Voltage reference
Encyclopedia
A voltage reference is an electronic device that 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 supply
voltage regulator
s, analog-to-digital converter
s and digital-to-analog converter
s, and myriad 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.
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 volt
is now defined by the Josephson Effect
to get a voltage to an accuracy of 1 parts per billion. 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 meter
s; mercury batteries had a very stable discharge voltage over their useful life.
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.
Zener diode
s are also frequently used to provide a reference voltage of moderate stability and accuracy, useful for many electronic devices. An avalanche diode
displays a similar stable voltage over a range of current.
The most common 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 added 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.
s and neon lamp
s 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 "Voltage regulator tube
" is an NE-16 neon lamp
which fires at 87 Volts and then holds 48–67 Volts across the discharge path.
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...
s, analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
s and digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
s, and myriad 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.
In metrology
The earliest voltage references or standards were wet-chemical cells such as the Clark cellClark cell
The Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell that produces a highly stable voltage usable as a laboratory standard.-Chemistry:...
and Weston cell
Weston cell
The Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters...
, which are still used in some laboratory and calibration applications.
Laboratory-grade Zener diode
Zener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
secondary solid-state voltage standards used in metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
can be constructed with a drift of about 1 part per million per year.
The value of the volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
is now defined by the Josephson Effect
Josephson effect
The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
to get a voltage to an accuracy of 1 parts per billion. The paper titled, "Possible new effects in superconductive tunnelling", was published by Brian David Josephson
Brian David Josephson
Brian David Josephson, FRS is a Welsh physicist. He became a Nobel Prize laureate in 1973 for the prediction of the eponymous Josephson effect....
in 1962 and earned Josephson the Nobel Prize in Physics in 1973.
Formerly, mercury batteries
Mercury battery
A mercury battery is a non-rechargeable electrochemical battery, a primary cell. Due to the content of mercury, and the resulting environmental concerns, the sale of mercury batteries is banned in many countries. Both ANSI and IEC have withdrawn standards for mercury batteries...
were much used as convenient voltage references especially in portable instruments such as photographic light meter
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
s; mercury batteries had a very stable discharge voltage over their useful life.
Solid state devices
Any semiconductor diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
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
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
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
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
in analog circuits.
Zener diode
Zener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
s are also frequently used to provide a reference voltage of moderate stability and accuracy, useful for many electronic devices. An avalanche diode
Avalanche diode
In electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...
displays a similar stable voltage over a range of current.
The most common voltage reference circuit used in integrated circuits is the bandgap voltage reference
Bandgap voltage reference
A bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits, usually with an output voltage around 1.25 V, close to the theoretical 1.22 eV bandgap of silicon at 0 K. This circuit concept was first published by David Hilbiber in 1964...
. 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 added 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
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, 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.
Gas filled devices
Gas filled tubeGas filled tube
A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal...
s and neon lamp
Neon lamp
A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...
s 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 "Voltage regulator tube
Voltage regulator tube
A voltage-regulator tube is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.Physically, these devices resemble vacuum tubes, but there are two main differences:...
" is an NE-16 neon lamp
Neon lamp
A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...
which fires at 87 Volts and then holds 48–67 Volts across the discharge path.
External links
- Understanding Voltage-Reference Topologies and Specifications
- The Design of Band-Gap Reference Circuits: Trials and Tribulations — Robert Pease, National Semiconductor
- LT Journal March 2009: How To Choose A Voltage Reference Brendan Whelan, Linear Technology Corporation