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: [email protected]

Web: www.flukecal.com

(UTC/GMT-5)

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Session Topic

Improved Metrology Measurements Using Precision Digital Multimeters (or Long Scale DMMs)

o o o o 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 ©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

• • • • • • • • • 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 >10 10  Low Bias Current - Typically <10pA Wide Dynamic Range 2x10 8 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 (>10 10 Ohm 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

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 (%) Rear Inputs 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 (>10 10 Ohm 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

• 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 .

x1

Current Source

x1

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Shunt

.

.

Multimeter

Buffer Shunt

.

.

.

Op -Amp

.

Resistance Applications

• Wide Measurement Ranges (2  to 20G  – Standard Resistor Comparisons in the 8508A) – 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

Input Hi R x Input Lo Sense Hi Sense Lo Constant Current Sink Lo Follower Ohms Range Control DC Voltage Pre-Amp • • • 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 (Rx) (Rs) V V Front

Active

Rear Rs = Standard Resistor Rx = Unknown Resistor

8508 Resistance Ratio Measurement Technique

Front Input INPUT Hi SENSE Hi SENSE Lo INPUT Lo Potential Difference Measurement

•

Stimulus Current Source (Reversing) Rear Input INPUT Hi 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 A T 1 T 2

Eliminating Common Measurement

V ab

Errors

V ab = Q ab (T 1 - T 2 ) Q ab is Seebeck coefficient of material A with respect to B

Seebeck Coefficients Relative to Copper Paired Materials

Cu-Cu CU-Ag

Seebeck Coefficient (Q ab )

< 0.2 uV/ o C 0.3 uV/ o C Cu-Au Cu-Pb/Sn Cu-Si Cu-Kovar Cu-CuO 0.3 uV/ o C 1-3 uV/ o C 400 uV/ o C 40-75 uV/ o C 1000 uV/ o C Cadmium-Tin Solder 0.2 uV/ o C

Eliminating Common Measurement

Tin-Lead Solder 5 uV/ o C

Errors

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

V UUT

+

V o

+

V UUT

+

V o

+

V actual = (V o + V uut )-(V o -V uut ) 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 Rx

S1

V V

1

= Current Off = S1 Open V

2

= Current On = S1 Closed

Eliminating Common Measurement Errors

sensitive to power dissipation changes R

x

= V

2

I - V

1

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 (V th ) Sense Hi Sense Lo UUT Resistor (R) Input Hi Reversal Switching PD Measurement (V) Current Source (I)

• With forward current: V 1 = I x R + V th • With reverse current: V 2 = -(-Ix R + V th ) • Averaging V 1 and V 2 : = 0.5(2 x I x R +V th –V th ) = I x R

Input Lo Eliminating Common Measurement

• Sense path reversal ensures V & V 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 (R Std = 10 Note: Thermal emf magnitude & rate of change greatly exaggerated….

 ):

Precision Dmm

V

Thermocouple Sense Hi

Allow setup to stabilise (V th <100  V) Plunge thermocouple into water bath at ~35  C Readings taken & stored automatically by PC

Standard Resistor Input Hi True Ohms Dmm Input Lo Sense Lo

Compare both Normal Ohms & True

Eliminating Common Measurement Errors

Results using Normal Ohms Normal Ohms 7.5 digit Fast Precision Dmm

V

Thermocouple

10.01

10.00

9.99

9.98

9.97

9.96

Resistance Reading Thermal Offset 0 -100 -200 -300 -400 -500

Standard Resistor Sense Hi Input Hi True Ohms Dmm Input Lo Sense Lo

9.95

0.00

1.00

2.00

3.00

4.00

5.00

6.00

-600 • 20 

Elapsed Time (minutes) Eliminating Common Measurement Errors



100



V



10m

 • Measured resistance value tracks thermal emf

Results using True Ohms Tru Ohms 7.5 digit Fast

10.01

10.00

9.99

9.98

9.97

9.96

9.95

0.00

Resistance Reading Thermal Offset 1.00

2.00

3.00

4.00

Elapsed Time (minutes)

5.00

0 -100 -200 -300 -400 -500 6.00

-600

Precision Dmm

V

Thermocouple Sense Hi Input Hi Standard Resistor True Ohms Dmm Input Lo Sense Lo

• Effect of changing thermal emf eliminated • Thermal emf initial rate of change extremely fast − initial cancellation less effective due to comparatively long

integration time

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 • 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 •

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 Errors

Any questions

©2010 Fluke Corporation Fluke Calibration Web Seminar 39

Thank you.

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