I have a circuit that needs a comparator where the negative input will go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp designed to work on +/- 15 volts into service in this singly-supply application and expect its negative input to withstand the negative 12 volts.snipped-for-privacy@billburg.com wrote in news: snipped-for-privacy@i42g2000cwa.googlegroups.com:
I think many op-amps and comparators can accept a small overshoot beyond their supply rails. If the intermittent low is short, and you don't need to know the value, and maybe only need to detect whether it goes below zero, you could clamp it on the input using a germanium or Schottkey diode. Anything that has around 0.3 V forward drop. I don't know how well the comparator will detect the difference between that and the actual zero, but often you'll get a better comparotr using a normal op-amp, just stick a
100K or 1M resistor between output and non-inverting input to accelerate the slew on the output for clean transitions.
Without a negative supply, no. The input voltage range has no idea what voltage you are calling ground. It only reacts to the its input voltage relative to its supply voltages.
If the negative supply is available, you could use a 2903 and adapt its output signal to the logic range you need. If a negative supply is available, you also have lots of other choices of comparators that have separate output stages that do this, internally.
You don't say what you are comparing (what level event is being captured).
Are you trying to capture the negative pulse? Where is the output of the comparator going?
If you are comparing a voltage within the normal range of the comparator, then it's a simple matter to clamp the input so the negative pulse does not result in a negative pulse at the device pin(s).
A little more information would help.
Cheers
PeteS
have you thought of using a clamping diode on the input?
-- "I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
I do not recommend an opamp, there might be more caveats than using a comparator. You do *not* need a negative voltage. A simple resistor of 33k followed by a schottky-diode to gnd will clamp the voltage to save levels, that are within the -.3V common range of the 2903. Even if you want to measure the negative voltage, you can put up a summing comparator without negative supply. Just look in the datasheet. To prevent spurious oscillation you might want to add some hysteresis and need anyway those input resistors. The On-semi datasheet LM339 has as an example a zero x-ing detector.
-- ciao Ban Apricale, Italy
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