Voltage reference

A voltage reference is an electronic device that ideally produces a fixed (constant) voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power supplies, analog-to-digital converters, digital-to-analog converters, and other measurement and control systems. Voltage references vary widely in performance; a regulator for a computer power supply may only hold its value to within a few per cent of the nominal value, whereas laboratory voltage standards have precisions and stability measured in parts per million.

The earliest voltage references or standards were wet-chemical cells such as the Clark cell and Weston cell, which are still used in some laboratory and calibration applications.

Laboratory-grade Zener diode secondary solid-state voltage standards used in metrology can be constructed with a drift of about 1 part per million per year.

The value of the " conventional" volt is now maintained by superconductive integrated circuits using the Josephson Effect to get a voltage to an accuracy of 1 parts per billion or better, the Josephson voltage standard. The paper titled, "Possible new effects in superconductive tunnelling", was published by Brian David Josephson in 1962 and earned Josephson the Nobel Prize in Physics in 1973.

Formerly, mercury batteries were much used as convenient voltage references especially in portable instruments such as photographic light meters; mercury batteries had a very stable discharge voltage over their useful life.

Any semiconductor diode has an exponential voltage /current characteristic that gives an effective "knee" voltage sometimes used as a voltage reference. This voltage ranges from 0.3 V for germanium diodes up to about 3 volts for certain light emitting diodes. These devices have a strong temperature dependence, which may make them useful for temperature measurement or for compensating bias in analog circuits.

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