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What does the Comparator with Hysteresis circuit do?

Latest Updated:04/01/2008

Question:

"APPLICATION CIRCUIT EXAMPLE (COMPARATOR with HYSTERESIS CIRCUIT)" is shown on the Data Sheet of the uPC393 single-power dual comparators. What operation does this circuit perform?

upc393_ex1.gif

Answer:

A feature of this circuit is that its output is not affected even if the input signal contains a certain amount of noise (the output is influenced not only by the present input condition but also by the past input condition, that is, hysteresis).
[Operation]
Hysteresis can be given to the threshold voltage by positively feeding back part of the voltage of OUTPUT to the positive (+) input pin with R2 and R1, and the influence of a noise voltage within the hysteresis width can be eliminated.
Specifically, when the INPUT voltage rises with a sufficiently low voltage (), OUTPUT goes high. Consequently, a positive voltage drop occurs across R1 (i.e., the potential on the pin of R1 connected the positive (+) input pin is higher than the potential on the pin connected to the VREF pin) and the potential on the positive (+) input pin goes VTH (High). This status is maintained until INPUT reaches this potential VTH (High). As soon as INPUT exceeds VTH (High) (), OUTPUT falls low. At the same time, the voltage drop across R1 becomes negative (i.e., the potential on the pin of R1 connected to the positive (+) pin is lower than the potential on the pin connected to the VREF pin), and the potential on the pos itive (+) input pin goes VTH (low). Even if INPUT is raised after that, OUTPUT remains low (). When INPUT is gradually lowered next, OUTPUT remains low until INPUT = VTH (low) (). When INPUT drops below VTH (low) (), however, OUTPUT goes high again, returning to the initial status.

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