Transcript a 21 st century nullmeter

THE NULLMETER
A 21st Century Instrument
Or
A 19th Century Instrument?
PPM, Inc.
October 2007
26-Mar-16
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
26-Mar-16
www.ppminc.com
Cleveland OH 44122
2
AGENDA
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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
26-Mar-16
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Cleveland OH 44122
3
TAKE AWAY
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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
26-Mar-16
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Cleveland OH 44122
4
WHO ARE YOU?
• Involvement With DC Calibration Techniques?
26-Mar-16
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Cleveland 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
26-Mar-16
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Cleveland OH 44122
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HOW?
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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
+
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Cleveland OH 44122
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NULLMETER
CHARACTERISTICS
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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
26-Mar-16
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Cleveland OH 44122
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THE NULLMETER
ADVANTAGE
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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
26-Mar-16
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Cleveland 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
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Cleveland OH 44122
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MORE EARLY
METERS
C 1930
C 1890
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Cleveland OH 44122
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NULLMETERS YOU
MIGHT OWN
HP 419A
HP 413A
1960s
1950s
FLUKE 845 AB
Mid-1960s
KEITHLEY 155
The 1970s
26-Mar-16
PPM AVM-100
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Cleveland OH 44122
Mid 1990s
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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
26-Mar-16
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Cleveland OH 44122
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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
26-Mar-16
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Cleveland OH 44122
RANGE 100 nV – 1 KV
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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
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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?)
26-Mar-16
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Cleveland 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
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Cleveland OH 44122
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BASIC BRIDGE
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Cleveland OH 44122
AVM-2000 NULL METER
0
-10
-3
+3
26-Mar-16
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
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DECADE DIVIDER
100 Volt Source
To Be
Calibrated
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Cleveland OH 44122
AVM-2000 NULL METER
-10
0
-3
+3
26-Mar-16
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
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A NULL METER &
A RATIO DIVIDER
-10
0
-3
+3
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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
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Cleveland OH 44122
STANDARD
10.000,000 V
SOURCE
RATIO : 0.9062441
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WHAT ARE THE
ALTERNATIVES?
• Direct Measurement With Long-Scale DMM
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–
–
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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
26-Mar-16
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Cleveland 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:
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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
26-Mar-16
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Cleveland OH 44122
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OBSERVING A
1µV NULL
26-Mar-16
ANALOG
DIGITAL
Range ± 3µV
Range ± 200.00 mV
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Cleveland OH 44122
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NULLMETER Vs. DMM
• Nullmeter
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–
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• 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?
26-Mar-16
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Cleveland OH 44122
23
ALL IS NOT
ROSY
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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
26-Mar-16
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Cleveland OH 44122
24
MR. JOHNSON’S
NOISE
Johnson Noise nV √Hz Vs. Resistance
RESISTANCE OHMS
en = (4kBTR)
-1/2
Noise (nV/√Hz)
26-Mar-16
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Cleveland OH 44122
25
WHEN A SHORT IS
NOT A SHORT
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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
26-Mar-16
Keithley Instruments
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Cleveland OH 44122
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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
26-Mar-16
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Cleveland OH 44122
27
COMMON MODE
VOLTAGES
• Another Source Of A DC Signal
=
VCM
26-Mar-16
VNM
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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
26-Mar-16
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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
26-Mar-16
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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
26-Mar-16
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
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Cleveland OH 44122
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SUMMARY
• The Nullmeter Is A 21st Century Instrument
(What else would we say!)
• Where You Need It:
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–
–
–
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
26-Mar-16
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
26-Mar-16
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Cleveland OH 44122
33
QUESTIONS???
26-Mar-16
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Cleveland OH 44122
34
QUESTIONS???
26-Mar-16
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Cleveland OH 44122
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26-Mar-16
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26-Mar-16
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Cleveland OH 44122
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