DC Voltage Offset

Discussion in 'General Electronics Chat' started by harman79, Dec 23, 2010.

  1. harman79

    Thread Starter New Member

    Dec 23, 2010
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    Hi all,

    I am v.v. new with electronics circuits and I have, what I think it is, a very basic problem..:confused:

    I have a 0-12V DC signal but the AI module of my DAQ is a -10 to +10 V one.

    How can I offset my signal by (say) -2V so that I can read its full range??? Is there a STD circuit to do that?

    Thanks a lot for your time,
    Harry
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    If you have no experience with electronics, it will be very difficult to help you build a circuit to do this. Can you not get a different input module? 0-20 or 4-20 mA inputs would be preferable.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    What about using voltage divider?
     
  4. harman79

    Thread Starter New Member

    Dec 23, 2010
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    Well,
    the idea is to stick with what we ve got.. I thought that maybe there is a standard circuit to do that, so that somebody could just provide a schematic for it..

    Thanks
     
  5. harman79

    Thread Starter New Member

    Dec 23, 2010
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    Shteii01,

    the voltage divider was what I tried initially. Just two resistors in series.. However, it turned out that the 0-12V signal strength is too low (low amps I guess) and the resistors were killing it..

    Thanks
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    How about using diodes in the signal path. by using three diodes you would drop around 2.1V (0.7V per diode)
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    Standard sort of job for an op amp. Google should help us op amp plus words like off set adjustment and input conditioning.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the part number of your DAQ? Can you provide a link to the datasheet?
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I don't mean to take the topic on a tangent, but what values of resistors did you use for voltage divider?
     
  10. gootee

    Senior Member

    Apr 24, 2007
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    Yes, there is a standard circuit to subtract a voltage. But it seems like you might actually want to fit the 0-12 into 0-10, instead. That lowers your resolution a little, but might make more sense. If you did want to just subtract 2V, to get -2 to 10 instead of 0 to 12, then why not also multiply as well as subtract, and get -10 to +10, to get the best resolution when you digitize?

    Anyway, the opamp circuits for doing arithmetic are relatively simple. But for gains of less than one, if your circuit can drive the impedance, your voltage divider should have worked well, too. I'm guessing you used resistances that were too low. You could insert a unity-gain (gain of 1) opamp amplifier (a buffer amplifier, with high input impedance) before the divider, or maybe just use higher-value resistors.

    Here are links containing the basic opamp circuits needed in order to perform analog arithmetic operations (adding and subtracting voltages, scaling voltages up or down by a factor, changing polarity, fine-adjust of offsets, etc) to condition signals to fit into ADC input ranges:

    http://www.national.com/an/AN/AN-20.pdf

    http://www.national.com/an/AN/AN-31.pdf

    Remember to use 0.1uF ceramic and 10uF electrolytic in parallel, from each opamp power pin to ground.

    I suggest taking the time to learn LTspice so you can simulate your opamp circuits, before building them. It's a free download from http://www.linear.com .

    Cheers,

    Tom
     
    Last edited: Dec 24, 2010
    harman79 likes this.
  11. bassplayer142

    Active Member

    Jan 2, 2007
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    Keep in mind if you just subtract 2 volts you will lose resolution. A better alternative is an op-amp with a gain < 1. This way you scale the entire 0-12V scale to 0-10. If you do this, there will be a linear mathematical trend which is changed which will have to be taken into account. I'm assuming that you are bringing the daq into your computer to be processed there.

    If you don't or can't do this in software then subtracting 2 volts will work. You just loose potential information which you may need.
     
  12. harman79

    Thread Starter New Member

    Dec 23, 2010
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    Guys thank you all for the very valuable info.

    I will follow gootee links as they seem very promising and will also help me understand some basics.

    Thanks to all again and merry xmas
     
  13. gootee

    Senior Member

    Apr 24, 2007
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    Actually, it seems like just subtracting 2 V would keep the voltage span the same and no ADC resolution would be lost (assuming his ADC really can digitize -10 to +10), right? Then he could just add the number of ADC counts for 2V and and have his 0-12 digitized. But scaling from 0-12 to 0-10 would shrink the voltage span and would lower the resolution, right?
     
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