Computer-aided DMM calibration software with enhanced AC precision

In recent years small computers have come to an important role in the calibration of test and measurement instrumentation. They have been very effective in reducing the cost and increasing the quality and repeatability of these calibrations. This paper describes a system for calibrating digital multimeters in which the software is menu-based to give the user flexibility and ease of use. The user needs no programming experience to produce test procedures for the unit under test; these units being either manual or programmable. The capabilities of the automated calibration system are limited only by the stimulus sources and the precision measuring equipment which exercises the unit under test. To accommodate for the limited accuracy of ac voltage sources, the system uses a programmable thermal transfer standard to enhance the accuracy of the ac source. This technique removes the burden of accuracy from the AC source and places it on the thermal transfer standard. This enables the calibration of high-precision ac voltmeters. The IEEE-488 interface bus is used in closed-loop mode to control all stimulus equipment and the unit under test. Special emphasis is placed on the user friendliness of this computer-aided calibration system and its cost effective application to calibration verification and quality assurance.