Algorithmic Trimming on Active Circuitry

The active circuitry for a digital autoranging multimeter has been fabricated on a single hybrie substrate measuring 2.8 X 3.8 cm. The hybrid is realized using tantalum nitride thin film technology. A high temperature passivation process stabilizes the tantalum nitride film. During this process, the absolute value of resistance changes substantially. This change makes the technology unusable in some precision applications. In order to overcome this problem and take advantage of the film\´s inherently excellent stability, laser trimming is utilized to adjust the absolute value. The result is a stable precision resistor. When the end result of the laser trimming process is a resistor of predetermined value, the operation can be performed by "passive trimming." In many cases, it is desirable to trim the resistor to an unknown value in order to obtain a given circuit response. When this is done to an operating circuit, the procedure is known as "active trimming." In general, a relationship can be determined between a known stimulus and its desired circuit response. The trim procedure can be designed to cause this desired relat onship. Specific algorithms and techniques have been developed to test the sensitivity of a circuit parameter to a given change in trim resistance. The computer controlled system uses this information to determine the succeeding trim magnitude. The use of a computer controller allows the system to handle complicated nonlinear stimulus-response data. Once a hybrid is trimmed, it is effectively a finished product. Since it has been calibrated during the trimming process, there is no need for manual adjustment. One difficulty associated with active laser trimming is duplicating the external circuit while at the same instant connecting the hybrid substrate to noisy probe rings, loading it with capacitances, and subjecting it to-the hostile environment of electromechanical controls. This paper will deal primarily with algorithms developed for active trim and the problems of electrically interfacing the system with the hybrid under test. Most of this experience was achieved during the development of the Hewlett-Packard Model 970A Probe Multimeter.