the nullmeter a 21 st century instrument or a 19 th century instrument?

Apr 22, 2023 www.ppminc.comCleveland OH 44122

1

THE NULLMETERA 21st Century Instrument

Or

A 19th Century Instrument?

PPM, Inc.

October 2007


2

INTRODUCTIONS

Presentation: Chas GilmoreManaging Director, Strategy

PPM, Inc.

Sponsor: Mark ReidtechniCAL

Copies of Presentation Available at techniCAL


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


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


5

WHO ARE YOU?

• Involvement With DC Calibration Techniques?


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


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

+

-

Source Voltage Vs

Vin = Vs

Output VoltageVO = Vin X G

DifferentialAmplifierGain = G

+10

0

-10


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


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


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


11

MORE EARLY METERS

C 1890

C 1930


12

NULLMETERS YOU MIGHT OWN

FLUKE 845 AB

Mid-1960s

PPM AVM-100

Mid 1990s

KEITHLEY 155The 1970s

HP 419A

1960sHP 413A

1950s


13

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


14

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


15

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?)


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

http://en.wikipedia.org/wiki/Image:Great_Western_Bridge_over_the_River_Kelvin.jpg


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

G

AVM-2000 NULL METER

HI LO

+10+3

0

-10

-3

R 1

R 3 R 4

R 2

A B


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

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


SOURCE


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

90.62441 Volt SourceTo Be

Calibrated

G

AVM-2000 NULL METER

HI LO

+10+3

0

-10

-3


SOURCE

KELVIN-VARLEYRATIO DIVIDER

RATIO : 0.9062441


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


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


22

OBSERVING A 1µV NULL

ANALOG DIGITAL

Range ± 3µV Range ± 200.00 mV


23

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?


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


25

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


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


27

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


28

COMMON MODE VOLTAGES

• Another Source Of A DC Signal

=

VCMVNM


29

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


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

Am

plif

ier

Source Voltage

Vs

Input Source Resistance Rs

Vin = Vs + Ib x Rs

Ib


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


-

+

+10

0

-10

Vin

Input ChopperVin = Vs

AC Amplifier With Demodulator

Iin

Vs


-

+

+10

0

-10

Vin

Input ChopperVin = 0

AC Amplifier With Demodulator

Iin

Vs


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


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


34

QUESTIONS???


35

QUESTIONS???


36


37

the nullmeter a 21 st century instrument or a 19 th century instrument?

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