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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 3616
Keywords: comparator, hysteresis, threshold, trip voltage
APPLICATION NOTE 3616
Adding Extra Hysteresis to Comparators
Sep 19, 2005
Abstract: Analog comparators are always in the shadow of their more popular cousin, the ubiquitous op
amp. Designers can draw on a wealth of application notes for op amps, but not so for comparators. One
consequence of that scarcity is that customers call the Maxim applications line for help when adding
comparator hysteresis. This note explains how to add hysteresis to some common comparator circuits,
providing increased noise immunity and stability.
A discussion of comparator hysteresis begins with a definition of the word. Like so many other scientific
words, it is derived from the Greek. In this case, it means to lag behind or follow, or to offer resistance to
change from a previous state. We use it in engineering to describe an operation that is not symmetrical,
i.e., its path from A to B is not the same as from B to A. Hysteresis is found in the phenomena of
magnetism and nonplastic deformation, and in electronic circuits such as comparators.
Hysteresis is designed into most comparators, usually with a value of 5mV to 10mV. Internal hysteresis
helps the comparator avoid oscillation due to small amounts of parasitic feedback. Though sufficient to
prevent the comparator from self-oscillating, such internal hysteresis can easily be swamped by any
external noise of greater amplitude. In that case, you can improve performance by adding hysteresis
externally.
First, consider the transfer function for an ideal comparator without internal hysteresis (Figure 1). In
contrast, the transfer characteristic for a real comparator (Figure 2) shows an output change that
requires an increase of about 2mV in the input voltage (V
IN
).
Figure 1. Transfer characteristic for an ideal comparator.
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