The DMM correction for each test voltage at frequency f; is then given by:
Cuseni = V (1 + Cogevi) - Rei Rew (13)
where R,, = the DMM reading of the test voltage at f,.
3.4.4 Voltages from 1 mV to 1 kV from 0.01 Hz to 1 kHz

A programmable digitally synthesized source (DSS) developed at NIST is employed in place of the
system calibrator to perform high accuracy, low-frequency tests of DMMs. The DSS synthesizes a
staircase approximation of a sine wave using a sine look-up table and a precision digital-to-analog
converter (DAC). Output voltages are programmable between 1 mV and 7 V (for higher voltages an
external de-coupled amplifier is used). A DMM (on its DCV function) is used to measure the de
voltage of each of the steps in the approximation. The rms value of the output voltage V, is computed
by this step calibration:

V, = (v2 IN”, (14)

where v,= the dc voltages of the steps
N= the number of step per period.

The frequency response rms error of the synthesized signal is measured between 20 Hz and 1 kHz
using a calibrated multirange TVC. Below 20 Hz, it is assumed that the rms value remains constant
to within +2 »V/V. At 7 V, the frequency response out to 1 kHz is flat to within + 5 "V/V. This
performance degrades at lower voltages; however, even at 1 mV, the DSS may be treated as an ac
calibrator with zero correction below 20 Hz. DMMs calibrated using the DSS are assigned
corrections C,,cy; by:

Carers = V, + Cosi) - Ra: WRois (15)

where R,,= the DMM reading at frequency f;
Coss: = the DSS correction to V, at frequency f, .

Control charts for the ACV function corrections of the system calibrator and DMMs are quite
complex covering voltage, frequency, and DMM type. A sample chart for the ACV calibrator
corrections at 10 V at 10 kHz is shown in Fig. 9. ACV corrections are less predictable than the DCV
corrections; however, until the drift patterns arc better understood, a least squares linear fit to the
correction time series is performed and a linear equation is used to predict the calibrator correction
between calibrations. The system software also linearly interpolates between calibration points to
compute corrections over the fullrange. Specific software corrections for the DMM type being tested
are applied where possible and all interconnection parameters are specified. In a similar manner,
when a calibrator is tested using a system DMM, the DMM type and all interconnection parameters
are specified. Measurements are performed at a specific set of voltages and frequencies trom 1 mV
to 1 kV and 0.1 Hz to 30 MHz. Measurements can be made at other than the calibrated points;

16


Table 5
MCS Type B Standard Uncertainties for DCI

Current Range Calibrated Points Full Range
1pA tolmA 20 pA/A+10nA 40 pA/A + 10 nA
1 mA to 10 mA 15 pA/A 30 pA/A
10 mA to 100 mA 10 pA/A 20 pA/A |
100 mA tol A 20 pA/A 40 pA/A
1Atol0A 25 pA/A 50 pA/A

3.3 DC Resistance (DCR)

The DCR function of the MCS calibrator and DMMs is periodically calibrated against the set of NIST
reference resistors described earlier. Each reference resistor is measured by the system DMMs using
the 4-wire DCR function. The dc resistance corrections to the DMM C,p-, are related to the DMM
reading R and the true de resistance Rp by:

Roc = ROI + Cancer). 6)

Once the DMMs are calibrated, they are used to measure the set of resistors in the system calibrator.
The corrections to the calibrator resistors are based on the average of values assigned by the system
DMMs. The dc resistance corrections to the calibrator C,p- are related to the calibrator setting S and
the actual calibrator de resistance R.pcp by:

Roce = SCL + Concer). (6)

A control chart with selected DCR calibrator corrections is shown in Fig. 6. DCR corrections also
tend to drift linearly with time, so a least squares fit to the correction time series is performed and a
linear equation is used to predict the calibrator correction between calibrations. The system software
also linearly interpolates between calibration points to compute corrections over the full range
(DMMs only). As with other functions, software corrections are applied during DMM and calibrator
tests.

Measurements are performed at nominal decade resistance values from 0.1 Q to 100 MQ using the
DMMs. Decade resistance values between 1 Q and 10 MQ are available on the system calibrator.

Other than decade values may be measured using the system DMMs, which demonstrate resistance
linearity as good or better than resistance stability. However, for measurement at other than the
calibrated points, the MCS corrections are interpolated estimates and uncertainties are somewhat
larger. The Type B standard uncertainties of the MCS DCR (which include the uncertainties of the

10


understood, a least squares linear fit to the correction time series is performed and a linear equation
is used to predict the calibrator correction between calibrations. The system software also linearly
interpolates between calibration points to compute corrections over the full range. Specific software
corrections for the DMM type being tested are applied where possible and all interconnection
parameters are specified. In a similar manner, when a calibrator is tested using a system DMM, the
DMM type and all interconnection parameters are specified.

3.5.2 Currents from 2 mA to 10 A from 0.01 Hz to 1 kHz

The NIST DSS and Transconductance Amplifier are employed to perform high accuracy, low
frequency current tests of DMMs. A DMM (on its DCI function) is used to measure the de current
of each of the steps in the waveform approximation. The rms value of the output current J, is
computed by this step calibration:

1, = (Bi AY", (19)

where i, = the de currents of the steps
N = the number of steps per period.

The frequency response error of the synthesized signal is measured between 20 Hz and 1 kHz using
acalibrated TCC. Below 20 Hz, it is assumed that the rms value remains constant to within +2 pA/A.
At currents between 100 mA and 10 A, the frequency response out to 1 kHz is flat to within
+100 pA/A. This performance degrades at lower currents; however, even at 1 mA, the DSS may be
treated as an ac calibrator with zero correction below 20 Hz. DMMs calibrated using the DSS are
assigned corrections Cyc, by:

Caren = I, 1 + Coss) - Roi Rein (20)

where R,;= the DMM reading at frequency /;
. Cpss; = the DSS correction to J, at frequency f; .

Again, a least squares linear fit to the correction time series is performed and a linear equation is used
to predict the calibrator correction between calibrations. The system software also linearly
interpolates between calibration points to compute corrections over the full range. Specific software
corrections for the DMM type being tested are applied where possible and all interconnection
parameters are specified. In a similar manner, when a calibrator is tested using a system DMM, the
DMM type and all interconnection parameters are specified.

Measurements are performed at a specific set of currents and frequencies from 1 mA to 1 A and
0.1 Hz to 1 kHz. Measurements can be made at other than the calibrated points; however, because
the corrections at these points are interpolated estimates, the MCS uncertainties are somewhat larger.
The Type B standard uncertainties of the MCS ACV function are given in Table 8.

20


TABLE OF CONTENTS
1. INTRODUCTION... 1... cee n nent e tent n eens 2
2. DESCRIPTION OF THE MCS

2.1 MCS Hardware ....
2.2 MCS Software

3. CHARACTERIZING THE MCS
3.1 DC Voltage (DCV) ...
3.2 DC Current (DCD) ..
3.3 DC Resistance (DCR) .
3.4 AC Voltage (ACV) ..... 0... cece cece eee eee .

3.4.1 Voltages from 1 mV to 1 kV from 10 Hz to 1 MHz ................. 12

3.4.2 Voltages from | mV to 250 mV from 10 Hz to | MHz............... 14

3.4.3 Voltage from 250 mV to 7 V from 10 Hz to 30 MHz ..

3.4.4 Voltages from 1 mV to 1 kV from 0.01 Hzto 1 kHz .
3.5 AC Current (ACI . 2.00... 00 c cece cece e cece e cece ees

3.5.1 Currents from 2 mA to 10 A from 10 Hz to 100 kHz .

3.5.2 Currents from 2 mA to 10 A from 0.01 Hz to 1 kHz

4, CALIBRATIONS AND TESTS USING THE MCS ............ cece eee eee eee eee 21
4.1 Calibration of DMMs Using the MCS Calibrator . . 21
4.2 Calibration of Calibrators using the MCS DMM ..... ~23

4.3 High Accuracy Special Test of DMMs and Calibrators . . .

4.4 Special Test of Low Voltage Thermal Voltage Converters . .24
4.5 Special Test of Micropotentiometers ......... 20... c cece cece ence nena 24
5. QUALITY CONTROL AND UNCERTAINTIES ...... 20.0000 cece cece cence eeeee 24
7. ACKNOWLEDGMENTS ......... 00.0. c cece cence teen een ene n ene eeee 25
8. REFERENCES .......0.0 0000 c cece cece cence cece ene e cece eee eseeeres 26


however, because the corrections at these points are interpolated estimates, the MCS uncertainties are
somewhat larger. The Type B standard uncertainties of the MCS ACV function are given in Table 7.

0 10 20 30 40 50 60 70 80 90 100
Time (weeks)

© 10V @ 10 kHz (measured) —* 10 V @ 10 kHz (regression)

Fig. 9. Control chart with measured and predicted
corrections for the MCS calibrator at 10 V and 10 kHz.

Table 7
MCS Type B Standard Uncertainties for ACV
Voltage Range Calibrated Points Full Range
1 mV to 10 mV (10/v)(70 + fp V/V (10/v)(200 + f)V/V
10 mV to 100 mV (70 + £/2)uV/V (200 +£/2 )wV/V
100 mV to 500 mV (20 + £/5)nV/V. (60 + £/5) V/V
500 mV to 100 V (10 + £/10)pV/V (30 + f/10)nV/V
100 V to 1 kV (20 + f/5)pV/V. _ (60 + f/5)nV/V |

Where v = test voltage in mV and f= test frequency in kilohertz.

3.5 AC Current (ACI)

The ACI function of the MCS calibrator and DMMs is periodically calibrated against a set of
reference thermal current converters (TCCs), ac shunts, and a calculable DSS. The ac-de differences
of these support instruments are ultimately referenced to a set of characterized TCCs that are the
NIST standards for ac-de difference [10]. The test procedures for characterizing the system calibrator

and DMMs for ACT are also described in detail.

17


Certain DMMs have software correction tables that are used to keep the instrument within a particular
specification. Correction tables are updated by applying a reference signal from the MCS calibrator
and then initiating a GPIB "correction" command that changes the appropriate correction value until
the DMM displays the proper value to within a selectable tolerance. Both the MCS software and the
Wavetek 4950 software have this capability. Repeat tests are performed to verify that the DMM
correction table has been accurately modified. This service represents a change in the long-standing
NIST policy of not adjusting instruments submitted for test. However, instead of making an analog
adjustment which can not generally be undone, in this case the adjustment is digital. The initial
corrections are stored in the instrument and the residual corrections are given in the test report. The
uncertainties of the residual corrections are based on the standard uncertainty of the calibrator and the
standard deviation of the repeat tests.

4.2 Calibration of Calibrators using the MCS DMM
Unless otherwise requested, calibrators are tested at their output terminals using the setup shown in
Fig.12. The interconnections are the same as those given in Tables 9 - 11. The test calibrator setting

Sis related to the system DMM reading R:

SU +Cread = RA + Cooma» (22)

NOSE TO NOSE FAST DMM OR FAST CALIBRATOR TEST SETUP

To COMPUTER f— — >} CALIBRATOR

hefa OM = TO COMPUTER
XN ADJUSTABLE

PLATFORM

MCS RACK

| qa

Fig. 12. Setup to calibrate a test DMM using the
characterized MCS calibrator.

where C7c,, is the test calibrator correction and Cypy, is the system DMM correction.

As with DMMs, certain calibrators have software correction tables that are used to keep the
instrument within a particular specification. Correction tables are updated by applying a nominal
signal from the test calibrator to the MCS DMM, and then initiating a GPIB "correction" command,

23


The DMM correction at each frequency C,,¢;; is given by:
Cuscn = (1 + Coren) - Rai (17)

As the frequency increases the calibrator becomes more sensitive to the load imposed by the TCC and
the test DMM. Therefore, for the highest accuracy test of a particular DMM type, the above
procedure must be performed using that DMM type. The second set of DMM readings R,, is used to
generate unique corrections C, 4c; for all DMMs of that type (type-X DMMs for this example).
These corrections apply only when a type-X DMM is connected to the calibrator using the same
connectors and cables that were used to obtain the set of R,, readings. These corrections are given by:

Cerctieg = Ceacti + Caicos (18)

where Cyigy = (Reign ~ Raigo)/I tepresents the additional correction needed to compensate for the
difference between the calibrator output current (programmed to /;) with the TCC and type-X DMM
connected, and with only the type-X DMM connected.

As with DCI, ACI measurements are made at various compliance voltages and a correction factor for
the calibrator compliance voltage is applied when testing a DMM type that has not been characterized
with the system calibrator.

Control charts for the ACI function corrections of the system calibrator and DMMs are also quite
complex covering current, frequency, and DMM type (or compliance voltage). A sample chart for
the ACI calibrator corrections at 1 A, at 1 kHz for one of the DMM types is shown in Fig. 11. ACI
corrections are less predictable than the DCI corrections; however, until the drift patterns are better

0 5 10 15 20 25 30
Time (weeks)

© 1A @1 kHz (measured) -* 1 A @ | kHz (regression)

Fig. 11. Control chart with measured and predicted
corrections for the MCS calibrator for ACI.

19


used to generate unique corrections C,,cyx) for all DMMs of that type (type-X DMMs for this
example). These corrections apply only when a type-X DMM is connected to the calibrator using the
same connectors and cables that were used to obtain the set of R,; readings. The unique set of
calibrator corrections for each DMM type are given by:

Corcrien = Ceacns + Coie: (9)

where Cyi, = (Reicn - Rai)/V; represents the additional correction needed to compensate for the
difference between the calibrator output voltage (programmed to V;) with the TVC and type-X DMM.
connected and with only the type-X DMM connected.

3.4.2. Voltages from 1 mV to 250 mV from 10 Hz to 1 MHz

A reference micropotentiometer (j1pot) is connected to the system calibrator output and a DMM is
connected to its output as shown in Fig. 8. The calibrator is set to the ACV function at 1 kHz and its
voltage is adjusted until the DMM reading ,, is within +t of the test voltage V. The DMM reading
R,, and the pot output emf e,, are recorded. The calibrator frequency is programmed to f, (the first
test frequency), the calibrator voltage is adjusted until the zpot emf e,, is within + of e,,, and the
DMM reading R,, is recorded. The calibrator is programmed to 1 kHz, the calibrator voltage is
adjusted until the nominal test voltage V +t is read by the DMM, and its reading R,, and the ppot emf
e,, are recorded. The procedure is repeated for frequencies f, through /f; interleaving a 1 kHz
measurement between measurements at each test frequency.

GPIB
MULTIFUNCTION
CALIBRATOR
POT

DMM fest sae

woe} bot

DVM bey
CONTROLLER

Fig. 8. Setup for characterizing the ACV function of
the MCS using a ppot.

The ppot is removed, the DMM is connected directly to the calibrator output, frequencies f, through
Jf, are set at the test voltage V, and the DMM readings R,, through R,,; are recorded.

14


DCR

Used to calibrate the resistance function of the MCS
calibrator and DMMs.

AC-DC diff

Used to determine the ac-de difference of thermal
voltage/current converters and micropotentiometers.

4950MTS

Software that uses a 4950 DMM to test a calibrator, and
store a new set of firmware corrections in Wavetek
calibrators.

3.1 DC Voltage (DCV)

3. CHARACTERIZING THE MCS

A Wavetek 4911 10 V zener reference is part of the MCS equipment used to support the DCV
function of the system calibrator and DMMs. This instrument is periodically compared to the NIST
10 V Josephson array [5]. A control chart with the zener reference corrections is shown in Fig. 3.

Correction (ppm)

= Nw

50 100 150 200 250
Time (weeks)

© Measured ~® Regression

Fig. 3. Control chart with measured and predicted
corrections for the zener reference.

Periodically, special resistive dividers, a Fluke 752, and another similar divider designed at NIST,
are used to compare five calibrator voltages (100 mV, 1 V, 10 V, 100 V, and 1 KV) to the 10 V zener
reference. The de voltage corrections to the calibrator C,y¢y are related to the calibrator setting S and
the actual calibrator output voltage Vp, by:

Vener = SL + Cover). qd)



(1]

[2]

B]

[4]

[5]

{6]

(7]

[8]

19]

8. REFERENCES
N. M. Oldham, M. E. Parker, A. M. Young, and A. G. Smith, “A High-Accuracy 10 Hz -
1 MHz Automatic AC Voltage Calibration System,” IEEE Trans. Instrum. Meas., IM-36,
pp.883-887, Dec. 1987.

N. M. Oldham, “New Low-Voltage Standards in the DC to 1-MHz Frequency Range,” IEEE
Trans. Instrum. Meas., Vol. 40, No. 2, pp. 368-372, Apr. 1991.

N. M. Oldham, P. S. Hetrick, “A Calculable, Transportable Audio-Frequency AC Reference
Standard,” IEEE Trans. Instrum. Meas., Vol. 38, No. 2, Apr. 1989.

N. M. Oldham, “A 50-ppm AC Reference Standard which Spans 1 Hz to 50 kHz,” IEEE
Trans. Instrum. Meas., IM-32, No. 1, Mar. 1983.

C.A. Hamilton et al., “The NBS Josephson Array Voltage Standard,” IEEE Trans. Instrum.
Meas., IM-36, No. 2, pp 258-261, June 1987.

B.N. Taylor, C. E. Kuyatt, “Guidelines for Evaluating and Expressing the Uncertainty of
NIST Measurement Results,” NIST Technical Note 1297, 1994 Ed.

R.F. Dziuba et al, "NIST Measurement Service for DC Standard Resistors," NIST Technical
Note 1298, 1992.

N.M. Oldham, P.S. Hetrick, and X. Zeng, "A Calculable, Transportable Audio-Frequency AC
Reference Standard,” [EEE Trans. Instrum. Meas., Vol. 38, No. 2, pp. 368-371, April 1989.

JR. Kinard et al, "NIST Measurement Services: AC-DC Difference Calibrations," NIST
Special Publication 250-27, May 1989.

26


2. DESCRIPTION OF THE MCS

2.1 MCS Hardware®

A block diagram of the MCS instrumentation is shown in Fig. 1. The basic system is quite simple,
consisting of a controller, a characterized calibrator (to test DMMs), and two characterized DMMs
(to calibrate calibrators and serve as check standards for the system calibrator).

CALIBRATOR, fos

CONTROLLER

Fig. 1. Simplified diagram of the MCS.

The NIST test facility consists of two such systems which utilize the instruments shown in Table 1:

Table 1
MCS Instrumentation

System 1 System 2
Fluke 5720A Multifunction Calibrator Wavetek 4808 Multifunction Calibrator
Hewlett Packard 3458A Digital Multimeter Hewlett Packard 3458A Digital Multimeter
Wavetek 4950 Digital Multimeter Wavetek 4950 Digital Multimeter
PC Controller with GPIB Card PC Controller with GPIB Card
Keithley 7001 Switch System. Keithley 7001 Switch System

°Commercial instruments and software are identified in this report to describe the measurement system.
Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the instruments
or software are necessarily the best available for the purpose.

3


NIST MULTIFUNCTION CALIBRATION SYSTEM

Nile Oldham and Mark Parker
Electronic Instrumentation and Metrology Group
Electricity Division
National Institute of Standards and Technology
Gaithersburg, MD 20899

Abstract - The NIST automated Multifunction Calibration System (MCS) for voltage, current, and
resistance is described. Developed primarily to calibrate digital multimeters and calibrators, the
system can also be used to test thermal converters, and micropotentiometers. Methods for
characterizing the MCS over a wide range of amplitudes at frequencies from de to 30 MHz are
described.

1. INTRODUCTION

Digital multimeters (DMMs) from a number of manufacturers claim uncertainties and stabilities that
approach those of the laboratory standards used to support them. This is possible because these
DMMs are essentially miniature standards labs with built-in zener references, stable resistors, and
ac-de transfer standards, all controlled by a central processor that applies corrections and performs
data analysis. In addition to being easier to use than most standards, DMMs are also quite rugged,
able to withstand the mechanical and thermal shocks of transportation from one laboratory to another
with little degradation in performance. These properties make them ideal transport standards to
provide traceability for the five quantities that most DMMs measure (de voltage, current, and
resistance, and ac voltage and current).

The National Institute of Standards and Technology (NIST) offers calibration services’ for de
voltage, de resistance, and the ac-de difference of thermal converters [1-4]. With thesc three services,
standards laboratories can provide direct NIST traceability for the functions measured by most
DMMs. Based on customer demand, special tests’ for DMMs (all functions) were offered beginning
in 1990 [2-4]. This technical note describes the NIST Multifunction Calibration System (MCS) that
is used to provide special test services for DMMs, multifunction calibrators (the programmable
sources designed to calibrate DMMs), and special tests for low voltage thermal converters, and
micropotentiometers. The MCS is periodically calibrated using reference electrical standards
available at NIST. Results of these system calibrations and of frequently performed self-tests are
analyzed statistically to maintain quality control of the MCS.

'NIST calibration services are well documented and analyzed measurements offered at a fixed cost.
NIST special tests are generally undocumented measurements that are charged "at cost."

2


The DMM corrections for each test voltage at frequency f, is described by the following relationship:
Cay = (Raj ~ Rai Rai + (Cai - Cn: (neg) + D; + Cans (10)

where Rj; = (Ryq. + R,)/2 {ave. 1-kHz reading}
Cn: = (C1) + @i)/2 {ave. wpot emf with 1 kHz applied}
n= the ppot power coefficient at output voltage V
D, = the ac 1-kHz difference of the pot at output V at f;
C,, = the DMM 1-kHz correction.

The calibrator correction is given by:
Cyig = (Ry; 1 + Cy) - V/V (¢5))
where R,, = the DMM reading of the test voltage at f,.
3.4.3 Voltage from 250 mV to 7 V from 10 Hz to 30 MHz

Measurements are performed using the calibrator 50-Q wideband output port. Special 50 Q TVCs,
calibrated out to 30 MHz, are used to measure the amplitude flatness of the calibrator using a
technique similar to that described in 3.4.1, except the calibrator is left on the ACV function and each
test frequency is referred back to 1 kHz.

A 50 Q TVC is connected to the wideband output of the MCS calibrator and to a wideband DMM
ina similar manner as shown in Fig. 7. The nominal voltage V at frequency /, is programmed and the
TVC emf e, is measured using the dc DVM. The calibrator frequency is set to 1 kHz and its
amplitude is adjusted to V,, to bring the TVC emf e,, to within +t of e,. The calibrator frequency is
set to f, and the TVC emf e,, and the DMM reading R,, are recorded. The calibrator is
reprogrammed to 1 kHz and adjusted to V,, to bring the TVC emf e,, to within +f of e,,. The above
procedure is repeated for frequencies f, through f; .

The TVC is disconnected, frequencies f, through f, are set at the same nominal voltage V, and the
DMM readings R,, through R,, are recorded.

The calibrator correction for each test voltage at frequency f, is described by:
Coscvi = Vai - VYV + (Cai - €ai (ney) + D; + Copy (12)
where Vp, = (Viq1-V,,)/2 {ave. 1-kHz voltage}
Cai = (sai + Ci )/2 {ave. 1-kHz TVC emf}
n= the TVC power coefficient at V

D, = the ac-de difference of the TVC at V at f;
C., = the calibrator 1 kHz correction at V.

15


10Hz - 30MHz 0.1 Hz - 1 kHz 10 Hz - 100 kHz
200 V1 kV 4 - 200m 1nv-7V 2mA - 208
THERMAL MICRO- DIGITALLY THERMAL
VOLTAGE POTENTIOVETERS ‘SYNTHESIZED CURRENT
CONVERTERS ‘SOURCE, CONVERTERS
avo) TM) (08s) (recy
CONTROLLER MULTIFUNCTION DIGTAL AUDILLARY
CALIBRATOR CAB MULTIMETERS INSTRUVENTS
AD ‘AND (VM DETECTOR
SOFTMARE ANVPLIFIERS omy ‘SMTCHES, ETC)
omer | mare ee | eee | ome
ARRAY ‘STANDARD pirtlaae RESISTORS
0-10V 40V 1000 = 1000-1 0012-1002 64 ko

Fig. 2. Block diagram of MCS support equipment.

controlled through the GPIB, most tests can be completely automated, requiring an operator only to
change connections between the calibrator and DMM for certain functions. Test data are numerically
processed using commercial spreadsheet software. In addition to the software developed at NIST,
the MCS also makes use of commercial software to control the 4950 DMM.

Table 3
A list of the software routines available in the MCS
Title Description
DMM cal Used to calibrate any of the functions on a test DMM using
the MCS calibrator.
Calibrator cal Used to calibrate any of the functions on a test calibrator

using one of the MCS DMMs.

Thermal V/I cal

Used to calibrate the ac voltage and current functions of
DMMs and calibrators using thermal converters.

DC V/ cal

Used to calibrate the dc voltage and current functions of the
MCS calibrator and DMMs.



Table 8
MCS Type B Standard Uncertainties for ACT

| Current Range Calibrated Points Full Range
1mA to 10 A (50 + 20pA/A (150+ 3 pA/A

Where f= test frequency in kilohertz.
4, CALIBRATIONS AND TESTS USING THE MCS

Once the system calibrator and DMMs have been characterized, the MCS is used to test customer's
DMMs, calibrators, and related instruments.

Prior to testing, the test instrument is energized in the MCS lab overnight, to allow it to stabilize and
to satisfy any warmup requirements. Then, self-calibration or auto-zeroing routines are performed
to bring the instrument to its rated uncertainty.

4.1 Calibration of DMMs Using the MCS Calibrator

Unless otherwise requested, the DMM is calibrated at its input terminals. Using an adjustable
platform, the DMM terminals are brought "nose-to-nose" with the MCS calibrator terminals so that
only a few centimeters of connectors are required to interconnect them as shown in Fig. 12. The
calibrators have six output terminals including outputs (hi and lo), sense (hi and lo), guard and
ground. Test DMMs have a similar set of input terminals. Tables 9 - 11 list the interconnections made
for typical DMM calibrations.

Table 9
Connections and configuration for a DCV or ACV calibration
From To
Calibrator voltage output hi DMM input hi
Calibrator voltage output lo DMM input lo
Calibrator guard and ground

The calibrator is programmed to Jocal sense and local guard, and the DMM is programmed or
switched to local guard.

21


NIST Measurement Services:

NIST Multifunction
Calibration System

NIST
Special
Publication
250-46

Nile Oldham and Mark Parker

U.S. Department of Commerce
Technology Administration
National Institute of Standards and Technology


description of these uncertainty types and methods of combining them is given in Section 5 and in
reference [6].

The Type B standard uncertainties of the MCS calibrator for DCV are given in Table 4. These figures
include uncertainties associated with the DCV support equipment described in Table 2. The
uncertainties in Table 4 are for the set of points where the MCS is routinely calibrated and monitored

(Calibrated Points) and for all other points within the range (Full Range) where corrections are
interpolated estimates.

Table 4
MCS Type B Standard Uncertainties for DCV

ee
Voltage Range Calibrated Points Full Range
ImV to 100 mV SyuVv/V+1pV 10 pV/V +1 pV
100 mV to 1 V. 2pV/V +1 pV 4yuv/V +1 nV
1Vtol0V 1pV/V +1 pV 2pv/V +1 pV
10 to 100 V 2pV/V 4 v/v
100 V to 1 kV 3nV/V 6pVviV

3.2 DC Current (DCI)

The DCI function of the MCS calibrator and DMMs is periodically calibrated by applying various
calibrator currents to a set of NIST reference, four-terminal resistors and measuring the voltage across
the resistors using a system DMM. The set of reference resistors consists of values between 0.1 Q
and 1 MQ that are maintained in oil and air baths with a temperature control of £0.1°C, and are
periodically calibrated against the NIST resistance standards [7].

To measure the influence of compliance voltage, calibrator currents are measured with the following
burdens:

a. reference resistor only,
b. reference resistor and each system DMM,
c. each system DMM only.

The compliance voltage is measured for each burden and the current differences vs compliance
voltage are plotted. If there is a significant difference, additional burden measurements are made to
cover the expected range of customer's DMM burdens. Linear interpolation between data points is
generally adequate to correct the calibrator current.


Acontrol chart with selected calibrator corrections is shown in Fig. 4. Over long intervals, calibrator
corrections tend to drift linearly with time, so a least-squares fit to the correction time series is
performed and a linear equation is used to predict the calibrator correction between calibrations.

S
a

ab

Correction (ppm)

rm

&
a

1 4 8 12 16 20 24 30 34 38 42
Time (weeks)

¥ 0.1 V (measured) —* 0.1 V (regression)

© 10 V (measured) -® 10 V (regression)

Fig. 4. Control chart with measured and predicted
corrections for the MCS calibrator for DCV.

The system DMMs are calibrated at 10 V directly against the zener reference and at other voltages
using the characterized calibrator. The DMM corrections C,,, are related to the DMM reading Rand
the true voltage Vp, by:

Ve = R(I + Caney): Q)

The system calibrator and DMMs rely on an internal zener voltage reference and timing to generate
and measure voltage. The calibrator uses a scaling technique known as time-division-multiplexing,
while the DMMs employ multislope integrating analog-to-digital converters. Both techniques are
capable of excellent linearity. The linearity of one of the system DMMs has been shown to be better
than | part in 10’ of full scale between 1 V to 10 V using the Josephson array [5]. The linearity of
other voltage ranges is measured using a calibrated DMM and resistive divider.

For both the calibrator and DMMs, calibration at full scale and 10% of full scale in each range is
generally adequate to characterize the full range. The system software linearly interpolates between
calibration points to compute calibrator corrections over the full range (>10’ discrete voltages). These
software corrections are applied during the test of a customer's DMM. DMM corrections are
processed in a similar manner and applied during the test of a customer's calibrator.

The uncertainty of DCV measurements performed using the MCS depends on uncertainties computed
during the test (Type A) and uncertainties associated with the calibration of the MCS (Type B). A

7


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National Institute of Standards

and Technology

Gaithersburg, MD 20899-0001

Offi Business
Penalty for Private Use $300


SP 250-1

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The system calibrator and DMMs are periodically characterized using the support equipment and
NIST reference standards shown in Table 2 and in Fig. 2.

Table 2
Support Equipment
Function Support Equipment and NIST Reference Standards

DC voltage NIST 10 V Josephson array

(DCV) Wavetek 4911 10 V zener reference

Fluke 752 resistive voltage divider (0.1:1 to 100:1)

DC current NIST standard resistors (0.1 Q to 100 MQ)

(DCD), System DMM (DCV)

DC resistance NIST standard resistors

(DCR) System DMM (RES)
AC voltage System calibrator (DCV)

(ACV) NIST thermal voltage converters (0.1 V to 1 kV, 10 Hz to 100 MHz)

Ballantine 440 and Holt 12 micropotentiometers
(1 mV to 200 mV, 10 Hz to 30 MHz)
Wavetek 4920 ac digital voltmeter (1 mV to 200 mV, 1 Hz to 30 MHz)
NIST DSS-S5 Digitally Synthesized Source (1 mV to 7 V, de to 1 kHz)
Keithley 181 and 182 de nanovoltmeters
Hewlett Packard 34420A nanovoltmeters

AC current System calibrator (DCI)
(ACT) NIST thermal current converters (2 mA to 20 A, 10 Hz to 100 kHz)
NIST DSS-5

NIST Transconductance Amplifier
NIST ac resistors (0.1 Q to 100 kQ)

Instruments are controlled using a PC (486 or higher) and a National Instruments GPIB-PCIIA
general purpose interface bus (GPIB) interface card.

2.2 MCS Software

The system software, developed in National Instruments LabVIEWTM, allows the operator to perform
all of the tests needed to characterize the system calibrator and DMMs, store and edit correction files,
and set up and perform tests of the instrument under test (UT). Since the instrumentation is


3.5.1 Currents from 2 mA to 10 A from 10 Hz to 100 kHz

The reference TCC is connected to the system calibrator and DMM (both set to the ACI function) as
shown in Fig. 10. The calibrator is programmed for the nominal current J at frequency f, and the TCC
oulpul emf e, is measured by the dc DVM. The calibrator function is switched to +DCI and
programmed to +Z The calibrator current is adjusted to /,,, to bring the TCC emf e,,, to within
+tyV/V ofe,. The calibrator is switched back to ACI, the TCC emf e,, and the DMM ACI reading
R,, are recorded. The calibrator is then switched to -DCI and the negative polarity dc current is
adjusted

to 1, to bring the TCC emf to within +r 1V/V of e,, . The above procedure is repeated for
frequencies f, through f.

MULTIFUNCTION
CALIBRATOR
DMM bee bite
SHUNT

Teo ‘pe pw fzzzz

CONTROLLER

Fig. 10. Setup for characterizing ACI function of MCS
calibrator using a TCC.

The TCC is removed, the calibrator is switched to ACI, frequencies f, through f; are set at the same
nominal current J, and the DMM readings R,, through R,, are recorded. The calibrator correction at
each test current at frequency J is described by:

Cesc = Ua WIE + (Cai - Cai (ney) + D; + Concer (16)

where I; = (1,4;-1,;)/2 {ave. de current}
Cu = (Cra: + @g;)/2 {ave. TCC emf with +DCI applied}
n= the TCC power coefficient at the nominal voltage
D, = the ac-de difference of the TCC at the nominal voltage at f;
C.ncr = the average calibrator DCI correction.

18


which changes the appropriate correction value until the DMM displays the proper value to within
a selectable tolerance.

4.3 High Accuracy Special Test of DMMs and Calibrators

The highest accuracy test of a DMM or calibrator involves direct comparisons against the standards
that are used to characterize the MCS DMMs and calibrator using the techniques described in 3.4.
These are inherently more time consuming and expensive than the automated tests described in 4.2.

4.4 Special Test of Low Voltage Thermal Voltage Converters

Low voltage thermal voltage converters are tested using the MCS multirange thermal voltage
converter. Tests are performed by placing the two instruments nose-to-nose, and then applying ac
and dc voltages, while monitoring the output of each device. The ac-dc difference corrections for the
TVC under test are calculated by using the techniques described in 3.4.

4.5 Special Test of Micropotentiometers

Micropotentiometers are tested using the MCS multirange thermal voltage converter. Tests are
performed by connecting the j:pot input to the calibrator output and the pot output to the thermal
converter input and then applying ac and dc voltages, while monitoring the output of each device.
The ac-de difference corrections for the spot under test are calculated using the techniques described
in 3.4.

5. QUALITY CONTROL AND UNCERTAINTIES

The MCS relies on standards that are calibrated in other laboratories in the NIST Electricity Division
and it is important to maintain traceability back to these standards. Each of the support artifacts or
instrument standards is calibrated on a regular interval (normally once a year) and a control chart
describing its value and uncertainty at any time is maintained. The MCS calibrator and DMMs are
normally characterized four times a year using the support standards. The calibrator and DMMs are
also intercompared several times per month to track long-term stability. The standard deviation of
this intercomparison represents a composite stability figure for the calibrator and two DMM types.

The standard uncertainty of each function in the MCS calibrators and DMMs is based on the standard
uncertainties of the associated support instrument, the repeatability of the transfer process, and the
long-term stability of the calibrator. The figures given in the tables in section 3 represent onc-
standard deviation (1-sigma) estimates of the system standard uncertainty. Note: These are
composite figures that serve for both the calibrator and the DMMs. Over a large parameter space
the figures apply equally to both; however, at high frequencies, the stability of the calibrator may be
significantly better than the DMMs. Under these conditions, a different set of uncertainty figures is
used. The total uncertainty of a test performed using the MCS includes the short-term stability of the
instrument under test. For example, the uncertainty of a test DMM calibrated by the MCS calibrator
on the ACV function depends on the uncertainties of the following parameters:

24


National Institute of Standards and Technology Special Publication 250-46
Natl. Inst. Stand. Technol. Spec. Publ. 250-46, 28 pages (Feb. 1998)
CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 1998

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325


DCR support equipment described in table 2) are given in Table 6.

Correction (ppm)
°

7 4 8 12 16 20 24 28 32 36 40
Time (weeks)

v_ 10 ohms (measured) —* 10 ohms (regression)
© 100 k-ohms (measured) -#- 100 k-ohms (regression)

Fig. 6. Control chart with measured and predicted
corrections for the MCS calibrator for DCR.

Table 6
MCS Type B Standard Uncertainties for DCR
Resistance Range Calibrated Points Full Range

1Q 20 pQ/Q 60 pQ/Q
10Q 10 pQ/Q. 30 pQ/Q
100 Q 6 pQ/Q 18 pQ/Q
1kQ 3 pad 9 pQ/Q
10 kQ 3 pQ/Q 9 pQ/Q
100 kQ 4 naa 12 pQ/Q
1MQ 5 nQ/O 15 pQ/Q
10 MQ 20 pQ/Q 60 pQ/Q
100 MQ 30 pQvQ 90 pQ/Q

11


The calibrator correction for each test voltage at frequency f; is described by:

Coron = Var - V/ Var + (Cai - Cai (ney) + D; + Covers 12)

where V,,
ui

=(Vi4- Vg )/2 {ave. de voltage}

€.4 + C4;)/2 {ave. TVC emf with +DCV applied}

n= the TVC power coefficient at the nominal voltage
D,= the ac-de difference of the TVC at the nominal voltage at f;
C.pcy = the average calibrator DCV correction.

The DMM correction for each test voltage at frequency f; is given by:

Cusen = (V1 + Coser) ~ Rai Rae (8)

where R,; = the DMM reading of the test voltage at f.

Notes: i.

ti.

tii.

iv.

The above procedure is designed to allow the MCS to be calibrated using the
reference TVCs as well as the multirange TVC. By driving the de voltage to
match the TVC output when ac voltage is applied, the importance of an
accurate knowledge of the TVC power coefficient n is minimized by
minimizing the numerator in the second term of (7). For the reference TVCs,
this coefficient varies between 1.6 and 2 as a function of the test voltage and
must be measured at each voltage. The multirange TVC has a nominal power
coefficient of 1, which is relatively insensitive to the test voltage, so the
tolerance t may be increased to reduce test time.

The reference TVC time constants are typically 1 s to 2 s; therefore, a wait
period of up to 30 s is required before recording the TVC emf. To minimize
the effects of drift, it is important that all of the wait periods are the same for
a particular set of measurements. The multirange TVC settles in about 10 s
and has a low drift rate so timing is not as critical.

It is important that the dead time in switching from the ACV to the DCV
functions is minimized. For the highest accuracy using reference TVCs, a
switch and auxiliary supply are used to keep the TVC energized while the
calibrator is switching between functions. The multirange TVC is less
sensitive to switching time.

The dc DVM should be able to resolve the TVC emf to within +1 uV/V. This
resolution corresponds to several nV for the reference TVCs (which have
output emfs from 2 mV to 10 mV) and several uV for the multirange TVC
(which has a full scale output of about 2 V).

Depending on the random error of the measurements, an actual calibration
consists of from 1-10 repeats of the described procedure.

As the test frequency increases the calibrator becomes more sensitive to the load imposed by the TVC
and the test DMM. Therefore, to obtain the lowest uncertainties, the calibrator must be characterized
for each DMM type using the procedure described above: The second set of DMM readings R,, is

13


NIST Special Publication 250-46

NIST MEASUREMENT SERVICES:
NIST Multifunction Calibration System

Nile Oldham and Mark Parker

Electronic Instrumentation and Metrology Group
Electricity Division

Electronics and Electrical Engineering Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899-0001

February 1998

gg OF Co,

”y

* 0
yn Oey,
“te, 3
len 4 3

States of ©

U.S. Department of Commerce
William M. Daley, Secretary

Technology Administration
Gary R. Bachula, Acting Under Secretary for Technology

National Institute of Standards and Technology
Raymond G. Kammer, Director


Table 10
Connections and configuration for a DCI or ACI calibration

From To
Calibrator current output hi DMM current input hi
Calibrator current output lo DMM current input lo
Calibrator guard and ground

The calibrator is programmed to Jocal guard, and the DMM is programmed or switched to local
guard.

Table 11
Conncctions for a DCR (4-wirc) calibration

From To
Calibrator resistance output hi DMM resistance input hi
Calibrator resistance output lo DMM resistance input lo
Calibrator resistance sense hi DMM resistance sense hi
Calibrator resistance sense hi DMM resistance sense hi
Calibrator guard DMM guard
Calibrator ground

The calibrator is programmed to remote sense and local guard, and the DMM is programmed or
switched to local guard.

Calibrations are performed by programming the MCS calibrator to the desired function and setting,
triggering the DMM (the first reading is ignored), and recording the next N readings.

The average reading of the DMM under test R, its associated correction Cypy,,, the system calibrator
setting S, and its correction C;,4, are related by the following equation:

ROU+ Crom) = SC + Cscar)- (21)

22


i he National Institute of Standards and Technology was established in 1988 by Congress to “assist industry in

the development of technology .

needed to improve product quality, to modernize manufacturing processes, to

ensure product reliability . . . and to facilitate rapid commercialization . . . of products based on new scientific

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‘At Boulder, CO 80303.
?Some elements at Boulder, CO.


1. The ac-de difference of the multirange TVC, U;,¢ (see 3.4)
The correction of the de voltage function of the calibrator, Up, (see 3.1).

3 The standard deviation of the process in which the calibrator ac voltage is compared to its dc
voltage, Us, (see 3.4).

4. The long-term stability of the calibrator/DMM at the test point, U,7; (the standard deviation
of periodic comparisons with a system DMM).

5. The short-term stability of the test, U;,; (the standard deviation of the measurement including
the MCS calibrator and the instrument under test).

These values (all 1-sigma estimates) are combined according to the procedures recommended in [6]
to give the expanded total uncertainty, U,

Uy = 2p" + Ups) + srs’ + Ung? + Urye')? (23)

where the first three terms are considered to be type A standard uncertainties and the last two are type
B standard uncertainties.

7. ACKNOWLEDGMENTS

The authors wish to acknowledge those people who helped in the development of the MCS: Barry
Bell, the group leader of the Electronic Instrumentation and Metrology Group, for his strong support;
Ron Dzuiba, and Dean Jarrett for sharing their expertise in resistance measurements; Dick Steiner and
June Sims for assisting in de voltage support: Joe Kinard and Tom Lipe for providing ac-dc transfer
calibrations and overall guidance in ac-de voltage measurement techniques; Robert Palm for
prototyping and drafting support; and Kathy Magruder for running tests, and performing much of the
data reduction necessary for the MCS.

25


The dc current corrections to the calibrator C,,,, are related to the calibrator setting S and the actual
calibrator output current J.p, by:

Tener = SL + Coer)- G)

The system DMMs are also calibrated at various currents by this procedure. The DMM corrections
Cipc; ate related to the DMM reading R and the true current J,,by:

Toc = ROL + Caner). (4)

A control chart with selected DCI calibrator corrections is shown in Fig. 5. DCI corrections tend to
drift linearly with time, so a least squares fit to the correction time series is performed and a linear
equation is used to predict the calibrator correction between calibrations. The system software also
linearly interpolates between calibration points to compute corrections over the full range. As with
DCV, software corrections are applied during DMM and calibrator tests.

1 4 8 12 16 20 24 28 32 36 40
Time (weeks)

¥ 10mA (measured) —*~ 10 mA (regression)

© 1A(measured)  -* 1 A (regression)

Fig. 5. Control chart with measured and predicted
corrections for the MCS calibrator for DCI.

Measurements are performed at cardinal currents between 1 pA and 10 A. Measurements at other
currents indicate that, for the calibrator and DMMs, the current linearity is generally as good or better
than the current stability. However, for measurement at other than the calibrated points, the MCS
corrections are interpolated estimates and uncertainties are somewhat larger. As with DCV, the
uncertainty of DCI measurements performed using the MCS depend on Type A and Type B standard
uncertainties. The Type B standard uncertainties of the MCS calibrator for DCI (which include the
uncertainties of the DCI support equipment described in table 2) are given in Table 5.

9


3.4 AC Voltage (ACV)

The low-frequency ACV function of the MCS calibrator and DMMs is periodically calibrated using
amultirange thermal voltage converter (TVC) that has been calibrated against a set of NIST reference
TVCs, micropotentiometers, and a calculable DSS [8] . The ac-de differences of these support
instruments are ultimately referenced to a set of characterized TVCs that are the NIST standards for
ac-de difference [9]. The test procedures for characterizing the system calibrator and DMMs for ACV
are more complex than the other functions and will be described in detail.

3.4.1 Voltages from 1 mV to 1 kV from 10 Hz to 1 MHz

The multirange TVC is connected to the MCS calibrator and to the DMM (both set to the ACV mode)
as shown in Fig. 7. The nominal ac voltage V at frequency /, is programmed and the output emf of
the TVC, e, is measured using the dc digital voltmeter (DVM). The calibrator function is switched
to +DCV and the amplitude is programmed to +V. The calibrator voltage is adjusted to V,,, to bring
the TVC emf e,,, to within +¢ of e, . [The operator selectable tolerance ¢ is typically between 10
pV/V and 50 1. V/V.] The calibrator is switched back to ACV and the TVC emf e,, and the DMM
ACV reading R,, are recorded. The calibrator is then switched to -DCV and adjusted to V’,, to bring
the TVC emf e_,, to within +1 of e,,. The above procedure is repeated for frequencies f, through f,

The TVC is disconnected, the calibrator is switched to ACV, frequencies /, through /; are set at the
same nominal voltage V, and the DMM readings R,, through R,, are recorded.

GPIB,

MULTIFUNCTION
CALIBRATOR
He .
DMM work a ae
Tvc
ew pzzzv

CONTROLLER

Fig. 7. Setup for characterizing the ACV function of the
MCS using a multirange TVC.

12