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Voltage reference

A voltage reference is an electronic device (circuit or component) that produces a fixed (constant) voltage irrespective of the loading on the device, power supply variation and temperature. It is also known as a voltage source, but in the strict sense of the term, a voltage reference often sits at the heart of a voltage source.

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The distinction between a voltage reference and a voltage source is, however, rather blurred especially as electronic devices continue to improve in terms of tolerance and stability.

Voltage references are used in ADCs and DACs to specify the input or output voltage ranges.

The commonest voltage reference circuit used in integrated circuits is the bandgap voltage reference. A bandgap-based reference (commonly just called a 'bandgap') uses analog circuits to add a multiple of the voltage difference between two bipolar junctions biased at different current densities to the voltage developed across a diode. The diode voltage has a negative temperature coefficient (i.e. it decreases with increasing temperature), and the junction voltage difference has a positive temperature coefficient. When edded in the proportion required to make these coefficients cancel out, the resultant constant value is a voltage equal to the bandgap voltage of the semiconductor. In Silicon, this is approximately 1.25V. Buried zener references can provide even lower noise levels, but require higher operating voltages which are not available in many battery-operated devices.

The earliest voltage references or standards were wet-chemical cells (like batteries), such as the Clark cell and Weston cell, used to this day in some laboratory and calibration applications.

Gas filled tubes and neon lamps have also been used as voltage references, primarily in tube-based equipment, as the voltage needed to sustain the gas discharge is comparatively constant. For example, the popular RCA 991 ^[1] "Voltage regulator tube" is actually a NE-16 neon lamp which fires at 87 volts and then holds 48-67 volts across the discharge path.

Zener diodes are also frequently used to provide a reference voltage.