www.ppminc.com

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THE NULLMETERA 21st Century Instrument OrA
19th Century Instrument?

• PPM, Inc.
• October 2007

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• INTRODUCTIONS
• Presentation Chas Gilmore
• Managing Director, Strategy
• PPM, Inc.
• Sponsor Mark Reid
• techniCAL
• Copies of Presentation Available at techniCAL

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AGENDA

• What Is The Nullmeter
• Why Is It Used
• How Is It Used
• Null Meter Basics (Principles Use)
• Alternative Techniques
• Comparison Of Techniques
• Common Measurement Problems
• Summary/Questions

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TAKE AWAY

• What The Nullmeter Is
• What The Nullmeter Is Not
• Where A Nullmeter Is Used
• Nullmeter Vs. Digital Instruments In Classic Use
• Practical Issues With Nullmeter Measurements
• When To Use A Null MeterAnd When Not To
• The Future Of Nullmeters

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WHO ARE YOU?

• Involvement With DC Calibration Techniques?

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WHAT/WHY/WHO?

• An Instrument To Compare Two Voltages (lt gt)
• Typically Moderate Voltages (Few to Few 10sHas a
  much wider dynamic range if needed)
• Compared Within Micro-Volts or few 100s of
  Nano-Volts
• Analog Display (Meter) For Ease Of Use
• Not A Precision/High-Resolution Voltmeter
• Compare An Unknown To A Known
• Standard Voltage, Current or Resistance To
  Unknown
• Calibration Laboratories
• Manufacturers
• Product Design Organizations

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HOW?

• High Gain, Low-Drift, DC Amplifier
• Unipolar (Amplifies Positive Negative Equally)
• Amplifier Output Drives A Zero Center Meter
• Input Can Be Attenuated Or Amplifier Gain Changed
  

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NULLMETER CHARACTERISTICS

• DC Voltmeter
• High Sensitivity (Few µV Minimum)
• Wide Range Of Sensitivities (Manually Selected)
• Low Drift (Holds Zero)
• Relatively High Input Impedance (1, 10 Or gt M?)
• Analog, BiPolar Display (Meter With Scales)
• High Common Mode (Often Via Battery Operation)
• Filtered For 2 5 Second Response (Or gt)
• Ability To Zero The Meter

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THE NULLMETER ADVANTAGE

• NULL Means Just That Null/Nada/Nothing
• OHMs Law I E/R
• At Null E 0 so I 0
• The Meter Draws No Current From The Circuit Under
  Test At Null
• Null Very Easy To See (Zero Center Meter)
• The Higher The Input Impedance, The Less The
  Off-Null Disturbance

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A BIT OF HISTORY

• The Tangent Galvanometer
• First Described In 1837 By Claude-Servais-Mathias
  Pouillet (1790-1868)
• Used Galvanometer To Verify Ohm's law
• Galvanometer Is On A Level Surface The Coil
  Aligned With Magnetic North-South
• Current Flow/Direction Observed As Compass Needle
  Movements

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MORE EARLY METERS

• C 1930

• C 1890

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NULLMETERS YOU MIGHT OWN
HP 419A 1960s
HP 413A 1950s
FLUKE 845 AB Mid-1960s
KEITHLEY 155 The 1970s
PPM AVM-100 Mid 1990s
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A 21ST CENTURYNULLMETER

• Traditional Features

MIRROR BACKED ZERO-CENTER METER WITH -3/3
-10/10 SCALES
ISOLATED OUTPUT
ZERO ADJUSTMENT
ZERO/OPERATE MODE
LO-THERMAL EMF INPUT
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A 21ST CENTURYNULLMETER
STATUS LCD

• 21ST CENTURY FEATURES

SELECTABLE OFFSET VOLTAGE 0 30 mV
SCALABLE ISOLATED OUTPUT
INPUT OFFSET (I V) ADJUSTMENT
INPUT IMPEDANCE SELECTION
FILTER SELECTION 0.1 S 100 S
RANGE 100 nV 1 KV
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SIMPLE COMPARISON

• An Un-Calibrated Source To A Standard Source
• Standard Is 2 µV lt Un-Calibrated Source ? 2
  Parts In 107
• Errors Nullmeter 100 nV, Noise/Thermals 500 nV
• Measurement Uncertainty 0.51 Parts In 106
• Major Uncertainty The Standard (A few ppm?)

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EXTENDING THE DYNAMIC RANGE

• Bridge Circuits (Wheatstone/Kelvin)
• General Ratio Measurements
• Ultra-Low Resistance
• Decade/Reference Divider
• High Precision Decade Division
• Ratio Divider
• High Precision 7-Digit Ratio Divide

Another Kelvin Bridge Scotland
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BASIC BRIDGE

• Compare Resistance Ratios
• Null Achieved When R 1 / R 3 R 2 / R4
• A - B Voltage Zero
• Extensions Compare
• Voltage Temperature
• Current Pressure
• Resistance Force

A
B
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DECADE DIVIDER

• Also Called Reference Divider
• Decade Input / Output Voltage Ratios
• 0.011, 0.11, 11,
• 101, 1001
• Accuracy gt 0.5 ppm
• Self Calibrating (With Nullmeter)
• Limited Comparisons

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A NULL METER A RATIO DIVIDER

• Kelvin-Varley Ratio Divider
• Ratios From 0 To 1.1
• Ratios 7 Decimal Places (Linearity 0.1 ppm or gt)
• Example 90.62441 V To 10 V 906 nV (1 ppm)
• Null Error 100 nV
• Thermals 500 nV
• Uncertainty 1.04 ppm

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WHAT ARE THE ALTERNATIVES?

• Direct Measurement With Long-Scale DMM
• DMM On 200 V Range (For 90 V Example)
• Resolution 8 Digits 90.624,410 (1 µV)
• Uncertainty 3 PPM (271 µV)
• Very Fast
• Ratio DMM 10 Volt Reference Standard
• Uncertainty 2 PPM
• SlowerBut Only Uses Two Pieces Of Equipment

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USE A DMM AS A NULLMETER?

• High End DMMs Have The Resolution
• High Sensitivity DMMs Have The Resolution
• Difficulties
• Identifying The Null
• Lack Of Filtering And Digits Flicker/Blur
• Sufficient Filtering And Changes Jump
• Common Mode (Usually Not Battery Operated)
• Generally Difficult To Accomplish

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OBSERVING A 1µV NULL
ANALOG
DIGITAL
Range 3µV
Range 200.00 mV
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NULLMETER Vs. DMM

• Nullmeter
• Easily 1 Part in 106
• Typically 5 Parts in 107
• 1 2 Parts in 107 With Care
• Preserves Existing Procedures
• Measurement Time 10 20 Minutes (experienced
  metrologist)
• Complex Setup (Lots Of Equipment)
• Lack Of Automated Data Recording

• DMM Direct
• Typically 3-5 Parts in 106
• Measurement Time 1 2 Minutes (minimal
  experience)
• Automated Data Capture
• DMM Ratio-meter
• Typically 2-3 Parts in 106
• Measurement Time 3 5 Minutes (moderate
  experience)
• Automated Data Capture

• How Good Do You Need To Be?
• Is The Change Justified?

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ALL IS NOTROSY

• Noise (Man Made Natural Johnson)
• Thermal Voltages
• Electro-Chemically Generated Voltages
• Input Bias/Offset Currents
• Pump Out Currents
• Impact Of Induced Voltages (Line Frequency)
• Impact Of Differing Input Impedances

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MR. JOHNSONS NOISE
Johnson Noise nV vHz Vs. Resistance
RESISTANCE OHMS
Noise (nV/vHz)
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WHEN A SHORT IS NOT A SHORT

• Links
• Straps
• Plugs
• Copper-to-Copper 200 nV/C
• Copper-to-Gold 300 nV /C
• Problems Heat From Your Hands
• Heat From Inserting A Banana Plug
• Handling Contaminants Make Batteries

Keithley Instruments
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OFFSET VOLTAGES

• Voltages From Multiple Sources

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COMMON MODE VOLTAGES

• Another Source Of A DC Signal

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COMMON MODE COUPLING

• Capacitive
• Inductive

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INPUT CURRENTS

• Current Generated By The Instrument Itself
• Current Flows Thru Resistances In MMT Circuit
• Creates Additional (Unknown) Voltage Drop
• Example 50 pA _at_ 100 k? 5 µV

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PUMP OUT CURRENT

• DC Amplifier Stabilized By Chopper
• Chopper Converts DC Signal To Square Wave
• Square Wave Amplified Without Amplifier Drift
• Chopping Generates Small (pA) Currents Charging
  Input Capacitor

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SUMMARY

• The Nullmeter Is A 21st Century Instrument
  (What else would we say!)
• Where You Need It
• Trend Indications
• Comparison Measurements From 1 To 0.1 ppm
• High Common Mode/Isolation Measurements
• When It Doesnt Make Sense To Change Procedures
• When Nearly Zero Isnt Good Enough
• Where You Dont Need It
• When Direct Measurements Will Do
• When Multi-Digit Measurements Are Required

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ITS INEVITABLE

• If You Havent Faced ItYou Will
• The Old Instruments Are Dying
• The New High-Accuracy, High-Sensitivity
  Instruments Require High-Precision Calibration
• Sub-Part Per Million Is Here
• Do I Totally Change Procedures/Processes?
• There Are Just Some Measurements You Cant Do
  Without A Nullmeter

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QUESTIONS???
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• QUESTIONS???

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