Electronics > Beginners

Comparator (LM393) not saturating at low frequencies

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ogden:

--- Quote from: Frongach on January 16, 2020, 11:46:19 pm ---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.

--- End quote ---
Right. This is textbook "above zero" detector circuit, single supply definitely is not an issue. My consideration is that particular circuit do not have *any* frequency-sensitive components that can distort waveform the way you show. Especially at low 200Hz(!), yet your scope has. Maybe AC/DC coupling of your scope is faulty? Try 2nd channel and 2nd probe, if any. [edit] Look at 200Hz square wave coming out of signal generator. Also double-check compensation of your probe.


--- Quote from: floobydust on January 16, 2020, 11:56:46 pm ---no proper clamp diodes/resistors to limit the comparator's input

--- End quote ---
Incorrect. Input 1M/10k resistor divider together with input clamp (10mA) diodes provides enough protection. [edit] Yet it appears that LM393 do not have input protection diodes

floobydust:

--- Quote from: ogden on January 17, 2020, 12:03:24 am ---
--- Quote from: floobydust on January 16, 2020, 11:56:46 pm ---no proper clamp diodes/resistors to limit the comparator's input

--- End quote ---
Incorrect. Input 1M/10k resistor divider together with input clamp (10mA) diodes provides enough protection.

--- End quote ---
Na, the Schottky diode before the input, clamps -ve excursions but there is nothing to deal with +ve overload.  See the input diodes in the LM393's internal schematic, it will not tolerate much maybe VCC-1.5V maximum before overriding the current source. That could explain very slow recovery times.
We don't know OP's input voltages or divider resistors.

ogden:

--- Quote from: floobydust on January 17, 2020, 12:36:10 am ---Na, the Schottky diode before the input, clamps -ve excursions but there is nothing to deal with +ve overload.  See the input diodes in the LM393's internal schematic, it will not tolerate much maybe VCC-1.5V maximum before overriding the current source. That could explain very slow recovery times.
We don't know OP's input voltages or divider resistors.

--- End quote ---
Yes you are right. I did not bother to check LM393 assuming that it is equivalent to TLV7011 (that has proper input protection). This is the case when simple copy-pasting of the circuits between parts that are not equivalent may go wrong.

floobydust:
TLV7011 "Input terminals are diode-clamped to VEE. Input signals that can swing 0.3V below VEE must be current-limited to 10mA or less." So why bother adding the Schottky?  :palm:

LM393 Datasheet: "Input current flows thorough parasitic diode to ground and turns on parasitic transistors that increases ICC and may cause output to be incorrect. Normal operation resumes when input current is removed."

Both parts don't like +ve overload.

StillTrying:
The 50k feedback resistance stopping it working makes me think the input resistors really are 1M/10k, even if the input voltage was the mains :o the max input voltage would be +3.3V and the highest input current -0.33mA.

"and have also put the output through a schmitt trigger, neither of which solved this."

It's bound to be the scope, definitely try channel 2. :)

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