508A as a precision thermometer • Direct temperature readout • 2, 3, & 4 wire PRT probe connections • 1mA excitation current • Current Reversal Tru Ohms • 8508A stores coefficients for up to 100 SPRT/RTD probes • ITS 90 & Callendar van Dusen • Optional SPRT & RTDs • -200C to 660C • 8508A - SPRT - Hart 5699 • 8508A - PRT - Hart 5626

  24. REFERENCE SPRT UUT RTD Using the 8508A to calibrate PRTs • Front and Rear inputs provide excellent Resistance transfer capability • Lo I excitation (1 mA) • 4-wire Ohms • Bipolar True Ohms • Use automation (MET/CAL), to cal SPRTs

  25. Reference Multimeters can replace Traditional Instruments • Long Scale Digital Multimeters • Null Detectors • Nanovoltmeters • Kelvin Varley Dividers • Resistance Bridges • Micro-ohmmeter • Precision Thermometers • Electrometers/Pico-ammeters • External shunts • Ammeters • AC/DC Transfer Standards • Multifunction Transfer Standards

  26. Eliminating Common Measurement Errors

  27. A B A T1 T2 Vab Watch Thermoelectric EMFs • Thermoelectric voltages (EMFs) are the most common source of errors in low-voltage measurements • Generated when • Different parts of circuit are at different temperatures • Conductors made of dissimilar materials are joined • Called the Seebeck effect Eliminating Common Measurement Errors Qab is Seebeck coefficient of material A with respect to B Vab = Qab (T1 - T2)

  28. Seebeck Coefficients Relative to Copper Paired Materials Seebeck Coefficient (Qab) Cu-Cu < 0.2 uV/oC CU-Ag 0.3 uV/ oC Cu-Au 0.3 uV/ oC Cu-Pb/Sn 1-3 uV/ oC Cu-Si 400 uV/ oC Cu-Kovar 40-75 uV/ oC Cu-CuO 1000 uV/ oC Cadmium-Tin Solder 0.2 uV/ oC Tin-Lead Solder 5 uV/ oC Ag=silver Au=gold Cu=copper CuO=copper oxide Pb=lead Si=silicon Sn=tin Eliminating Common Measurement Errors

  29. Other Precautions for Making Low Level Measurements • Crimp copper sleeves or lugs on copper wires • Use low thermal solder (Cadmium-Tin) • Clean connections and remove oxides (0.2uV vs. 1000uV!) • Keep ambient temperatures constant, equipment away from direct sunlight, exhaust fans. • Wrap connections in insulation foam.

  30. Vo + + VUUT Perform Reverse Measurement and Average Results Vo + VUUT Vactual = (Vo + Vuut)-(Vo-Vuut) + 2

  31. S1 Rx V I V1 = Current Off = S1 Open V2 = Current On = S1 Closed V2 - V1 Rx = I Avoiding Thermal Errors in Resistance Measurements • Cancelling static & dynamic thermal emfs • True Ohms • Offset Compensated Ohms • Effectively measures and removes thermal offsets • current ON & OFF measurements • But the technique modulates stimulus current at the reading rate • can lead to errors if UUT resistor sensitive to power dissipation changes Eliminating Common Measurement Errors

  32. Improved True Ohms, availablein 8508A Reference Multimeter • Actual bipolar current stimulus • Allows for constant heating of the UUT • Especially important when measuring temperature sensitive devices • Essential for precision temperature measurements 0 Original True Ohms Improved Bipolar True Ohms

  33. Thermal Emf (Vth) Sense Hi PD Measurement (V) Sense Lo Reversal Switching Current Source (I) UUT Resistor (R) Input Hi Input Lo Current Reversal True Ohms • With forward current: V1 = IxR + Vth • With reverse current: V2 = -(-IxR + Vth) • Averaging V1 and V2: = 0.5(2xIxR +Vth –Vth) = IxR Eliminating Common Measurement Errors • Sense path reversal ensures V1 & V2 same polarity for ADC • Offsets in Potential Difference (PD) measurement path after reversal are not cancelled • removed by zero calibration and input zero operations

  34. Precision Dmm Thermocouple Sense Hi Input Hi True Ohms Dmm Standard Resistor Input Lo Sense Lo V Experimental Confirmation Experimental procedure (RStd= 10): Note: Thermal emf magnitude & rate of change greatly exaggerated…. • Allow setup to stabilise (Vth<100V) • Plunge thermocouple into water bath at ~35C • Readings taken & stored automatically by PC • Compare both Normal Ohms & True Ohms measurement results Eliminating Common Measurement Errors

  35. Precision Dmm Thermocouple Sense Hi Input Hi True Ohms Dmm Standard Resistor Input Lo Sense Lo V Results using Normal Ohms Eliminating Common Measurement Errors • 20 range stimulus current = 10mA • 100V  10m • Measured resistance value tracks thermal emf

  36. Precision Dmm Thermocouple Sense Hi Input Hi True Ohms Dmm Standard Resistor Input Lo Sense Lo V Results using True Ohms • Effect of changing thermal emf eliminated • Thermal emf initial rate of change extremely fast • initial cancellation less effective due to comparatively long integration time

  37. 8508A Feature Summary • 1 year Absolute Specifications: • DCV: 3 ppm • ACV: 65 ppm • DC Current: 12 ppm • AC Current: 280 ppm • Resistance: 7.5 ppm • “2s” Ranges • 1000VAC RMS • 200 uA to 20 A ranges • Two channel Ratio • Spot Frequency • Bipolar True Ohms • Lo I Ohms • Hi V Ohms • 2 ohm to 20 Gohm ranges • Ohms Guard • Precision SPRT support • Comprehensive Self-Test Eliminating Common Measurement Errors

  38. Conclusion • A very cost effective addition to the Cal Lab • Increases efficiency • Easy to automate • Replaces a number traditional standards • Low Maintenance cost • Long-Scale DMMs and NOW Reference Multimeters are a Credible and Essential part of the Laboratory Equipment Eliminating Common Measurement Errors

  39. Any questions

  40. Thank you. Please visit our web site: http://www.flukecal.com

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