Evaluating a State-of-the-art Digital Multimeter

About this Article
This article by Bill Bruce was presented at the Measurement Science Conference in 1989, however it did not appear in the published Proceedings.

Abstract

The introduction of a state of the art digital multimeter presents a real challenge to the standards lab to provide external verification of its performance. The advent of internal standards and automatic internal calibration of all the DMM’s functions from those standards pits the calibration expert, his methods and equipment against an effective machine capable of extreme levels of accuracy. Conventional metrology methods if used very carefully are capable of verifying the DMM’s performance but there is very little room for error. The mindset of the metrologist must change because sources of error which may have been negligible in the past are now significant. The main purpose of this paper is to describe some methods which have been found adequate to verify the performance of a very accurate automatically calibrated DMM, the HP 3458A. The verification processes have been automated as much as is practical. With well characterized processes in place it is possible to develop history on the DMM’s performance. Data have been obtained on a number of HP 3458A’s which indicate that the autocalibration processes work extremely well and that the instrument definitely has the capability to perform standards lab measurements at very high levels of accuracy. Also included is a description of use of a 10 volt Josephson junction array to measure linearity of the A/D converter of the 3458A, and some results of these measurements. Verification of A/D converter linearity is imperative, since the A/D in the 3458A is used to provide all the ratios needed for the internal autocalibration processes.

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3458A Evaluation