Reference voltage source

A reference voltage source is a voltage source whose voltage is used as a reference for a measurement or control process. As a rule, reference voltages have a fixed voltage due to the design. Depending on the internal structure, the value of the reference voltage source can also be adjustable, e.g. B. to switch to another measuring range.

Reference voltage sources are normally not intended to be able to drive high loads. The output resistance and the thermal load due to the output current can reduce the accuracy.

Simple reference voltage source

Voltage divider circuit with Zener diode

A simple reference voltage source can be generated with a resistor and a Zener diode . The special property of the Zener diode of becoming conductive at a certain voltage is used here. This voltage can be tapped at the Zener diode.

This voltage remains roughly constant when the input voltage is increased . Essentially the voltage drop across the resistor changes . ${\displaystyle U}$ ${\displaystyle R}$

A smoothing factor can be specified for the stress stabilization :

${\displaystyle G={{\Delta U} \over {\Delta U_{z}}}=1+{{\Delta I_{z}} \over {\Delta U_{z}}}\cdot R}$

with the differential resistance it becomes ${\displaystyle r_{z}={{\Delta U_{z}} \over {\Delta I_{z}}}}$

${\displaystyle G=1+{R \over r_{z}}}$

The greater the ratio , the better the stabilization . The accuracy depends, among other things, on the Zener diode characteristic, which is relatively flat with low reverse voltages. Furthermore, there is a temperature dependency which has a negative temperature coefficient below around 5 V and above that, due to the avalanche effect, a positive temperature coefficient. Reference voltage sources with Z-diodes and minimal temperature influence can best be realized in the range of 5 V. A reduction in the temperature dependency at higher Zener voltages can be achieved by replacing the Zener diode with a reference diode . ${\displaystyle R \over r_{z}}$

A further improvement is achieved by the use of shunt regulators ( English shunt regulator reached diode Z) instead of: It is not a diode in the strict sense, but rather an electronic circuit with at least two terminals, which is similar to a Zener diode behaves in the blocking range: It keeps the voltage at its two connection terminals constant over a relatively wide range of the current, but with a much higher accuracy and less temperature dependence than is the case with Zener diodes. These circuits are available in temperature-stable versions with tolerances down to the range below 0.01%. Some shunt regulators have an additional, third connection for setting the reference voltage. One example is the LM431 integrated circuit .

Precision reference voltage sources

Band Gap Reference

For higher requirements, reference voltage sources such as the band gap reference are used, which contain further semiconductor-based components such as bipolar transistors . The functionality is based on the band gap which is material dependent. By cleverly combining opposing temperature coefficients in a semiconductor circuit, a temperature-stable reference is created, some of which can get by with just 1 V supply voltage. Integrated circuits with buried Zener reference that are also temperature-compensated and calibrated provide better long-term stability and, above all, lower noise . The disadvantage is the high supply voltage of 8 V (almost 7 V at the Zener diode), the higher power loss and the higher price. A more recent development is the XFET with excellent stability, an equally low and, above all, linear temperature response, low power dissipation, lower price, usable from 4.1 V. In this type, two JFETs with different pinch-off voltages are similar to the band gap reference interconnected.

Precision reference voltages are used, for example, in high-resolution analog-to-digital converters (ADC). Many manufacturers of these components integrate a reference voltage on their chip so that it cannot be affected by EMC influences.

Overview

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

• If the accuracy requirements are low, a resistor in series with the Zener diode and / or the temperature-compensated and stabilized form with reference diodes.
• The reference voltage source most widely used today in integrated circuits is the band gap reference , which can be easily implemented in a temperature-stabilized form.
• Floating gate analog also found in integrated circuits; a floating gate transistor is used for trimming .
• For high accuracy requirements, such as when defining the unit volt , quantum mechanical effects in superconductors such as the Josephson effect are used as a reference.

Historical reference voltage sources are:

• The glow stabilizer , a special form of glow lamp .
• Electrochemical normal elements such as the Weston normal element , which was previously used to determine the unit volt, or the Clark normal element .
• Magnetic voltage stabilizer

Individual evidence

1. ↑ Voltage References , overview table, Analog Devices.
2. ↑ LM431 Adjustable Precision Zener Shunt Regulator. Texas Instruments , accessed June 15, 2013 . 
3. ↑ Bob Dobkin, Jim Williams: Analog Circuit Design: A Tutorial Guide to Applications and Solutions , Elsevier, 2011, ISBN 978-0-12-385185-7 , limited preview in Google Book Search.
4. ↑ Linden T. Harrison: Current sources & voltage references , Elsevier, 2005, ISBN 0-7506-7752-X , limited preview in the Google book search.
5. ↑ Precision voltage reference uses EEPROM and floating gate trim. (No longer available online.) Planet Analog , formerly in the original ; Retrieved April 18, 2009 .  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.