[SOLVED] How to explain the Hystersis of a comparator?

[alanbrooke]

I think the hystersis circuit in a comparator is used to immune the unwanted output pulse when the input signal is noisy.
So the hystersis circuit is used to block high frequency noise signal.
But I find that hystersis is one of DC spec in those IC produces data sheet.
I got confused!

Anyone can kindly help me to explain this question?

 

[Hughes]

Hystersis is introduced to increase the noise margin. It is not limited to block high frequency noise signal. Any noise level between lower and higher trip points will be blocked whatever frequency they are.

 

[ramone]

The hysterisis phenomenon at a comparator is used to get a fixed and not delayed pulse on the output allthough you may have a big and noise line... It does reduces noise but it is a side effect (allthough it is usefull)... If you want someone to explain you how hysterisis is introduced by a comparator you must upload your circuit... But for the functionality of hysterisis you may think of a rough hysterisis loop as a chopper that tells when we are going to have a rising pulse or a falling one in a very certain way( so it reduces the delays)....

 

[fashion]

The purpose of hysterisis is the same as schmitt trigger.

 

[mady79]

Hysteresis comparator could also be used to avoid sequence of the pulses which happens at Vin =Vref (Trip point).Hysteresis is obtained by adding a positive feedback .Any false triggering at the out put could be avoided by setting the hyst width .

 

[CDRCDR]

Hysteresis is to avoid the comparator staying at the same level and not changed as the input changes

 

[djalli]

How to explain the Hystersis of a comparator?

If a signal (the input signal) is connected to the inverting terminal and the threshold voltage is set by non-inverting input. This has the feedback in most cases unless is some complex tri-memory comparator.

There will be a feedback to the +terminal is present (positive feedback) in the circuit. The opamp itself is in positive saturation region, when only and only when input signal voltage is increased to larger than Vt (threshhold voltage) and then Vout output is Vc- of opamp (depends of how you supply the opamp). Inverted.

When and only when input signal voltage is decreased to less than V threshold you shall observe output is Vc+ the op-amp is said to be in negative saturation region.

In such circuit two stable states exist. Present state is depended on both present input and the past history of the circuit.

All this property is called hysteresis (the feedback happening to) of opamp. Has nothing to do with noise even if it said Schmidt Trigger cleans up the noise. It looks like is cleaning only thing is doing is smoothing by comparing.

 

[layes]

use very large time step eg : tran 1 10m to simulate your comparator