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Maxim > Design Support > Technical Documents > Tutorials > Voltage References > APP 719
Keywords: voltage reference, tutorial, topologies, two-terminal, three-terminal, shunt, buried zener,
bandgap, series
TUTORIAL 719
Understanding Voltage-Reference Topologies and
Specifications
By: Bill Laumeister, Strategic Applications Engineer
David Fry, Strategic Applications Engineering Manager
Mar 06, 2013
Abstract: This document discusses the three most popular topologies of voltage references. These
include bandgap and buried zener topologies in three-terminal series voltage references, and zener
topologies in two-terminal shunt voltage references. Also demystified are the definitions of common
voltage-reference parameters.
Introduction
The first considerations in choosing a voltage reference are output voltage and initial accuracy. Often
overlooked, however, are the various other data sheet parameters that can assume major importance in
specific applications. Also, be sure to take into account the error budget when evaluating a data
converter (ADC or DAC) together with a voltage reference. (See application note 4300, “Calculating the
Error Budget in Precision Digital-to-Analog Converter (DAC) Applications.”)
The following discussion of voltage-reference basics will help you better understand the common types of
voltage references and the performance parameters associated with the most common voltage-reference
topologies: the two-terminal shunt and the three-terminal series designs. (For guidance in choosing
between a series and a shunt voltage reference, see application note 4003, “Series or Shunt Voltage
Reference?” and application note 2879, “Selecting the Optimum Voltage Reference.”)
Common Types of Voltage References
There are three common types of voltage references: charged capacitor, zener, and bandgap. The
charged capacitor is little used (particularly in safety applications) because of instability caused by
ionizing radiation. Alpha, beta, gamma, and cosmic rays or common x-rays in airports, hospitals, and
transport security all discharge the capacitor (typically a 7mV change to the capacitor at each discharge).
Zeners, the second type of common voltage reference, are used in and out of avalanche mode. Most
avalanche mode zeners are used where they would be most stable (i.e., at a sharp knee), above
approximately 5.5V depending on the semiconductor process. True zeners at lower voltages work
because of quantum mechanical tunneling. The majority of zener noise issues are due to associated
impurities on the die surface, which are overcome with buried zeners by burying the zener inside or
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