AC-DC Standards Funck - National Physical Laboratoryresource.npl.co.uk/docs/networks/electromagnetics/071129/dclf... · 3 P BT EM Day 2007 AC-DC current transfer standards Introduction

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CALIBRATION OFAC-DC CURRENT TRANSFER STANDARDS

BASED ONCALCUABLE THERMAL CONVERTERS ON

QUARTZ SUBSTRATE

Torsten Funck

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EM Day 2007 ACEM Day 2007 AC--DC current transfer standardsDC current transfer standards

Contents

• Introduction

• Quartz-PMJTCs for current transfer

• Calculation of the current transfer difference

• Validation of the model used for calculation

• Measurement setup

• AC-DC current transfer standards

• Step-up chains

• Improved measurement uncertainties

• Conclusions

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Introduction

Electrical quantities are defined, realized an maintained at direct current (dc).

For alternating current (ac) calibrations, traceability is given due to ac-dc transfer.

For current, the measured quantity is δ i , the ac-dc current transfer difference:

dc

dcaci I

II=δ −

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Quartz-PMJTCs

Planar multijunction thermal converters (PMJTCs) on a quartz substrate are used as a calcuable standard

+ Very low conductivity of the substrate

+ Low dielectric constant εr

+ Low, calculable transfer difference

- Difficult to manufacture

- Low thermal time-constant

- Low sensitivity

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Calculation of δi due to heater impedance

{ } 1Re

Hi1 −

ZR=δ

LH /10

LH /10

RH/10

RH

/10

C2/10

C2/10

C1/10

0

0

0

0

G 2vs (f)‏

G2vs (f)‏

G1vs (f)‏

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Calculation of δi due to stray capacitances

LH

LH

LCu

LCu

RCu

RCu

LAu

LAu

RAu

RAu

RHCwAuGquartz

CcarrierGcarrierI If, 0

| || | 1

2 213

432i2 −

⋅⋅

⋅

)Z+Z(Z)Z+Z(Z

=δ

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Current transfer differences

0 ,1 1-5

0

5

1 0

1 5

2 0

2 5

3 0

δ i

f re q u e n c y

R H = 7 0 0 Ω R H = 3 5 0 Ω R H = 2 4 0 Ω R H = 1 1 7 Ω

M H z

μ A /A

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Validation of the calculation

0,1 1-16

-14

-12

-10

-8

-6

-4

-2

0

2μA/A

MHz

117 Ω - 700 Ω measured calculated

Δδ i

frequency

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Uncertainties using built-in Tee

0,1 1

0,2

0,3

0,4

0,5

0,6

0,7

u

frequency MHz

μA/A

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Measurement setup using driven guards

• Symmetrical setup• Upper standard connected „upside down“• Constant voltage across parasitic capacitances =>

equal transfer differences in “upper position” and “lower position”

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AC-DC current transfer standards

• Low currents: PMJTC with low heaterresistance

Problem: Technology limits 90 Ω < RH < 900 Ω

• Medium currents: Shunt + PMJTCProblems: Shunt inductance

Current dependence

• High currents: Cooled shunt + PMJTCProblems: Shunt inductance

Current dependenceCompliance voltage of current source

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Shunts used at PTB

Up to 300 mA: PTB design (top)‏

500 mA to 5 A: Holt design (middle)‏

10 A and 20 A: Fluke design (bottom) ‏

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Shunts in Justervesenet design

Resistors

Current input

Output toTCC

+ Low cost materials

+ Large number of resistors ensure equal current distribution

+ Very small area of current path yields low inductive coupling

- No caseJustervesenet

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High current shunts

Set of four shunts:• 10 A to 30 A (25 mΩ)‏

• 30 A to 50 A (10 mΩ)‏

• 50 A to 80 A (5 mΩ)‏

• 80 A to 100 A (3 mΩ)‏

EL-9800

(NIST Design) ‏

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New high current shunts

+ Precision resistors provide also dc accuracy

+ Large number of resistors ensure equal current distribution

+ Small area of current path yields low inductive coupling

- No case

- Quite expensive

Proven design scaled for higher currents

800 mV nominal output voltage

Available up to 100 A

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Step-up chains using different standards

5 A72 A6

5 A1 A53 A5 A500 mA41 A1 A200 mA3

300 mA500 mA100 mA2100 mA100 mA50 mA130 mA30 mA20 mAstart

CBAStep \ Chain

Chains A & B: Self-made and Holt shuntsChain C: Justervesenet-Shunts (two sets)‏Comparison of the chains: 1 A at 1 MHz: Δ < 10 µA/A

5 A at 100 kHz: Δ < 5 µA/A

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Measurement of the level dependence

Assumption:Current level dependence of shunts

results from self-heating

Measurement approach:

Shunt with low voltage drop used as a standard

• => Low power => low self-heating => low current level dependence

• Low voltage drop => voltage-amplifier necessary to operate PMJTC

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Measurement setup: Shunt with Amplifier

DCVoltage

ACVoltage

AC-DC Switch

x11

Amplifierwith PMJTCShunt

TransconductanceAmplifier

PMJTC

nV nV

Shunt

Hi

Low

Hi

Low

Device under test Standard

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Uncertainty of measurement

The following sources of uncertainty of the step-up procedure can be identified for each step:

u (δstd) Standard uncertainty of the (calculable) transfer difference of the standard used, i.e. uncertainty of previous step

u (δC) Standard uncertainty of the comparison procedure

u (δA) Type A standard uncertainty, i.e. the standard deviation of the mean

u (δLev) Standard uncertainty due to the current level effects in the shunt

u (δLF) Standard uncertainty due to PMJTC low frequency effects, which are level dependent

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Improvements in measurement uncertainties

Expanded uncertainty of measurement in µA/A at the frequencies Current 10 Hz 1 kHz 10 kHz 100 kHz 500 kHz 1 MHz

5 5 5 10 - - 30 mA 1 1 1 1 1 2 5 5 5 10 - - 100 mA 2 2 2 2 2 5 10 10 10 20 - - 300 mA 2 2 2 2 3 5 10 10 10 30 - - 1 A 4 3 3 4 6 10 20 20 20 60 3 A 4 4 4 8 25 25 25 80 10 A 12 12 12 25 - - - - 30 A 30 30 60 120 - - - - 100 A 40 40 80 150

Red : old (2002)Blue: (2006) ‏

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Conclusions

• PMJTCs on quartz substrate developed at PTB are well suited for current transfer at low current up to 1 MHz

• Potential driven guarding reduces the uncertainty of the comparison measurements

• New shunts with low current level dependence and low transfer difference allow further reduction of the step-up procedure's uncertainty

• New approaches for the determination of the current level dependence allow for corrections and therefore for further reduction of the uncertainty

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Thank you for your attention!

Torsten Funck

Documents

AC-DC Standards Funck - National Physical Laboratoryresource.npl.co.uk/docs/networks/electromagnetics/071129/dclf... · 3 P BT EM Day 2007 AC-DC current transfer standards Introduction