Electronics > Beginners

Comparator (LM393) not saturating at low frequencies

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jmelson:
You only have positive voltages supplied to the LM393, so it can only operate with positive signal voltages.  Since the other input is grounded, it will just about always say the + input is greater than the minus.  If you want to detect the zero-crossing of the input, you need either a comparator with + and minus power supplies, or to shift BOTH inputs positive to be within the common-mode
rejection range of the comparator.

Jon

Bud:
I always ask the posters a same question: where did you get  the part from, if from China, throw it away right now and buy from a reputable supplier before even thinking about doing anything else.

ogden:

--- Quote from: Frongach on January 16, 2020, 11:01:55 pm ---The issue is in no way about AC coupling of the scope!! The waveform outline is identical with DC or AC coupling.

--- End quote ---
Yet only proof we have is picture where scope is in AC coupling ;) To show that we all are on the same page, could you possibly show waveform & switch "AC/GND/DC" (below volts/div dial) in DC position?

Frongach:
I'm away from the shop now, but I will post pictures of DC coupled waveform tomorrow and scope screen at higher frequencies.

Jon, I had thought that the problem might be with the single supply, but firstly the circuit is from a pretty good source so I had confidence in it. Secondly, the output is really clean, and conforms to calculated values (I measured frequency of the square wave output with microcontroller), at higher frequencies. Finally, I was avoiding a split supply as the output is used for a microcontroller (arduino) which cannot take negative voltages, or at least not below -0.3v I believe. I will try a split supply tomorrw for sure, thanks.

Bud, the part is from Farnell, TI I think, or at least a reputable manufacturer, and I've switched it out several times for new ICs, plus it only suffers at low frequencies so fairly sure this is not the problem.

floobydust:
Give us the schematic you have, it's not making sense.

I say the TI circuit is crap (as a starting point even) because it has no hysteresis, no low-pass filter to stop noise from false triggering, and no proper clamp diodes/resistors to limit the comparator's input overload.
If you tell us what you are trying to do, probably have better circuits in mind.

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