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voltage reference


Voltage Reference

Introduction

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.

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 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.

The Design of Band-Gap Reference Circuits

Bipolar implementation
The band-gap reference has been a popular analog circuit for many years. In 1971, Robert Widlar introduced the LM113, the first band-gap reference.1 It used conventional junction-isolated bipolar-IC technology to make a stable low-voltage (1.220 V) reference. This type of reference became popular as a stable voltage reference for low-voltage circuits, such as in 5-volt data acquisition systems where zener diodes are not suitable. Band-gaps are also used in digital ICs such as ECL, to provide a local bias that is not adversely affected by ambient noises or transients.

The principle of the band-gap circuit is well known and will be mentioned here in the briefest terms. The circuit relies on two groups of transistors running at different emitter current densities. The rich transistor will typically run at 10 times the density of the lean ones, and a factor of 10 will cause a 60 millivolt delta between the base-emitter voltages of the two groups. This delta voltage is usually amplified by a factor of about 10 and added to a Vbe voltage. The total of these two voltages adds up to 1.25 volts, typically, and that is approximately the band-gap of silicon.

More details at http://www.national.com/rap/Application/0,1570,24,00.html

Bandgap Reference Notes quick overview Holberg

basic tutorial on Bandgap reference design

Curvature Corrected CMOS Bandgap Reference ; research paper

Low Voltage Low-Power CMOS Bandgap References

PHD thesis CMOS Bandgap Reference ; bandgap variance and noise

design of Band-Gap Reference Circuits, app note

curvature correcting bandgap architecture

Low Voltage Bandgap research paper

collection of practical voltage reference and bandgap design notes

Micropower Voltage Reference article from national semiconductor

National
Semiconductor Bandgap Notes

References Lecture discusses different bandgap references

Effect
of Ground Drop on Bandgaps routing bandgap currents and Kelvin connections

Info
on Bandgap References Basic bandgap info

BetaMultiplier Self-Referencing Vt/R voltage reference scheme

Self-Biased Referencing

The making of a bandgap reference design a bipolar bandgap
Maxim’s

voltage
reference application notes

Error Sources in 1st order bandgap references

different types of voltage references

voltage references article

Bandgapcircuit design

Many papers on bandgap design NEC Research Index

Many good papers on bandgap designESSCIRC
Website

band gap lecture notes from cae.wisc.edu

bandgap class notes from McGill

bandgap notes from Georgia Tech

Voltage Regulators and References

A Noise-Shaped Switched-Capacitor DC-DC Voltage Regulator

Low-Drift Bandgap Voltage References

Precision and Robust CMOS Voltage Reference Based on the Work Function Difference of Poly Si Gate

A New Voltage Reference Topology Based on Subthreshold MOSFETs





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  1. Guru

    Voltage references are electronic circuits that produce constant output voltages (references) that are used to compare other voltages in a system. There are two general types of voltage references: shunt and series. Shunt references operate in parallel with the load. The most common types of two-terminal shunt reference are based on the Zener principle, in which the current in a reverse-biased diode begins to flow at a certain voltage threshold and then increases dramatically with an increase in voltage. Resistors in series with the diode establish a constant current, allowing the Zener to achieve a stable reference voltage. The Zener reference behaves like a typical shunt or two-terminal reference.

    Subsurface or “buried” Zener references are more stable and accurate than the simple Zener-reference. They consist of diodes with the correct value of reverse-breakdown voltage, formed below the surface level of the integrated-circuit chip; then covered by a protective diffusion to keep the breakdown below the surface. At the surface of a silicon chip there are more impurities, mechanical stresses and crystal-lattice dislocations than within the chip. Since these contribute to noise and long-term instability, the buried breakdown diode is less noisy and much more stable than surface Zeners. This is the preferred on-chip reference source for accurate IC devices. However, its breakdown voltage is normally about 5 V or more and it must draw several hundred microamperes for optimum operation, so the technique is not suitable for references that must run from low voltage and have low power consumption. For such applications, the “bandgap” reference is preferred.






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