Comparing Small Voltages

Discussion in 'General Electronics Chat' started by mpuvdd, Jun 30, 2008.

  1. mpuvdd

    Thread Starter Active Member

    Feb 11, 2007
    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
  2. hgmjr


    Jan 28, 2005
    If you can post a schematic of the circuit you are currently using we can possibly see where it can be improved?

  3. mpuvdd

    Thread Starter Active Member

    Feb 11, 2007
    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?
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    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.
    Last edited: Jun 30, 2008
  5. pntrbl

    Active Member

    Apr 21, 2008
    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 ...

  6. Audioguru

    New Member

    Dec 20, 2007
    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.