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comparator question

comparator question

comparator question

(OP)
Hi all,

I have a question about comparator. I need to design a circuit that will detect a negative voltage. I have the positive input tied to GND and the negative input is tied to the signal. The comparator I am planning to use is the LMV761 from National Semiconductor. I don't have negative supply in my circuit to power up the comparator. I am using +5V and GND. My question is: Can I sense the negative voltage (it is about -0.7V) without damaging the comparator? If this is going to damage it, is there a way to sense it?

Regards,

Fabio Pereira

RE: comparator question

Since you tied the positive input to ground, you will compare to ground. So it won't work.

You can solve the problem by balancing your negative input voltage with a voltage divider that divides your +5 V and your input signal so that the result is zero when the input is -0.7 V.

Tie a 10 kohm resistor between +5 V and negative input. The tie a 1.2 kohm resistor from input signal to negative input. That will give you zero output from the comparator for all voltages more positive than about -0.61 V

Adjust (increase the 1.2 kohms until you have the correct level.



RE: comparator question

(OP)
Thanks skogsgurra,

It worked out fine.

RE: comparator question

Tested already? That's fast!

RE: comparator question

(OP)
I haven't breadboarded yet. I've just tested using Multisim. It seems to work fine. The real test will be next week.

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