Bill GaviriaRegional Product Manager, Electrical, RF and SoftwareFluke CalibrationOffice: +1.321.574.0728Direct: +1.425.446.6031Cell: +1.321.626.7845  Email:  Bill.gaviria@flukecal.com  Web:  www.flukecal.com (UTC/GMT-5)

Welcome to this Seminar from Fluke Calibration…..

Improved Measurement Techniques In DC & Low Frequency

AC Metrology

©2010 Fluke Corporation Fluke Calibration Web Seminar 2

• 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

Session Topic

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, 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

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

DifferenceMeasurement10V-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

B = +10.000 000 0 VA = + 1.018 165 2 V

V

Front10V

RatioMeasurement10V:1.018V

Front Inputs

A/B(%)= + 10.181 652 %

A/B(%)/Z= + 1.018 165 2 (%)

Rear Inputs

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 techniquesShunt 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

Current Source

Multimeter

Shunt

.

.Buffer

. .

. .

. . . .

Current Source

Multimeter

Shunt

.

.

. .

. .

. . . ..

.

Reference Multimeter

Op -Amp

.

x1

x1

Resistance Applications

• Wide Measurement Ranges (2to 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

ConstantCurrentSink

Lo Follower

Ohms RangeControl

• 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 – 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

V (Rx)

V (Rs)

Rs = Standard ResistorRx = Unknown Resistor

FrontActive

Rear

8508 Resistance Ratio Measurement Technique

Front Input

Rear Input

INPUT Lo

SENSE Lo

SENSE Hi

INPUT Hi

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– -200C to 660C

– 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

Eliminating Common Measurement Errors

• 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 AB

Vab

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

Seebeck Coefficients Relative to Copper

Eliminating Common Measurement Errors

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

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

Vactual = (Vo + Vuut)-(Vo-Vuut)

Vo +

VUUT

+

Vo +

VUUT

+2

Avoiding Thermal Errors in Resistance Measurements

Eliminating Common Measurement Errors

• 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 OpenV2 = Current On = S1 Closed

VRx

S1

I

Rx =V2 - V1

I

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

Current Reversal True Ohms

Eliminating Common Measurement Errors

• 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

Sense Lo

Sense Hi

Input Hi

Input Lo

ReversalSwitching

PDMeasurement(V)

CurrentSource (I)UUT

Resistor (R)

ThermalEmf (Vth)

• 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

Eliminating Common Measurement Errors

Experimental procedure (RStd = 10):

Note: Thermal emf magnitude & rate of

change greatly exaggerated…. • Allow setup to stabilise

(Vth<100V)

• Plunge thermocouple into water bath at ~35C

• Readings taken & stored automatically by PC

• Compare both Normal Ohms & True Ohms measurement results

Precision Dmm

Thermocouple

StandardResistor

Sense Hi

Sense Lo

Input Hi

Input Lo

True OhmsDmm

V

Results using Normal Ohms

Eliminating Common Measurement Errors• 20 range stimulus current = 10mA

Þ 100V 10m• Measured resistance value tracks thermal emf

Normal Ohms7.5 digit Fast

9.95

9.96

9.97

9.98

9.99

10.00

10.01

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Elapsed Time (minutes)

Mea

sure

d r

esis

tan

ce

(Oh

ms)

-600

-500

-400

-300

-200

-100

0

Th

erm

al o

ffse

t (m

icro

Vo

lts)

Resistance Reading

Thermal Offset

Precision Dmm

Thermocouple

StandardResistor

Sense Hi

Sense Lo

Input Hi

Input Lo

True OhmsDmm

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

Tru Ohms7.5 digit Fast

9.95

9.96

9.97

9.98

9.99

10.00

10.01

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Elapsed Time (minutes)

Mea

sure

d r

esis

tan

ce

(Oh

ms)

-600

-500

-400

-300

-200

-100

0

Th

erm

al o

ffse

t (m

icro

Vo

lts)Resistance Reading

Thermal Offset

Precision Dmm

Thermocouple

StandardResistor

Sense Hi

Sense Lo

Input Hi

Input Lo

True OhmsDmm

V

8508A Feature Summary

Eliminating Common Measurement Errors

•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

Conclusion

Eliminating Common Measurement Errors

• 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

©2010 Fluke Corporation Fluke Calibration Web Seminar 39

Any questions

Thank you.

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