NIST results lead to improvement in Zener voltage reference. (News Briefs).

Measurements made at NIST for the characterization of Zener voltage standards have enabled the manufacturer to redesign their instruments for improved behavior. As part of NIST's efforts to develop a 10 V measurement assurance program (MAP), NIST undertook an extensive study of the characteristics of eight Zener voltage standards under a wide range of differing environmental conditions. One striking feature of the measurements was that all units showed a strong correlation between the Zener voltage output and relative humidity. The time constant for response to humidity change was relatively short, on the order of about 2 to 3 weeks. This behavior would have seriously limited the use of these instruments as transport standards for the MAP.

The NIST measurement results were shared with the manufacturer in hope that the detailed characterization might point to the source of sensitivity to environmental humidity. A preliminary analysis identified certain epoxy-encapsulated resistors as likely candidates for the problem. When relative humidity changed, the plastic molding would either absorb or desorb water. The resulting swelling or shrinking would change stress patterns across the resistor array chip. Numerical simulation predicted resulting voltage changes in reasonable agreement with the NIST measurements. A retrofit to the product line was designed and implemented on four of the NIST units as a test. Subsequent measurements at NIST confirmed a large reduction in the sensitivity to humidity changes, greatly improving the potential for use of these devices as transfer standards for the NIST l0V MAP. The combination of critical measurement data from NIST and the manufacturer's timely response resulted a significant improvement in the performance of these high precision voltage standards.

CONTACT: Michael Kelley, [phone]; michael. [e-mail].