Could anyone tell me how to compare small voltages (.354V vs. .454V) with a comparator, or how to boost them so that I can compare them? Currently I'm using a lm339 comparator, but I can't get it to compare those low voltages, though in the datasheet, it says that the input offset voltage is +/- 2 mVdc (which I think is the input voltage difference?).

Thanks a lot,
Seth Haddix

 

If you can post a schematic of the circuit you are currently using we can possibly see where it can be improved?

hgmjr

 

It's basically just two voltages being put into a comparator, both of which are .354V, and when the one on the + input becomes higher (via control of another circuit), the comparator should output a high voltage.
Is a lm339 fit for such small voltages?

 

Small voltages in themselves are not a problem. You can always multiply them up (amplify them).

The problem comes when comparing the difference of two similar quantities and getting a small difference output.

for example 0.456 volts minus 0.455 volts.

The inputs have 3 decimal place precision, the output 1 decimal place.

 

Have you got a pull up resistor on the output of the 339? SgtWookie once told me a hundred ohms per volt of the supply. I'm using a 339 on 12V so I use 1.2K pullups.

You can also use Positive Feedback to get a cleaner switch by looping the output back to the positive input with a large resistor. Once the output switches up it "feeds" a small amount of voltage back thru the resistor and bumps the input up. 220K's are working for me.

Hope that helps ...

SP

 

The LM339 has a max input offset voltage of 5mV. It will probably oscillate if its input voltages are within about 10mV so a small amount of hysteresis (positive feedback) is used to create a snap-action.
With an input offset voltage of 5mV and 10mV of hysteresis added then the output will switch when one input is 15mV higher than the other input.