comparator question

[Fabioedl]

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

 

[Skogsgurra]

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.

 

[Fabioedl]

Thanks skogsgurra,

It worked out fine.

 

[Skogsgurra]

Tested already? That's fast!

 

[Fabioedl]

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