The Ultra-Zener . . . is it a portable replacement for the Weston cell?

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Abstract
Twenty-four samples of the so-called Ultra-Zener, an integrated circuit containing a buried Zener junction, type no LTZ 1000, have been tested since March 1988. To measure the promised low rate of change of voltage with time (aging), a voltage measuring system with a resolution of 0.1 mu V at the 7 V level has been developed and is described. Preliminary results are that half of the samples have an aging rate of less than1.5 PPM/year from about two months after being assembled onto their circuit boards. Both intermittent operation and setting a lower chip temperature have been found to reduce the aging rate. The standard deviation of a set of 24 readings of the Zener voltage over half a minute is typically 0.05 PPM (0.3 mu V on a 7 V level). For a 'serious' voltage standard, a 1 PPM/year aging rate has been stated to be a desirable maximum level. This is attainable with the Ultra-Zener. In addition, its low noise, excellent portability and good electrical robustness make it a replacement for a standard cell bank as a voltage transfer standard.

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