Electronics > Projects, Designs, and Technical Stuff
Strange behavior comparator hysteresis - solved
HendriXML:
My circuit consists of 5 reflective optical sensors. One is used as a reference (reflects gray). 3 respond to signal tracks. 1 responds to a trigger track, triggering the processing of the signal sensors.
I don't want the trigger to bounce, so I added some hysteresis via a mosfet.
However the trigger output oscillates before settling. Which is shown on the scope.
Channel 1 is probing the inverting input, channel 2 is probing the non inverting input, channel 3 is probing the drain (3) of the mosfet (thus the hysteresis offset), channel 4 is probing the trigger output.
What I can't explain is why channel 3 seems first to rise (several times) when the mosfet starts conducting.
The circuit at this page https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3616.html would use a p-mosfet on the lowside, which seems odd to me. So I'm probably misunderstanding something.. (But the eventual voltage drop is as I would expect.)
HendriXML:
Could this be an effect of the gate drain capacitance of the mosfet?
OM222O:
it just seems like a really odd way to add hysteresis ... you can make a schmitt trigger like so:
or like this:
depending on your application and desired waveform. I have not seen a mosfet used for adding hysteresis but it seems possible, however the schmitt circuit has always worked very well for me.
Edit: a simpler solution would be using schmitt input inverters after the comparators. if you don't want inversion, just invert it twice :D
HendriXML:
--- Quote from: OM222O on July 29, 2020, 08:44:57 am ---I have not seen a mosfet used for adding hysteresis but it seems possible, however the schmitt circuit has always worked very well for me.
--- End quote ---
The non inverting input is shared among 3 other comparators, so I did not want to disturb (load) that voltage. Hence this approach.
OM222O:
--- Quote from: HendriXML on July 29, 2020, 10:36:40 am ---
--- Quote from: OM222O on July 29, 2020, 08:44:57 am ---I have not seen a mosfet used for adding hysteresis but it seems possible, however the schmitt circuit has always worked very well for me.
--- End quote ---
The non inverting input is shared among 3 other comparators, so I did not want to disturb (load) that voltage. Hence this approach.
--- End quote ---
you can eliminate the hysteresis there and use schmitt input inverters like I mentioned. cleans up the signal without interfering with anything or needing additional circuitry. a simple hex inverter allows you to clean up 6 signals at the same time but like I said, the downside is the inversion so if that matters, just invert it again to get the original signal :-+
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