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Electronics => Electronics => Topic started by: mpuvdd on June 29, 2008, 10:03:31 PM

Title: ComparingSmallVoltages
Post by: mpuvdd on June 29, 2008, 10:03:31 PM

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, 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.

Thanks a lot,
Seth Haddix

Title: Re: ComparingSmallVoltages
Post by: bens on June 29, 2008, 10:05:27 PM

You could use an op amp.

Title: Re: ComparingSmallVoltages
Post by: mpuvdd on June 29, 2008, 10:06:43 PM

What kind of configuration, I'm new to electronics, especially op amps?

Title: Re: ComparingSmallVoltages
Post by: bens on June 30, 2008, 12:23:59 AM

By itself an op amp will amplify a voltage difference by maybe a factor of 1000, which usually means it will just go to the maximum or minimum supply voltage to the op amp.  By adding a feedback loop with some key resistors you can control the gain of the op amp to amplify the voltage difference by a factor of your choice.

Take a look at the non-inverting amplifier circuit on this page (you will probably want some variation of this):

http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

You might also want to check out the wikipedia page on op amps:

http://en.wikipedia.org/wiki/Operational_amplifier

You could also consider purchasing a fixed-gain amplifier.  For example:

http://www.national.com/pf/LM/LMP8270.html

- Ben

Title: Re: ComparingSmallVoltages
Post by: ALZ on June 30, 2008, 02:26:59 AM

What is the problem you are having? A .1 of a volt "compare small voltages (.354V vs. .454V) with a comparator" is more than enough for a comparator to work with. Are you sure it is not working? With a comparator you will only get a high or low voltage, there is no in between like with an op amp.

http://robotics.scienceontheweb.net

Title: Re: ComparingSmallVoltages
Post by: mpuvdd on June 30, 2008, 08:14:12 AM

Exactly, I'm only trying to get a one or a zero, depending on which voltage is higher.
Is the comparator not fit for such small voltage?

Title: Re: ComparingSmallVoltages
Post by: Admin on July 05, 2008, 08:50:57 PM

It sounds like your circuit is wrong . . . whats your schematic look like?

Like I tell everyone, this should help:
http://www.societyofrobots.com/robot_faq.shtml#circuit_debug