Voltage reference

A reference voltage source is a voltage source whose voltage is used as a reference for a measurement or control process. In general, reference voltages have a fixed voltage by design. Depending on the inner structure of the value of the reference voltage source may also be adjustable, for example to switch to a different measurement range.

Reference voltage sources are not provided in the normal case for being able to drive high loads. The output resistance and the thermal stress due to the output current can reduce the accuracy.

Simple reference voltage source

A simple reference voltage source can be produced with a resistance and a Zener diode. Here, the special characteristic of the Zener diode to become conductive at a given voltage, be utilized. This voltage can then be tapped at the Z- diode.

This voltage is constant with an increase in input voltage to about. It changes at substantially the voltage drop across the resistor.

For the voltage stabilization can specify a smoothing factor:

To the differential resistance thereof

The stabilization is better, the higher the ratio. The accuracy depends among other things on the Zener diode characteristic which is relatively flat with small reverse voltages. Furthermore a temperature dependency is present, which has a negative temperature coefficient, and In, according to the avalanche effect, has a positive temperature coefficient is less than about 5 volts. Reference voltage sources with Z- diode and minimum temperature influence can thus be implemented in the field by 5 V at best. A reduction of the temperature dependence Z at higher voltages can be obtained by replacing the zener diode with a reference diode.

A further improvement is through the use of shunt regulators (English shunt regulator ) reaches the place of the zener diode: This is not a diode in the strict sense, but an electronic circuit having at least two terminals, which are similar like a Zener diode in the stop band behavior: It holds over a relatively wide range of current, the voltage at its two connection terminals constant, but with a considerably higher accuracy and lower temperature dependency than that in the case of zener diodes. These circuits are available in designs with temperature-stable up to the area tolerances of less than 0.01 %. Some shunt regulators have an additional third connector on the setting of the reference voltage. One example is the integrated circuit LM431.

Precision reference voltage sources

For higher reference voltage sources, such as the band gap reference is used which contain other semiconductor -based components, such as bipolar transistors. The operation is based on the band gap which is material dependent. By cleverly combining contrasting temperature coefficient in a semiconductor circuit produces a temperature- stable reference which do partly with only 1 V supply voltage. Improved long term stability and especially provide reduced noise integrated circuits with buried zener reference, which are also temperature compensated and calibrated. A disadvantage is the high supply voltage of 8 V ( almost 7 V across the zener diode), the higher power dissipation and higher price. A more recent development is the XFET with excellent stability, an equally small and mainly linear temperature response, low power consumption, reduced price, can be used from 4.1 V. In this type two JFETs with different pinch-off voltage are similar to the bandgap reference interconnected.

Precision reference voltages are used for example in high-resolution analog -to-digital converters ( ADC). Many manufacturers of these devices incorporate a reference voltage to the chip, so that it can not be affected by the EMC influences.

Survey

Reference voltage sources are divided into different types depending on the requirements.

  • At low accuracy requirements, a resistor in series with the zener diode and or the temperature-compensated and stabilized form with reference diodes.
  • The now widespread in integrated circuits on the furthest reference voltage source is the bandgap reference, which can be easily implemented in temperature- stable profiled shape.
  • Wherein the integrated circuit also encountered analog floating gate, is used to trim a floating gate transistor.
  • At high demands on precision, as in the definition of the unit volt, quantum mechanical effects are used in superconductors, such as the Josephson effect as a reference.

Historical reference voltage sources are:

  • The Glimmstabilisator, a special form of a glow lamp.
  • Standard electrochemical elements such as the normal Weston element which has been used previously to determine the unit volt, or the Clark standard element.
  • Magnetic voltage stabilizer

Itemization ( s)

  • Voltage
Clark cell
608273
de