Calibration of Resistive Voltage Dividers

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Calibration of Resistive Voltage Dividers

Richard Timmons, P.Eng.richard.timmons@guildline.com

APMP 2019

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Different Techniques

• Background

• Calibration Techniques

• Comparison Method

• Bootstrap Method

• Ratiometric Method

• Self-Alignment Method

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Voltage Dividers

DC Voltage Dividers Are Primarily

Resistor BasedVout = Vin [R2 / (R1 + R2)]

Good Divider Design Should Also Allow For:

• Low Output Impedance

• High Input Impedance

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What Is A Voltage Divider and Why are they Important?

• Provide a known voltage, based on a voltage reference and ratio division

• Used to Calibrate or Verify high accuracy standards while providing minimum uncertainty contribution to the measurement process

Voltage Reference(10 Volts)

Voltage Divider 0.1V – Output

0.01V – Output

1.0V – Output

10V – Output

10:1

100:1

1000:1

1:1

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Voltage Divider as a Comparator

Voltage Standard(10 Volts)

Voltage Source

(eg 100 V)

Voltage Divider

(10:1)

Null Detector

Voltage Divider Allows Highly Accurate Comparison of Source Voltage to Reference Voltage

• For Example, 100 V Source compared to a 10 V Reference adds only a Small Uncertainty Contribution

10 V Out 100 V In10 V Out

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Why Is Resistance Network Calibration So Important?

Looking on the Inside of a Typical Voltage Divider

There are Many Resistors Inside!

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Not Just Resistors!

• Switches

• Potentiometers (Adjustments)

• Connections (Wire, Solder Points - etc.)

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Why is Calibration so Important

• Because the Divider Network is highly dependent on a Resistive Network – Low Uncertainty Dividers have to be calibrated each time before they are used

• Calibration Historically and Today Requires External Standards

• Voltage Source

• Voltage Meter, DMM, or Null Detector

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Resistance Drift Challenge

• Even Modern Resistors Have Material Short Term Drift

Why Many Dividers Just Have 8 Hour Specs

8 2412 16 204

Time (in Hours)

0.0

0.05

0.05Short Term Drift

in ppm

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Calibration ConsiderationsSub-ppm Measurements

• EMI Shielding

• Polarity Reversal

• Remove EMF Effects

• Temperature Stability

• Input Impedance

• Measure Offsets

• Short, Shielded Leads

• Measure Resistance

• Common Ground

• Run Voltage Reference and Null Detector off Battery

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Calibration by Comparison Overview

• Calibrate a Reference Voltage Divider and use to calibrate Voltage Divider Under Test (DUT)

• Use the same test setup

• Measure with Reference Voltage Divider then measure with DUT Voltage Divider and adjust DUT to match Reference

• Reference Voltage Divider needs to have ½ to ¼ uncertainty of DUT target uncertainty

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Calibration by Comparison

• Connect the Test Setup as Shown• Twist signal leads in pairs

• Set Divider mode to appropriate ratio(e.g. 1:1)

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Calibration by Comparison

Set V Source (e.g. +10 V (U5700+)) and Record the Null reading UND+

Reverse Source Polarity (e.g. –10V (U5700–)) and Record the Null reading UND–

Calculate the ratio error

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Calibration by Comparison1:1 & 10:1 Uncertainty

StandardUncertainty

(µV/V)

Total

Uncertainty

(µV/V)

1:1 Ratio Uncertainty Calculation

7520 DUT 1:1 Ratio Uncertainty 0.02

Voltage Source Short Term Stability (732A) 0.01

Null Detector (845AB) Short Term Stability 0.02

Total 1:1 Ratio Uncertainty 0.03

10:1 Ratio Uncertainty Calculation

7520 Reference 10:1 Ratio Uncertainty 0.05

7520 DUT 10:1 Ratio Uncertainty 0.05

Voltage Source Short Term Stability (732A) 0.01

Null Detector (845AB) Short Term Stability 0.02

Total 10:1 Ratio Uncertainty 0.07

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Calibration by Comparison100:1 & 1000:1 Uncertainty

StandardUncertainty

(µV/V)

Total

Uncertainty

(µV/V)

100:1 Ratio Uncertainty Calculation

7520 Reference 100:1 Ratio Uncertainty (4) 0.10

7520 DUT 100:1 Ratio Uncertainty 0.10

Voltage Source Short Term Stability (5720) 0.15

Null Detector (845AB) Short Term Stability 0.02

Total 100:1 Ratio Uncertainty 0.20

1000:1 Ratio Uncertainty Calculation

7520 Reference 1000:1 Ratio Uncertainty 0.25

7520 DUT 1000:1 Ratio Uncertainty 0.25

Voltage Source Short Term Stability (5720) 0.30

Null Detector (845AB) Short Term Stability 0.02

Total 1000:1 Ratio Uncertainty 0.46

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Calibration by Bootstrap Overview

• Calibrate DUT at 1 Voltage and Ratio

• Change Voltage, Recalibrate comparing to previous measurement using same ratio

• Change Ratio, Recalibrate comparing to previous measurement using same voltage

• Each stage uses measurements from previous stage, changing just one variable (i.e. voltage or ratio to ‘Bootstrap’ on previous measurements)

• Uncertainties build up at each stage

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Calibration by Bootstrap Process

Set Ref and DUT VDs to 1:1 Ratio and 10 V

• Use Measurement Offsets to determine actual input V to Dividers and Output DMM

Set Ref and DUT VDs to 1:10 Ratio and 10 V

• Use 1:1 Measurements to determine actual input V to Dividers and Output DMM

• Determine DUT 1:10 Calibration Coefficients by comparing Ref Div to DUT via Output DMM

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Calibration by Bootstrap Description

Set Ref and DUT VDs to 1:10 Ratio and 100 V

• Use 1:1 Measurements to determine actual input V to Dividers and Output DMM @ 100 V

• Use 10:1 Ratio Error from 10 V measurements to adjust 100 V measurements

• Determine DUT 1:10 Calibration Coefficients by comparing Ref Div to DUT via Output DMM

Continue for 100:1 @ 10 V; 100:1 @ 1000 V; 1000:1 @ 1000 V

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Calibration by BootstrapCalibrate Reference Standards

Calibrate Digital Volt Meters (DVM) and Reference Voltage Divider

(Reverse Polarity for ALL Measurements)

PJVS(Programmable Josephson

Voltage Standard)

ZenerRef (V)

Output DVM(3458A)

Ref DVM(3458A)Zener

Ref (V)

Div Ref(752A)

Manually Calibrate

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Calibration by BootstrapCalibrate Measurement Offsets

(Reverse Polarity for ALL Measurements)

V Source (Zener or 5440)

DUT (7520)

Determine Adjustments (e.g. lead resistance, input impedance of DUT

@ Diff V and Diff Ratios)

DUT (7520)Output DVM

(3458A)

Determine Adjustments (e.g. lead resistance, input impedance of Output

DVM@ Diff V and Diff Ratios)

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Calibration by BootstrapMeasure DUT

Calibration Setup

(Reverse Polarity for ALL Measurements)

Output DVM(3458A)

Ref Div(752A)

Ref V(Zener

or 5440)

Apply Adjustments to Ref Div / DUT

Input and Output Voltage Measurements

Ref DVM(3458A)

DUT(7520)

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NMI Calibration Certificate

NominalRatio

Nominalinput voltage

(V)

Nominaloutput voltage

(V)

Ratioerror

(µV/V)

Expandeduncertainty

(µV/V)

0.1 100 10 -0.30 0.43

0.01 100 1 -0.06 0.45

0.001 1000 1 0.62 0.59

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Calibration by Ratiometric Buildup Overview

• Use a Voltage Reference and calibrated 1:10 Reference Voltage Divider (720A Kelvin Varley)

• Use Fine Tuning Resistor to ensure voltage input to DUT (adjusted by appropriate ratio) is the same as voltage input to the Ref VD

• Calibrate each 1:10 Ratio/Resistance Network (i.e. 1:10, 10:100, 100:1000) separately against the Kelvin Varley

(i.e. Ratiometric Buildup from Kelvin Varley)

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Calibration by Ratiometric Buildup

Kelvin Varley (KV) 1:10 Ratio Calibrated at NMI and Fixed

DUT 1:1, 1:10, 10:100, 100:1000 Ratios Calibrated

1:1 Ratio @ 10 V

Compare directly to Zener Reference, with and without DUT

1:10 Ratio @ 10 V

Set DUT to 1:1 Ratio and balance at Null Detector (ND/3458A) by adjusting SOURCE (fine tune with Fine Tuning Resistor (FT) (i.e. adjust 10 V source used by DUT to be the same as Zener used by KV)

Set DUT to 1:10 Ratio and balance at ND against KV by adjusting DUT 1:10 calibration coefficient

Repeat for 10:100 Ratio @ 100 VRepeat for 100:1000 Ratio @ 1000 V

Reverse Polarity for All Measurements

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Calibration by Ratiometric Buildup

1V, 10V(Guildline

4410 ZENER)

+

–

SOURCE(5700)

DMM

KELVIN-VARLEY(1:0.1)

Hi

Lo

Hi

Lo

IN

OUTMUX

DUT (7520)Hi

Hi

Lo

Lo1:10

1:1001:1000OUT

IN

FT = Guildline Fine Tuning Resistor (< 0.1 µΩ/ Ω)DMM = 3458A used as a Null DetectorMUX = Guildline 6664C Scanner

Reverse Polarity for All Measurements

HHi

LoOFT

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NMIA Voltage Traceability

• Up to 10 V Traceable to Programmable JJ

• Up to 1100 V Comparison to Guildline 9700PL

• 55 Years of continuous operation

• Calibrated via Ratiometric Build-Up and Inductive Voltage Divider

Thanks to NMIA For Picture

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Self-Alignment Method

• Patented Design Incorporates True “Self-Alignment”

• Once Initial Factory Calibration Done, Only Self-Alignments Are Needed

• Initial calibration determines leakage and offsets

• Self alignments adjust for drift of resistive divider network

• Self-Alignment

• Incorporates internally All Required Standards

• Internal standards are also Temperature Controlled

• Fully automated Via push of a button

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7520 Voltage Divider

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Self-Alignment Modules

• New Design Incorporates a True “Self-Alignment” / “Self-Calibration” Process

• Even Checks Internal Temperature (to see if Stable)

Menu Operation

(Procedure)Manual,

Automated, Time Based

Divider Self-Alignment Modules

Internal Voltage References

Internal Null Detector

Internal Resistive Network

Temperature Stabilized Environment

Internal Wheatstone

Bridge

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100R

300R

HI

HI

LOLO

OUTPUT

INPUT

300R

300R

CALIBRATESWITCH

Reconfigurable Divider NetworkSelf-Alignment Mode AND Divider Mode

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10:1 DIVIDER CALIBRATION CIRCUIT

100R

300R

VOLTAGESOURCE

Nk

CALIBRATESWITCH

300R

300R +10:1 POT

HI LONULL

DETECTOR

Nk + BALANCE POT

7520 True Self-AlignmentDynamically Configured Wheatstone Bridge

Divider Network Reconfigured in Hamon Configuration

AFTER CALIBRATION RECONFIGURE LEFT SIDE OF BRIDGE TO GIVE 1:10 RATIO

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100R

1000R HI

HI

LOLO

OUTPUT

INPUT

10:1 VOLTAGE DIVIDER

Vin x RoutVout =

Rin

Vin x 100R=

1000R=

Vin

10

Divider Operation 1:10 Ratio

300R

300R

300R

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VOLTAGE DIVIDER Operation

Divider Operation 2

R

HI

HI

LOLO1000:1 OUTPUT

100:1 OUTPUT3R

3R

3R

30R

30R

30R

300R

300R

300R

10:1 OUTPUT

HI

HI HI

1:1 OUTPUT

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Questions

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