I am studying comparators and Schmitt triggers. And I did not find anywhere how to choose the value of hysteresis. I understand its purpose and I know how to calculate it. But what value is needed for a particular scheme? Is any rule to choose that value?
Electronic – Schmitt Trigger hysteresis
comparatorhysteresisschmitt-trigger
Related Solutions
An op-amp isn't a schmitt trigger. If you don't have a schmitt trigger in your set of models, you can model one using an op-amp, but you need positive feedback (output sent to input) to make that work.
See this for ways to do so:
http://en.wikipedia.org/wiki/Schmitt_trigger#Op-amp_implementations
Design will generate dual frequency signal, but due to asymmetric nature it will change duty cycle more and it would be hard to obtain high hi/lo frequency ratio (precise parts needed).
There are other ways.
- Use additional capacitor switched by mosfet. It may me parallel or serial to main (parallel shown).
simulate this circuit – Schematic created using CircuitLab 2. Use alternating oscillator topology.
Here on schematic second 'slave' oscillator work in two modes: when 'master' is high it is two invertor oscillator (NAND is working as second invertor); when 'master' is low NAND gate is blocked and is is acting as single schmitt oscillator.
Second schematic was discovered while playing with CD4093 quad NAND schmitt. Drawback of it that its frequency ratio hardcoded by schmitt thresholds.
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Best Answer
The correct value of hysteresis voltage depends entirely on your application, and you haven't told us anything about that.
Basically, you should consider three factors:
You probably want to make sure that the hysteresis voltage is greater than your noise voltage to prevent false triggering. Hopefully, the magnitude of the noise is less than the difference between your input high and low levels. If so, then set the hysteresis trigger points at the high and low levels you specified.