# Comparing Small Voltages

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

1. ### mpuvdd Thread Starter Active Member

Feb 11, 2007
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0
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,

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
If you can post a schematic of the circuit you are currently using we can possibly see where it can be improved?

hgmjr

3. ### mpuvdd Thread Starter Active Member

Feb 11, 2007
50
0
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
5,005
515
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
123
0
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

6. ### Audioguru New Member

Dec 20, 2007
9,411
896
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.