MAPAN-Journal
of Metrology Society of India
DOI 10.1007/s12647-012-0023-z
ORIGINAL PAPER
A Step-Down Technique to Calibrate AC Current Down to 10 lA
Using a Precision 10 mA Current Shunt
M. Halawa1* and M. Rahal2
1
National Institute for Standards (NIS), Tersa St., Giza, Box: 136, El-Ahram 12211, Egypt
2
Department of Electrical Engineering, Hail University, KSA, Hail, Saudi Arabia
Received: 26 April 2012 / Accepted: 19 July 2012
Metrology Society of India 2012
Abstract: This paper describes a calibration procedure for AC current measurements at 1 mA, 100 lA and 10 lA using a
NIST-calibrated 10 mA current shunt as a reference standard. The procedure involves a step-down technique using the
reference transfer standards (RTS) as a precision current divider. The RTS is used at values of 370 X, 3.3 and 30 kX to
provide the intended currents. Uncertainty calculations are estimated for calibrating the AC current of 10 mA, 1 mA, 100
lA and 10 lA at 55 Hz and 1 kHz. The expanded uncertainties are around the values of 37 nA and 1 nA for the ranges of
1 mA–10 lA respectively.
Keywords: AC current measurements; Thermal current converters; AC–DC difference; Step-down technique;
Uncertainty budget
1. Introduction
The calibration of AC current using precision calibrators,
transconductance amplifiers and measuring instruments
requires high accuracy and wide frequency range. Consequently, in the last few years, efforts have been made to
improve these measurements [1, 2]. In primary metrological
laboratories, AC value is commonly measured by comparing
an unknown AC current with a known DC current using
thermal current converters (TCCs) [3]. A TCC is usually
composed of a thermoelement (TE) in combination with a
shunting resistor or with a current shunt for currents[20 mA.
The effective value of an AC quantity is considered
equivalent to a DC value when their AC and DC powers
are indicated by the output of TE, at its resistive element
(heater) [4]. These standards can also be compared in a
step-up technique, which allows extending the traceability
from the basic reference to 20 A and beyond. The AC–DC
difference of this TCC depends upon the combined characteristics of the TE and the shunt. More details about the
principle of AC–DC difference can be found in [1–4].
Low AC current measurement have become more
challenging with the availability of precision digital
*Corresponding author, E-mail: mamdouh_halawa@yahoo.com
multimeters (DMMs) and calibrators in the commercial
market. As the recent AC shunt products, like Holt Model
HCS-1 and Fluke Model 40 A can only measure down to
the 10 mA current level [5], many techniques to calibrate
the lower levels of AC current, are still under investigation
by the metrological laboratories. For instance, this technique is used for the first time in NIS, Egypt to extend the
range of AC current calibration down to 10 lA.
This paper introduces a step down technique to calibrate
AC current at 1 mA, 100 lA and 10 lA levels related to
10 mA current shunt (Holt, HCS-1), calibrated by National
Institute for Standards and Technology (NIST). The
described technique consists of a calibrated TCC, TE in a
parallel connection with a current shunt (&41 X), combined with the resistance transfer standards (RTS) (esi,
SR1010). RTS acts in this technique as a precision current
divider through resistance values of 370 X, 3.3 and 30 kX
to cover the current values of 1 mA, 100 lA and 10 lA,
respectively. Uncertainty budget of this method is also
discussed and presented in this paper.
2. Set-Up of the Step-Down Technique
Step-calibration methods are used in many NMIs for the
extension of measurements to quantities which are ignored
123
M. Halawa, M. Rahal
Keithley 182
DVM
TE
S1
HCS-1
S2
Fluke
5720A
Calibrator
RTS1
S3
RTS2
TE) where long settling times allow no evident gain in the
operational time, the automatic system has many advantages. It does not require the full attendance of one or more
operators and it guarantees a better definition of the calibration procedure.
The system consists of a calibrated 10 mA TCC and
three separate boxes of RTS connected in parallel with the
TCC through four switches, S1, S2, S3 and S4. Fluke 5720A
calibrator generates the traceable DC current and the AC
signals at 55 Hz and 1 kHz. A Keithley 182 sensitive
digital voltmeter is used to measure the output electromotive force (EMF) of the TE while a calibrated Fluke
8508A reference multi-meter is used to measure the AC
and DC voltage drops across the RTS.
S4
RTS3
3. RTS as a Precision Current Divider
Computer
Interface
Fluke 8508A
Reference Multimeter
Fig. 1 Set-up of the step-down technique to calibrate 1 mA, 100 lA
and 10 lA AC current using RTS as a precision current divider
from the computation of measurement uncertainty at which
absolute determinations are made. The excellent precision
of repetitive substitution procedures is exploited by step-up
or step-down methods to extend measurements to higher or
lower magnitudes without serious degradation of accuracy
[6]. The application of step-down techniques to the calibration of low levels of AC current up to 100 lA is
described in [5]. However, this system has been used the
same technique but up to 10 lA level. In addition, different
current dividers RTS are investigated in this system to get
higher accuracy and less uncertainty.
The diagram in Fig. 1 illustrates the full calibration
system used in the step-down technique. The design of this
system allows the implementation of this calibration
manually. However, automatic calibration systems have
been developed to replace the manually operated systems.
The software has been built in laboratory virtual instrument
engineering workbench (LabVIEW) and controlled by
GBIB Bus to synchronize the system software and hardware. The system configuration can be selected on the basis
of the specific operative needs. Another facility is introduced by the description of the proper setting of the
instruments (such as bus address, selected ranges, and filtration) or by the type of features for the calibration
operations (such as the number of repetitions, the operation
timing, and the parameters to be evaluated). In addition,
when the system operates only with fully programmable
instruments like multimeters and calibrators, the operations
can be performed more rapidly than by manual procedures.
However, also in the case of nonautomatic devices (such as
123
RTS is a resistance box containing twelve nominally equal
precision resistors (Fig. 2). The RTS boxes are available in
various resistance values, depending upon the level of
accuracy required [7]. In most applications, the resistor
adjustment accuracy is high enough that the resistors can
be assumed to have exactly their nominal values. In electrical metrology, the major application for using RTS is
transferring the resistance calibration from the 1 X reference to the entire chain of resistance measuring equipment
and standards in the laboratory with an ultimate accuracy.
The resistors of RTS are unifilar-wound on specially
processed mica cards, and use a special alloy resistance
wire, which has excellent stability, extremely low temperature coefficient and negligible thermal emf to copper.
This special design assures the maintenance of high accuracy between calibration periods and over normal temperature ranges [6].
The part per million accuracy of the RTS is assured as
the series value is equal to 100 times the parallel value to
better than 1 part per million (1 ppm) [7]. The accuracy
and precision of the individual resistors make the Model
SR1010 (Fig. 3) a very accurate device for use as a multivalue standard resistor or current divider. Simply, it is
carefully built and inspected to insure maximum control of
quality.
Fig. 2 RTS circuit diagram
A Step-Down Technique to Calibrate AC Current Down to 10 lA
uncertainty that arises from the use of shunts is due to the
power dissipated in the resistive element which increases with
the square of the current to be measured [8]. The phase shift
error of the non-resistive input impedance of the DMM is
around the value of 10 ppm [5]. For each step of this method,
the following uncertainty contributions are taken into account:
•
•
Fig. 3 Internal design of model SR1010
4. Measurement Procedure: Step-Down Technique
Referring to Fig. 1, the calibration sequence is as follows:
•
•
•
•
•
•
Close switch S1 and use the 10 mA of the calibrated
TCC to calibrate the AC current source (Fluke 5720A)
at 10 mA current level.
Close switch S2 and use the 90 % of full-scale input of
the 10 mA AC current shunt to calibrate the AC–DC
Difference of the RTS1 (dRTS1) with a parallelconnected, DMM (Fluke 8508A).
Open switch S1 and use RTS1 with the parallelconnected DMM to calibrate the 1 mA current source
using dRTS1 and the actual value of the 1 mA DC
current source through the comparison of the AC and
DC voltage drops across the RTS1.
Close switch S3 to calibrate the AC–DC Difference of
RTS2 (dRTS2) with a parallel-connected DMM at
100 lA AC current level.
Open switch S2 and use RTS2 with the parallelconnected DMM to calibrate the 100 lA current source
using dRTS2 and the actual value of the 100 lA DC
current source through the comparison of the AC and
DC voltage drops across the RTS2.
Repeat the previous sequence using RTS3 to calibrate
the 10 lA AC current source. This sequence is
performed at frequencies of 55 Hz and 1 kHz because
the TCC is calibrated only at these frequencies.
The above described procedure was only performed at
frequencies of 55 Hz and 1 kHz because the TCC used in
this work was calibrated only at these frequencies.
5. Uncertainty Estimation
Calibration uncertainties have been estimated for this technique. Besides the uncertainty value of the calibrated 10 mA,
there are several sources for the uncertainty budget [3]. Error
contributions arise from the AC and DC measurements, and
current instability. However, the principle source of
Type A uncertainty of the mean calculated from a set of
determinations of the calibrated value [9], (10 times in
this work).
The global type B uncertainty for each step in this
technique where the following uncertainty components
are taken into account:
(i)
Uncertainty of the reference multimeter (Fluke 8508A
DMM);
(ii) Uncertainty of the calibrated current shunt;
(iii) Uncertainty of the specified values of the RTS;
(iv) Uncertainties of the traceable DC current source
(Fluke 5720A Calibrator);
(v) Uncertainty due to the short-term stability of the
source (Fluke 5720A Calibrator);
(vi) Uncertainty of phase shift error of the non-resistive
input impedance of the DMM.
(vii) Uncertainty due to the resolution of DMMs.
For instance, the uncertainty budgets for the highest
current (10 mA) and the lowest current (10 lA) for DC and
AC current at 55 Hz and 1 kHz are reported in Tables 1, 2,
3. Finally, typical values of AC and DC current calibrations, which performed during this technique associated
with the expanded uncertainties, (for confidence
level = 95 %), are reported in Table 4.
6. Conclusion
A step-down technique for AC current calibration using
RTS as a precision current divider (at values 370 X, 3.3 and
Table 1 Uncertainty budget of 10 mA DC current
Uncertainty component
Contribution, ppm
10 mA
10 lA
Repeatability of 10 times
0.3
0.5
Reference multimeter calibration
(Fluke 8508A)
3
7
Reference multimeter specification
(Fluke 8508A)
4.7
Reference multimeter resolution
(Fluke 8508A)
0.03
0.3
Short term stability of the DC current
source (Fluke 5720A)
1.7
2
Expanded uncertainty, (k = 2)
±12 ppm
±35 ppm
(±120 nA)
(±0.35 nA)
16
123
M. Halawa, M. Rahal
Table 2 Uncertainty budget of 10 mA AC current
Table 4 Summary of the results at 55 Hz and 1 kHz
Uncertainty component
Function
Nominal
value
Actual value
Expanded
uncertainty (nA)
DC current
10 mA
9.99999 mA
±120
1 mA
1.000007 mA
±11
100 lA
100.0051 lA
±1.4
10 lA
10.00403 lA
±0.35
10 mA
9.999710 mA
±160
1 mA
0.999387 mA
±38
100 lA
10 lA
99.5758 lA
9.6328 lA
±5.5
±1.02
Contribution, ppm
55 Hz
1 kHz
Repeatability of 10 times
4
3
Thermal current converter (TCC)
calibration: (combination of
TE ? HCS-1)
2
3.5
DC current source calibration
(Fluke 5720A)
6
6
Short term stability of the AC current
source (Fluke 5720A)
2
2
Short term stability of the DC current
source (Fluke 5720A)
1.7
1.7
Expanded uncertainty, (k = 2)
±16 ppm
±16 ppm
(±160 nA)
(±160 nA)
Table 3 Uncertainty budget of 10 lA AC current
Uncertainty component
AC current (1 kHz)
10 mA
9.99982 mA
±160
1 mA
0.999442 mA
±37
100 lA
99.5811 lA
±5.3
10 lA
9.582 lA
±1
References
Contribution, ppm
55 Hz
1 kHz
Repeatability of the DC voltage drop
for 10 observations
8
8
Repeatability of the AC voltage drop
for 10 observations
30
29
Calibration of DC current source at
100 lA (Fluke 5720A)
14
14
Calibration of DC current source at
10 lA (Fluke 5720A)
35
35
Calibration of AC current source at
100 lA (Fluke 5720A)
55
53
Calibration of RTS
20
20
DMM phase shift error
10
10
Resolution of DVM (Keithley 182)
0.4
0.4
Expanded uncertainty, (k = 2)
±102 ppm
±100 ppm
(±1.02 nA)
(±1.0 nA)
30 kX) has been performed and investigated for current
level of 1 mA, 100 lA and 10 lA at 55 Hz and 1 kHz. AC
current calibration based on step down technique has been
performed using 10 mA current shunt as a reference standard. Expanded uncertainties have been estimated as 38 nA
and 37 nA for 1 mA level at 55 Hz and 1 kHz respectively.
The uncertainty changes to about 1 nA for 10 lA level.
123
AC current (55 Hz)
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