CALIBRATION OF AC-DC CURRENT TRANSFER STANDARDS ...

CALIBRATION OF
AC-DC CURRENT TRANSFER STANDARDS
BASED ON
CALCUABLE THERMAL CONVERTERS ON
QUARTZ SUBSTRATE
Torsten Funck
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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).
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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:
I
δ=
i
ac
−I
I
dc
dc

Quartz-PMJTCs
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Planar multijunction thermal converters (PMJTCs) on a quartz
substrate are used as a calcuable standard
+ Very low conductivity of the substrate
+ Low dielectric constant ε r
+ Low, calculable transfer difference
- Difficult to manufacture
- Low thermal time-constant
- Low sensitivity

Calculation of δ i due to heater impedance
L H /10
L H /10
R
Re
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δ
R H /10
R H /10
i1
=
0
C 2 /10
0
C 1 /10
C 2 /10
{ } 1 H −
Z
0
0
G 2 vs (f)‏
G 1 vs (f)‏
G 2 vs (f)‏
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Calculation of δ i due to stray capacitances
R Cu
I I
f, 0
R Cu
L Cu
L Cu
C carrier
δ
i2
=
R Au
R Au
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L Au
G carrier
L Au
C wAu
| |
| | 1
Z2⋅(
Z3+
Z4
)
−
Z ⋅(
⋅Z+
Z )
3
2 1 2
L H
G quartz
L H
R H
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Current transfer differences
30
μ A/A
25
δ i
20
15
10
5
0
-5
R H = 700 Ω
R H = 350 Ω
R H = 240 Ω
R H = 117 Ω
0,1 MHz 1
frequency
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Validation of the calculation
μA/A
Δδ i
2
0
-2
-4
-6
-8
-10
-12
-14
-16
117 Ω - 700 Ω
measured
calculated
0,1 MHz 1
frequency
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Uncertainties using built-in Tee
μA/A
0,6
u
0,7
0,5
0,4
0,3
0,2
0,1
frequency
MHz 1
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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 heater
resistance
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Problem: Technology limits 90 Ω < R H < 900 Ω
• Medium currents: Shunt + PMJTC
Problems: Shunt inductance
Current dependence
• High currents: Cooled shunt + PMJTC
Problems: Shunt inductance
Current dependence
Compliance voltage of current source

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
Current
input
+ Low cost materials
Resistors
Output to
TCC
+ Large number of resistors ensure equal
current distribution
+ Very small area of current path yields low
inductive coupling
- No case
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Justervesenet

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)‏
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EL-9800
(NIST Design)‏
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New high current shunts
Proven design scaled for
higher currents
800 mV nominal output
voltage
Available up to 100 A
+ 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
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Step-up chains using different standards
Step \ Chain
start
1
2
3
4
5
6
7
1 A
2 A
5 A
20 mA
50 mA
100 mA
200 mA
500 mA
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1 A
5 A
100 mA
500 mA
Chains A & B: Self-made and Holt shunts
A
30 mA
Chain C: Justervesenet-Shunts (two sets)‏
B
C
1 A
3 A
5 A
30 mA
100 mA
300 mA
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
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• => Low power => low self-heating => low current level dependence
• Low voltage drop => voltage-amplifier necessary to operate PMJTC

Measurement setup: Shunt with Amplifier
Device
under test
AC
Voltage
Transconductance
Amplifier
PMJTC
AC-DC Switch
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Hi
Low
Shunt
Shunt
x11
DC
Voltage
Amplifier
with PMJTC
nV nV
Hi
Low
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 (δ std ) Standard uncertainty of the (calculable) transfer difference of
the standard used, i.e. uncertainty of previous step
u (δ C ) Standard uncertainty of the comparison procedure
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u (δ A ) Type A standard uncertainty, i.e. the standard deviation of the
mean
u (δ Lev ) Standard uncertainty due to the current level effects
in the shunt
u (δ LF ) Standard uncertainty due to PMJTC low frequency
effects, which are level dependent

Improvements in measurement uncertainties
Current
30 mA
100 mA
300 mA
1 A
3 A
10 A
30 A
100 A
Expanded uncertainty of measurement in µA/A at the frequencies
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10 Hz 1 kHz 10 kHz 100 kHz 500 kHz 1 MHz
5 5 5 10 - -
1 1 1 1 1 2
5 5 5 10 - -
2 2 2 2 2 5
10 10 10 20 - -
2 2 2 2 3 5
10 10 10 30 - -
4 3 3 4 6 10
20 20 20 60
4 4 4 8
25 25 25 80
12 12 12 25
-
30
-
30
-
60
-
120
Blue: (2006)‏
- - - -
40 40 80 150
Red : old (2002)

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
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attention!
Torsten Funck
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