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Keysight Technologies
Fully-Automatic DMM Calibration System
White Paper
Abstract
This paper describes a fully-automatic calibration system for digital
multimeters (DMMs), and uncertainty estimation of the DCV
measurements of the system. A connection mechanism whose rating
is more than 1000 V/1 A/1 MHz was developed for the system.
By using compressed air to drive the mechanism, we were able to
eliminate operator bias, as well as improve the thermal electromotive
force and measurement repeatability of the system. For the standard
in the system, an 8 1/2 digital multimeter was used, which was
calibrated by the standards laboratory of Keysight Technologies Japan
Ltd. The measurement uncertainty was estimated by referring to the
ISO Guide to the Expression of Uncertainty in Measurement[1] (GUM).
Introduction
Issue for Designing Fully-Automatic DMM Calibration System
There are some points to be considered in order to design a digital multimeter
(DMM) calibration system, such as traceability to an upper level standards
laboratory, selection of a stable standard, and evaluation of the measurement
repeatability. Furthermore, for a fully -automatic DMM calibration system, it
is necessary to consider signal switching mechanisms, system noise, and
countermeasures for abnormal conditions.
Generally, a relay switch is used to route test signals between the unit under
test (UUT) and signal sources. However, a relay switch isn't suitable for
precision measurement of low voltage because thermal electromotive force
(thermal-emf) is caused by self-heating due to the driving current of the relay.
Therefore, a commercially available low thermal-emf scanner is widely used
for a precision measurement system for a standard cell or a Zener voltage
standard. On the other hand, as maximum DC voltage and DC current of the
DMM calibration are 1000 V and 1 A, the scanner whose maximum rate is 24
V 0.5 A is unlikely to be used in the fully-automatic DMM calibration system.
Besides, a fully-automatic calibration system is physically larger than a
manual calibration system. Because the cables between the equipment
become long and are easily affected by environment noise, the problem of
measurement repeatability can occur.
Furthermore, as the fully-automatic calibration system can perform calibration
without operators, it is important to take countermeasures for unexpected
damage to the equipment or the software into consideration.
Authors: Yuko Hirota, Tos hiaki Aoki and Masao Noguchi
Keysight Technologies Japan, Ltd.
2000 NCSL Workshop & Symposium
Design for Fully-Automatic DMM Calibration System
We developed the fully-automatic system in order to improve upon operation
quality and throughput of 81/2-digit DMM calibration, considering the above-
mentioned problems. The calibration parameters are DCV, DCI, DCR, ACV, ACI
and frequency at 1 MHz. For example, calibration ranges are from 100 mV to
1 kV for DCV and from 100