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Protecting op amp input from negative voltages
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guymo:
I'm playing with a circuit in which I use an LM324 powered from 12V and 0V. The negative rail needs to be 0V for this purpose because one thing I am interested in is getting a "good zero" from the op amp in a comparator configuration.

The plan, however, is to expose one input to the outside world. The LM324 data sheet gives the "absolute maximum" low input voltage as -0.3V. It's possible that someone could accidentally connect a signal that goes to -5V or so there.

How should I go about protecting the input? A simple diode from ground will clamp at -0.7V -- is that good enough in practice? I'd like to avoid adding a further input conditioning stage if I can but happy to be told that I have to!
David Hess:
Adding a series resistance to limit the current is a good place to start.  A schottky diode will make a better clamp because of its lower forward voltage drop.
Kleinstein:
For the LM324 the negative input limit at -0.3 V is rather conservative for high temperature. At least in the NS data-sheet, there is another line: up to 50 mA of current is allowed when the voltage is negative. There is a kind of internal clamping diode and this "diode" can be used to 50 mA. The -0.3 V are only a safe limit not to reach this current even at high temperature.  So the main part would be to have a series resistor to limit the current to a safe level (e.g. 20 mA) when it turns too negative. There is little need for an extra diode with the LM324/LM358 - this could be different for another type.
hugo:
By adding a (5K) resistor in series with the input pin, when the signal accidentally goes to -5V, the current is then limited to less than 1mA ( there is a .65V voltage drop on the bottom steering diode inside LM324).
guymo:
Thanks everybody. I’m now much more confident about the circuit I have in mind: the input goes via a (Thevenin equivalent) 50k resistor so the current is limited already. Great!
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