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)
Session Topic
•
Improved Metrology Measurements Using Precision
Digital Multimeters (or Long Scale DMMs)
o
Describe the long scale DMM & its metrology application
o
Introduce new capabilities of a new class of Multimeter, the
Fluke 8508A Reference Multimeter
o
Provide Technical Overviews of Improved Measurement
Capabilities
o
Eliminating Common Measurement Errors
©2010 Fluke Corporation
Fluke Calibration Web Seminar
2
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
•
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•
•
•
•
•
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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
Rear Inputs
V
B = +10.000 000 0 V
A = + 1.018 165 2 V
A/B(%)= + 10.181 652 %
A/B(%)/Z= + 1.018 165 2 (%)
Ratio
Measurement
10V:1.018V
Front Inputs
Front
10V
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
• 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
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
.
• Pico-ammeter
topology
• Minimal burden
current
• Easier to guard
(virtual ground)
• Higher bandwidth
(100 kHz)
• 8508A has 10 pA
resolution
.
Multimeter
..
.
x1
• But, more practical
for higher currents
• 8508A has internal
20A shunt
Feedback Ammeter
.
Shunt
Buffer
.
Current Source
.
.
.
.
.
.
Reference
Multimeter
Multimeter
. .
x1
Shunt
.
Current Source
.
.
.
Op -Amp
.
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
Input Hi
Sense Hi
Rx
Sense Lo
Input Lo
Lo Follower
Constant
Current
Sink
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
• Two input channels
(Rx)
V
Front
Active
(Rs)
V
Rear
– 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
Rs = Standard Resistor
Rx = Unknown Resistor
8508 Resistance Ratio
Measurement Technique
INPUT Hi
SENSE Hi
Front Input
SENSE Lo
INPUT Lo
Potential
Difference
Measurement
•
Stimulus Current
Source
(Reversing)
INPUT Hi
Rear Input
SENSE Hi
SENSE Lo
INPUT Lo
• 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
A
B
T1
A
T2
Eliminating Common Measurement
Vab
Errors
Vab = Qab (T1 - T2)
Qab is Seebeck coefficient of
material A with respect to B
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
Eliminating
Common Measurement
5 uV/ C
Errors
Ag=silver Au=gold Cu=copper CuO=copper
oxide
Tin-Lead Solder
Pb=lead
Si=silicon Sn=tin
o
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
S1
Rx
V
• Effectively measures and removes
thermal offsets
• current ON & OFF measurements
V1 = Current Off = S1 Open
V2 = Current On = S1 Closed
• But the technique
modulates
stimulusMeasurement
V2 - V1
Eliminating
Common
Rx =
current at the reading rate
I
Errors
• can lead to errors if UUT resistor
sensitive to power dissipation changes
I
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
0
Original True
Ohms
Improved
Bipolar True
Ohms
Current Reversal True Ohms
Thermal
Emf (Vth)
Sense Hi
PD
Measurement
(V)
Sense Lo
Reversal
Switching
UUT
Resistor (R)
Input Hi
• With forward current:
V1 = IxR + Vth
• With reverse current:
V2 = -(-IxR + Vth)
Current
Source (I)
• Averaging V1 and V2:
= 0.5(2xIxR +Vth –Vth)
= IxR
Input Lo
Eliminating Common Measurement
• Sense path reversal ensures V1 & V2 same polarity for ADC
Errors
• 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
Precision Dmm
V
Thermocouple
change greatly exaggerated….
Sense Hi
• 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
Errors
Input Hi
Standard
Resistor
True Ohms
Dmm
Input Lo
Sense Lo
Measurement
Results using Normal Ohms
Precision Dmm
Normal Ohms
V
7.5 digit Fast
Thermocouple
0
Resistance Reading
10.00
9.99
-200
9.98
-300
9.97
-400
9.96
-500
1.00
2.00
3.00
4.00
5.00
Input Hi
-100
Thermal Offset
9.95
0.00
Sense Hi
Thermal offset
(microVolts)
Measured resistance
(Ohms)
10.01
Standard
Resistor
True Ohms
Dmm
Input Lo
Sense Lo
-600
6.00
Elapsed Time (minutes)
Eliminating Common Measurement
• 20 range stimulus current = 10mA
Errors
100V  10m
• Measured resistance value tracks thermal emf
Results using True Ohms
Precision Dmm
Tru Ohms
7.5 digit Fast
V
10.01
0
10.00
-100
Resistance Reading
9.99
-200
Thermal Offset
9.98
-300
9.97
-400
9.96
-500
9.95
0.00
1.00
2.00
3.00
4.00
5.00
Sense Hi
Input Hi
Thermal offset
(microVolts)
Measured resistance
(Ohms)
Thermocouple
Standard
Resistor
-600
6.00
Elapsed Time (minutes)
• Effect of changing thermal emf eliminated
• Thermal emf initial rate of change extremely fast
− initial cancellation less effective due to comparatively long
integration time
True Ohms
Dmm
Input Lo
Sense Lo
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 toEliminating
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
Common
Measurement
Errors
Conclusion
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•
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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
Any questions
©2010 Fluke Corporation
Fluke Calibration Web Seminar
39
Thank you.
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http://www.flukecal.com