17-jan-14 cleveland oh 44122 1 the nullmeter a 21 st century instrument or a 19 th century...
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Apr 10, 2023 www.ppminc.comCleveland OH 44122
1
THE NULLMETERA 21st Century Instrument
Or
A 19th Century Instrument?
PPM, Inc.
October 2007
Apr 10, 2023 www.ppminc.comCleveland OH 44122
2
INTRODUCTIONS
Presentation: Chas GilmoreManaging Director, Strategy
PPM, Inc.
Sponsor: Mark ReidtechniCAL
Copies of Presentation Available at techniCAL
Apr 10, 2023 www.ppminc.comCleveland 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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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 Meter—And When Not To• The Future Of Nullmeters
Apr 10, 2023 www.ppminc.comCleveland OH 44122
5
WHO ARE YOU?
• Involvement With DC Calibration Techniques?
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
+
-
Source Voltage Vs
Vin = Vs
Output VoltageVO = Vin X G
DifferentialAmplifierGain = G
+10
0
-10
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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 > 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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
11
MORE EARLY METERS
C 1890
C 1930
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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NULLMETERS YOU MIGHT OWN
FLUKE 845 AB
Mid-1960s
PPM AVM-100
Mid 1990s
KEITHLEY 155The 1970s
HP 419A
1960sHP 413A
1950s
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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A 21ST CENTURYNULLMETER
• Traditional Features MIRROR BACKED ZERO-CENTER METER WITH -3/+3 & -10/+10 SCALES
ZERO/OPERATE MODE
ZERO ADJUSTMENT
ISOLATED OUTPUT
LO-THERMAL EMF INPUT
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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A 21ST CENTURYNULLMETER
• 21ST CENTURY FEATURES
INPUT IMPEDANCE SELECTION
FILTER SELECTION 0.1 S – 100 S
SELECTABLE OFFSET VOLTAGE 0 – 30 mV
STATUS LCD
INPUT OFFSET (I & V) ADJUSTMENT
SCALABLE ISOLATED OUTPUT
RANGE 100 nV – 1 KV
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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STANDARD10.000,000 V
SOURCE
G
AVM - 2000 NULL METER
HI LO
0
3 µV
+
-
SOURCE TO BE
CALIBRATED(10.000,002 V)
+
-
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?)
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
G
AVM-2000 NULL METER
HI LO
+10+3
0
-10
-3
R 1
R 3 R 4
R 2
A B
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
100
Vol
t Sou
rce
To
Be
Cal
ibra
ted
G
AVM-2000 NULL METER
HI LO
+10+3
0
-10
-3
360 kΩ
40 kΩ
STANDARD10.000,000 V
SOURCE
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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 >)• Example 90.62441 V To
10 V ± 906 nV (~1 ppm)• Null Error 100 nV• Thermals 500 nV• Uncertainty 1.04 ppm
90.62441 Volt SourceTo Be
Calibrated
G
AVM-2000 NULL METER
HI LO
+10+3
0
-10
-3
STANDARD10.000,000 V
SOURCE
KELVIN-VARLEYRATIO DIVIDER
RATIO : 0.9062441
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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– Slower—But Only Uses Two Pieces Of Equipment
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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OBSERVING A 1µV NULL
ANALOG DIGITAL
Range ± 3µV Range ± 200.00 mV
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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?
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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MR. JOHNSON’S NOISE
J ohnson Noise nV/√Hz Vs. Resistance
1
10
100
1,000
10,000
100,000
1,000,000
10,000,000
0.1 1.0 10.0 100.0 1000.0
Noise (nV/√Hz)
1
µV
en = (4kBTR)-1/2
RE
SIS
TA
NC
E O
HM
S
Noise (nV/√Hz)
Johnson Noise nV √Hz Vs. Resistance
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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OFFSET VOLTAGES
• Voltages From Multiple Sources
Am
plifi
erG
ain
= G
Source Voltage VsVin = Vs ± Voff ± VPOff ± VNOff
± VAOff ± VBOff
+ Input Terminal Offset Voltage
VPOff Output VoltageVO = Vin X G
- Input Terminal Offset Voltage
VNOff
Amplifier Input Offset Voltage
VOff
Zero Offset Voltage VZOff
-
+
Circuit Offset VAOff
Circuit Offset VBOff
+10
0
-10
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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COMMON MODE VOLTAGES
• Another Source Of A DC Signal
=
VCMVNM
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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COMMON MODE COUPLING
• Capacitive
• Inductive
Source Voltage
Vs
Vin = Vs + VCM (CCM1 – CCM2)
G
AVM-2000 NULL METER
HI LO
+10+3
0
-10
-3
Common Mode Voltage VCM
Indicated Null Meter VoltageVO = (Vin + VCM (CCM1 – CCM2)) X Range Gain
CCM1
CCM2
Source Voltage
Vs
Vin = Vs + VCM (LCM1 – LCM2)
G
AVM-2000 NULL METER
HI LO
+10+3
0
-10
-3
Common Mode Voltage VCM
Indicated Null Meter VoltageVO = (Vin + VCM (LCM1 – LCM2)) X Range Gain
LCM1
LCM2
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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 @ 100 kΩ = 5 µV
Am
plif
ier
Source Voltage
Vs
Input Source Resistance Rs
Vin = Vs + Ib x Rs
Ib
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Output VoltageVO = Vin X G
-
+
+10
0
-10
Vin
Input ChopperVin = Vs
AC Amplifier With Demodulator
Iin
Vs
Output VoltageVO = Vin X G
-
+
+10
0
-10
Vin
Input ChopperVin = 0
AC Amplifier With Demodulator
Iin
Vs
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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 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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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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
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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QUESTIONS???
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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QUESTIONS???
Apr 10, 2023 www.ppminc.comCleveland OH 44122
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Apr 10, 2023 www.ppminc.comCleveland OH 44122
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