Different Ground Potentials

Discussion in 'The Projects Forum' started by bricke, Aug 15, 2011.

  1. bricke

    Thread Starter New Member

    Mar 1, 2011
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    I am working on a project that has 4 sets of batteries as the power source. Each battery pack (3.6V) is in series with one another so that the power source equals 14.4V. I have to use the negative terminal of each battery pack as my ground in order to measure the correct voltage. (If I measure a voltage point with respect to the power source as a whole's ground, it will give me the measurement of that battery pack as well as those between my point and the pack's ground) The problem is that I am interfacing each battery pack with an ATtiny13a microcontroller. When I connect the positive voltage to the Tiny13's ADC input, it reads a different value because it is trying to read the value compared to the 5V regulator's ground. I know this is confusing but hopefully the schematic helps (I only show 2 of the 4 batteries). I need to find a way to get the readings from the shunt regulator and voltage divider in so that the microcontroller can read it correctly. Is there a way of changing ground potentials like this? The VCC shown is a 5V buck regulator that is taken from the 14.4V supply and the VDD is the converted side of a DC/DC converter which has the buck regulator's 5V on the other side. Thank you!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Use four difference amplifiers:


    [​IMG]

    R1 = R2 = R3 = R4 = 10K will give a gain of 1.
     
    Last edited: Aug 15, 2011
  3. bricke

    Thread Starter New Member

    Mar 1, 2011
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    Thanks for the response MrChips. I had actually tried this already, and unfortunately, it didn't work very well. When I inserted the difference amplifier, the voltage on the output dropped from around 3.6V to 3.1V. I did exactly as you have it in your diagram as well. I also added a voltage follower amplifier to the output to see if that cleared it up but had no luck with that as well.

    I've seen these pre-built differential amplifiers, but I didn't know if those were any better than ones built with external resistors. Could the LM324 possibly be a bad amplifier to use for the differential amplifier? Thanks.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    What were the values of the resistors in the circuit you tried?
     
  5. bricke

    Thread Starter New Member

    Mar 1, 2011
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    Initially, I had them at 560 ohms but changed them all to 10k and still had the same result.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    10K should have worked fine.
    Next question, what opamp were you using and what power supply voltages?
     
  7. bricke

    Thread Starter New Member

    Mar 1, 2011
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    I was using the LM324 just because this is what I had on me at the time. Power supply voltages were from the 5V buck regulator so 5V and circuit ground. I know the input and output voltages are within the chips limits, but I'm wondering if maybe this isn't the best op amp to use for this purpose.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    LM324 will not give an output voltage higher than VCC - 2V.
    You can either increase the supply voltage or change to a rail-to-rail opamp.
    Another simple solution is to reduce the gain of the opamp to reduce the output signal.
     
  9. bricke

    Thread Starter New Member

    Mar 1, 2011
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    I'll give it a try with a rail-to-rail then. I didn't see anything about that on the datasheet but I'll take your word on it. Thanks again for all your help.
     
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