Improved Measurement Techniques In DC & Low Frequency AC Metrology 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  Email:  Bill.gaviria@flukecal.com  Web:  www.flukecal.com (UTC/GMT-5) Begin talking to attendees soon after they join. This at least allows them to set their audio. Heres the tricky bit – you’ll need to un-mute the Teleconference, un-mute your VOIP mic and (if not the Host) hold down the “Ctrl” key whilst you are talking. The presenter should monitor the VOIP, slide update delays, incoming Chat and Q&A and teleconference feedback while the host presents, and vice versa when the roles ultimately reverse. Always nerve racking to get going, starting lines might be….. This is a Fluke Calibration web seminar, welcome to any early participants. The presentation will begin in xxxx minutes, please use this announcement to check your audio settings”. May wish to welcome the geographies targeted by this event timing, but acknowledge attendees wherever they may be and at whatever time in their day it might be. Avoid references to morning, afternoon, night and day. The first four slides are intended to be used for these pre-seminar announcements, which will likely be repeated 3-times. After each, set back to this slide ready for the next. The seminar will ultimately begin with these three slides. When finished don’t forget to mute everything again

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

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

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

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

New Class of Multimeter, the 8508A 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

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”

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

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)

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

Using Rear Inputs to Compare Voltage Standards 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 (%)

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

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

Outperforms Traditional AC/DC Transfer Standards 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

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

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

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

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

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)

Resistance Measurement Topology DC Voltage Pre-Amp Sense Lo Sense Hi Input Hi Rx Input Lo Constant Current Sink Lo Follower Ohms Range Control 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

Traditional DMM Resistance Ratio Measurement Techniques V (Rx) (Rs) Rs = Standard Resistor Rx = Unknown Resistor Front Active Rear 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

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

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

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 REFERENCE SPRT UUT RTD

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

Eliminating Common Measurement Errors

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 T1 T2 A B Vab Eliminating Common Measurement Errors Qab is Seebeck coefficient of material A with respect to B Vab = Qab (T1 - T2)

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

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.

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

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 V1 = Current Off = S1 Open V2 = Current On = S1 Closed V Rx S1 I Rx = V2 - V1 Eliminating Common Measurement Errors

Improved True Ohms, available in 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 Original True Ohms Improved Bipolar True Ohms

Current Reversal True Ohms Sense Lo Sense Hi Input Hi Input Lo Reversal Switching PD Measurement (V) Current Source (I) UUT Resistor (R) Thermal Emf (Vth) With forward current: V1 = I x R + Vth With reverse current: V2 = -(-I x R + Vth) Averaging V1 and V2: = 0.5(2 x I x R +Vth –Vth) = I x R 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

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 Precision Dmm Thermocouple Standard Resistor Sense Hi Sense Lo Input Hi Input Lo True Ohms Dmm V Eliminating Common Measurement Errors

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

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

Eliminating Common Measurement Errors 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

Eliminating Common Measurement Errors 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

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