Electronics > Beginners
Protecting op amp input from negative voltages
guymo:
--- Quote from: Hero999 on July 20, 2018, 09:46:39 am ---If you only need it to be powered from 5V maximum, the list of CMOS comparators expands considerably, as they're optimised of interfacing with digital logic.
--- End quote ---
Thanks again. Just to check my understanding of these devices: if I want the outputs to be 0V and 5V, I had better run them from a 0V--5V power supply, right? In that case, yes, these 5V-powered devices are all potential winners.
I have ordered some TLC3704 to experiment with -- chosen for no better reason than that they are available in DIP which makes prototyping easier, at least for me.
Peabody:
With respect to the excessive negative input voltage, I think it might help if the input in question is the inverting input, and the feedback resistor from the output connects there too. As the input goes negative, the output goes positive, and the feedback resistor allows the "-" input to stay near the negative rail. Ideally, it's supposed to stay at 0V up to a certain point, but it doesn't really stay there.
I had the same issue driving an LM358 from the audio output of a walkie-talkie. Volume is supposed to be set to 1, but several times I've found it at 4 or 5, and so far the LM358 has survived. The input resistors are mainly responsible, but I think the feedback also helps. Circuit attached.
I should say that I'm not an EE, so if this is completely wrong, I hope someone will correct me.
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