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THE NULLMETER A 21st Century Instrument Or A 19th Century Instrument? PPM, Inc. October 2007 23-May-17 www.ppminc.com Cleveland OH 44122 1 INTRODUCTIONS Presentation: Chas Gilmore Managing Director, Strategy PPM, Inc. Sponsor: Mark Reid techniCAL Copies of Presentation Available at techniCAL 23-May-17 www.ppminc.com Cleveland OH 44122 2 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 23-May-17 www.ppminc.com Cleveland OH 44122 3 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 23-May-17 www.ppminc.com Cleveland OH 44122 4 WHO ARE YOU? • Involvement With DC Calibration Techniques? 23-May-17 www.ppminc.com Cleveland OH 44122 5 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 23-May-17 www.ppminc.com Cleveland OH 44122 6 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 Output Voltage VO = Vin X G 0 +1 0 Vin = Vs Source Voltage Vs Differential Amplifier Gain = G -1 0 + 23-May-17 www.ppminc.com Cleveland OH 44122 7 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 23-May-17 www.ppminc.com Cleveland OH 44122 8 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 23-May-17 www.ppminc.com Cleveland OH 44122 9 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 23-May-17 www.ppminc.com Cleveland OH 44122 10 MORE EARLY METERS C 1930 C 1890 23-May-17 www.ppminc.com Cleveland OH 44122 11 NULLMETERS YOU MIGHT OWN HP 419A HP 413A 1960s 1950s FLUKE 845 AB Mid-1960s KEITHLEY 155 The 1970s 23-May-17 PPM AVM-100 www.ppminc.com Cleveland OH 44122 Mid 1990s 12 A 21ST CENTURY NULLMETER • Traditional Features MIRROR BACKED ZERO-CENTER METER WITH -3/+3 & -10/+10 SCALES ISOLATED OUTPUT ZERO ADJUSTMENT ZERO/OPERATE MODE LO-THERMAL EMF INPUT 23-May-17 www.ppminc.com Cleveland OH 44122 13 A 21ST CENTURY NULLMETER 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 23-May-17 www.ppminc.com Cleveland OH 44122 RANGE 100 nV – 1 KV 14 SIMPLE COMPARISON AVM - 2000 NULL METER + + 0 0 -1 +1 -3 SOURCE TO BE CALIBRATED (10.000,002 V) 3 µV 0 +3 STANDARD 10.000,000 V SOURCE - • • • • • HI LO G - 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?) 23-May-17 www.ppminc.com Cleveland OH 44122 15 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 23-May-17 www.ppminc.com Cleveland OH 44122 16 BASIC BRIDGE www.ppminc.com Cleveland OH 44122 AVM-2000 NULL METER 0 -10 -3 +3 23-May-17 R2 HI LO 0 – Voltage – Temperature – Current – Pressure – Resistance – Force R1 +1 • Compare Resistance Ratios • Null Achieved When R 1 / R 3 = R 2 / R4 • A - B Voltage = Zero • Extensions Compare: G A B R3 R4 17 DECADE DIVIDER 100 Volt Source To Be Calibrated www.ppminc.com Cleveland OH 44122 AVM-2000 NULL METER -10 0 -3 +3 23-May-17 360 kΩ HI 40 kΩ LO +10 • 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 G STANDARD 10.000,000 V SOURCE 18 A NULL METER & A RATIO DIVIDER -10 0 -3 +3 23-May-17 AVM-2000 NULL METER HI LO +10 • 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 KELVIN-VARLEY RATIO DIVIDER G 90.62441 Volt Source To Be Calibrated www.ppminc.com Cleveland OH 44122 STANDARD 10.000,000 V SOURCE RATIO : 0.9062441 19 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 23-May-17 www.ppminc.com Cleveland OH 44122 20 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 23-May-17 www.ppminc.com Cleveland OH 44122 21 OBSERVING A 1µV NULL 23-May-17 ANALOG DIGITAL Range ± 3µV Range ± 200.00 mV www.ppminc.com Cleveland OH 44122 22 NULLMETER Vs. DMM • Nullmeter – – – – – • DMM Direct 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 – 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? 23-May-17 www.ppminc.com Cleveland OH 44122 23 ALL IS NOT ROSY • • • • • • • 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 23-May-17 www.ppminc.com Cleveland OH 44122 24 MR. JOHNSON’S NOISE Johnson Noise nV √Hz Vs. Resistance RESISTANCE OHMS en = (4kBTR) -1/2 Noise (nV/√Hz) 23-May-17 www.ppminc.com Cleveland OH 44122 25 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 23-May-17 Keithley Instruments www.ppminc.com Cleveland OH 44122 26 OFFSET VOLTAGES Amplifier Input Offset Voltage VOff + Source Voltage Vs Circuit Offset VBOff 0 0 +1 Circuit Offset VAOff Output Voltage VO = Vin X G -10 + Input Terminal Offset Voltage VPOff Amplifier Gain = G • Voltages From Multiple Sources Vin = Vs ± Voff ± VPOff ± VNOff ± VAOff ± VBOff - Input Terminal Offset Voltage VNOff Zero Offset Voltage VZOff 23-May-17 www.ppminc.com Cleveland OH 44122 27 COMMON MODE VOLTAGES • Another Source Of A DC Signal = VCM 23-May-17 VNM www.ppminc.com Cleveland OH 44122 28 COMMON MODE COUPLING CCM1 AVM-2000 NULL METER -3 +3 -10 0 Source Voltage Vs +10 • Capacitive Indicated Null Meter Voltage VO = (Vin + VCM (CCM1 – CCM2)) X Range Gain Vin = Vs + VCM (CCM1 – CCM2) HI G LO CCM2 Common Mode Voltage VCM • Inductive LCM1 Indicated Null Meter Voltage VO = (Vin + VCM (LCM1 – LCM2)) X Range Gain AVM-2000 NULL METER -3 +3 +10 -10 0 Source Voltage Vs Vin = Vs + VCM (LCM1 – LCM2) HI LO G LCM2 Common Mode Voltage VCM 23-May-17 www.ppminc.com Cleveland OH 44122 29 INPUT CURRENTS Input Source Resistance Rs Source Voltage Vs Ib Amplifier • Current Generated By The Instrument Itself • Current Flows Thru Resistances In MMT Circuit • Creates Additional (Unknown) Voltage Drop Vin = Vs + Ib x Rs • Example: 50 pA @ 100 kΩ = 5 µV 23-May-17 www.ppminc.com Cleveland OH 44122 30 PUMP OUT CURRENT Output Voltage VO = Vin X G + 0 -10 AC Amplifier With Demodulator Vin Vs - Input Chopper Vin = 0 Output Voltage VO = Vin X G + AC Amplifier With Demodulator 0 -10 Iin 0 +1 23-May-17 Input Chopper Vin = Vs Iin 0 +1 • 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 Vin Vs www.ppminc.com Cleveland OH 44122 - 31 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 23-May-17 www.ppminc.com Cleveland OH 44122 32 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 23-May-17 www.ppminc.com Cleveland OH 44122 33 QUESTIONS??? 23-May-17 www.ppminc.com Cleveland OH 44122 34 QUESTIONS??? 23-May-17 www.ppminc.com Cleveland OH 44122 35 23-May-17 www.ppminc.com Cleveland OH 44122 36 23-May-17 www.ppminc.com Cleveland OH 44122 37