1. 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:
Direct: Cell:    
Web: 
(UTC/GMT-5)
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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 ±
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 – or )
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, 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

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. Using Rear Inputs to Compare Voltage Standards
Ratio
Measurement
10V:1.018V
Front Inputs
Front
10V
B = V
A = V
A/B(%)= %
A/B(%)/Z= (%)

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 (± )
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
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. 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

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. 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

21. Traditional DMM Resistance Ratio Measurement TechniquesV
(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

22. 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

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. 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

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. 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)

28. Seebeck Coefficients Relative to Copper
Paired Materials Seebeck Coefficient (Qab)
Cu-Cu < 0.2 uV/oC
CU-Ag uV/ oC
Cu-Au uV/ oC
Cu-Pb/Sn uV/ oC
Cu-Si uV/ oC
Cu-Kovar uV/ oC
Cu-CuO 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. Perform Reverse Measurement and Average ResultsVo +
VUUT Vo +
VUUT
Vactual = (Vo + Vuut)-(Vo-Vuut) + 2

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. Any questions

40. Thank you.
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