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THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument?

THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument?. PPM, Inc. October 2007. INTRODUCTIONS Presentation: Chas Gilmore Managing Director, Strategy PPM, Inc. Sponsor: Mark Reid techniCAL Copies of Presentation Available at techniCAL. AGENDA. What Is The Nullmeter

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THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument?

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  1. THE NULLMETERA 21st Century InstrumentOrA 19th Century Instrument? PPM, Inc. October 2007 www.ppminc.com Cleveland OH 44122

  2. INTRODUCTIONS Presentation: Chas Gilmore Managing Director, Strategy PPM, Inc. Sponsor: Mark Reid techniCAL Copies of Presentation Available at techniCAL www.ppminc.com Cleveland OH 44122

  3. 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 www.ppminc.com Cleveland OH 44122

  4. 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 Meter—And When Not To • The Future Of Nullmeters www.ppminc.com Cleveland OH 44122

  5. WHO ARE YOU? • Involvement With DC Calibration Techniques? www.ppminc.com Cleveland OH 44122

  6. WHAT/WHY/WHO? • An Instrument To Compare Two Voltages (< = >) • Typically Moderate Voltages (Few to Few 10s—Has 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 www.ppminc.com Cleveland OH 44122

  7. 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 www.ppminc.com Cleveland OH 44122

  8. 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 > MΩ) • Analog, BiPolar Display (Meter With ± Scales) • High Common Mode (Often Via Battery Operation) • Filtered For 2 – 5 Second Response (Or >) • Ability To “Zero” The Meter www.ppminc.com Cleveland OH 44122

  9. THE NULLMETER ADVANTAGE • NULL Means Just That Null/Nada/Nothing • OHM’s 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 www.ppminc.com Cleveland OH 44122

  10. 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 www.ppminc.com Cleveland OH 44122

  11. MORE EARLY METERS C 1890 C 1930 www.ppminc.com Cleveland OH 44122

  12. NULLMETERS YOU MIGHT OWN HP 419A 1960s HP 413A 1950s FLUKE 845 AB Mid-1960s KEITHLEY 155 The 1970s PPM AVM-100 Mid 1990s www.ppminc.com Cleveland OH 44122

  13. 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 www.ppminc.com Cleveland OH 44122

  14. 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 www.ppminc.com Cleveland OH 44122

  15. SIMPLE COMPARISON • An Un-Calibrated Source To A Standard Source • Standard Is 2 µV < 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?) www.ppminc.com Cleveland OH 44122

  16. 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 www.ppminc.com Cleveland OH 44122

  17. 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 www.ppminc.com Cleveland OH 44122

  18. DECADE DIVIDER • Also Called Reference Divider • Decade Input / Output Voltage Ratios • 0.01:1, 0.1:1, 1:1, • 10:1, 100:1 • Accuracy > 0.5 ppm • Self Calibrating (With Nullmeter) • Limited Comparisons www.ppminc.com Cleveland OH 44122

  19. 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 >) • Example 90.62441 V To 10 V ± 906 nV (~1 ppm) • Null Error 100 nV • Thermals 500 nV • Uncertainty 1.04 ppm www.ppminc.com Cleveland OH 44122

  20. 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 • Slower—But Only Uses Two Pieces Of Equipment www.ppminc.com Cleveland OH 44122

  21. 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 www.ppminc.com Cleveland OH 44122

  22. OBSERVING A 1µV NULL ANALOG DIGITAL Range ± 3µV Range ± 200.00 mV www.ppminc.com Cleveland OH 44122

  23. 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 NULLMETER Vs. DMM • How Good Do You Need To Be? • Is The Change Justified? www.ppminc.com Cleveland OH 44122

  24. 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 www.ppminc.com Cleveland OH 44122

  25. MR. JOHNSON’S NOISE Johnson Noise nV √Hz Vs. Resistance RESISTANCE OHMS Noise (nV/√Hz) www.ppminc.com Cleveland OH 44122

  26. 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 www.ppminc.com Cleveland OH 44122

  27. OFFSET VOLTAGES • Voltages From Multiple Sources www.ppminc.com Cleveland OH 44122

  28. COMMON MODE VOLTAGES • Another Source Of A DC Signal www.ppminc.com Cleveland OH 44122

  29. COMMON MODE COUPLING • Capacitive • Inductive www.ppminc.com Cleveland OH 44122

  30. INPUT CURRENTS • Current Generated By The Instrument Itself • Current Flows Thru Resistances In MMT Circuit • Creates Additional (Unknown) Voltage Drop • Example: 50 pA @ 100 kΩ = 5 µV www.ppminc.com Cleveland OH 44122

  31. 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 www.ppminc.com Cleveland OH 44122

  32. 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 Doesn’t Make Sense To Change Procedures • When “Nearly Zero” Isn’t Good Enough • Where You Don’t Need It: • When Direct Measurements Will Do • When Multi-Digit Measurements Are Required www.ppminc.com Cleveland OH 44122

  33. ITS INEVITABLE • If You Haven’t Faced It—You 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 Can’t Do Without A Nullmeter www.ppminc.com Cleveland OH 44122

  34. QUESTIONS??? www.ppminc.com Cleveland OH 44122

  35. QUESTIONS??? www.ppminc.com Cleveland OH 44122

  36. www.ppminc.com Cleveland OH 44122

  37. www.ppminc.com Cleveland OH 44122

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