Voltage Offset and Scaling

Discussion in 'General Electronics Chat' started by cgha20@yahoo.com, Oct 21, 2009.

  1. cgha20@yahoo.com

    Thread Starter Active Member

    Oct 21, 2009
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    I need to use OP184 opamp to scale a 0.5-4.5 Voltage range to -3.000 to +3.000 voltage range. The opamp will be powered by +/- 12V. I have another available 5V line i can use. please help.
     
  2. cgha20@yahoo.com

    Thread Starter Active Member

    Oct 21, 2009
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    Clarification - The input to the opamp is the output of an accelerometer so it will vary with rotation. The output of the acceleromer is a DC +0.5 to +4.5V. I need to use OP184 to turn this voltage to -3.000 to +3.000 V. Thanks.
     
  3. cgha20@yahoo.com

    Thread Starter Active Member

    Oct 21, 2009
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    The input is to enter the (+) noninverting pin.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The input range Δ is 4.5-0.5 = 4

    The output range Δ is 3 - (-3) = 6

    Therefore, the gain = 6/4 = 1.5.

    The gain from the + input to the output is 1+R2/R1,
    so 1.5 = 1 + R2/R1, or R2/R1 = 0.5.

    The center of the input range is (0.5+4.5)/2 = 2.5V.

    If we apply 2.5V to the + input, what voltage on the gain setting resistor connected to the - input would cause the output to go to zero? The gain from the inverting input to the output is R2/R1 = 0.5, so to subtract out the 2.5V offset, you must supply an offset of 2.5 + 2.5/0.5 = 7.5V to the resistor.

    I would use 10.00K for R1, so that makes R2 5.00K.

    Since you dont have a 7.5V source, and 5V is too low, you can transform the 7.5V and 10K input resistor to 12V and a two resistor voltage divider by using Thevenin's Theorem. Look at the sim.
     
    Last edited: Oct 21, 2009
  5. cgha20@yahoo.com

    Thread Starter Active Member

    Oct 21, 2009
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    The only problem now is that the accelerometer I will use is very sensitive to Voltage changes in the millivolt range. My +12 volt is not as stable as I would like. My 5 volt will be very precise +/- 1 to 2 millivolts. Would you recommend to use OP284 and use the second opamp to amplify the 5 Volts to 10 Volts or even 7.5V and use that voltage? BTW this project is very important to me, and I would like to offer you a small donation for your help. What is the best way we can do this?
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    For the 7.5 volts bias you could use the 12Volts supply with a TL431 voltage reference adjusted to 7.5 Volts.
    See the datasheet for more info.

    Greetings,
    Bertus
     
  7. cgha20@yahoo.com

    Thread Starter Active Member

    Oct 21, 2009
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    Here is problem #2. The accelerometer outputs 1V/G. 1 G equals gravity which would really be my maximum reading needed. If I decrease my range to +/- 1.5 g, I should be able to take advantage of the 5V instead of needing a 7.5V right?
     
  8. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    You do not need to decrease your range. There is another way:

    Use one Op Amp in a differential configuration and use your 5V clean reference to offset it (see attached schematic).

    How did I calculate the resistors? Read my article
    Solving the Differential Amplifier - Part 1 for a simple math solution, or
    Solving the Differential Amplifier - Part 3 for an intuitive solution.

    In short, take the differential amplifier transfer function

    Vout = V1 * R2/(R1+R2)*(1+R4/R3) - V2*R4/R3

    and write two equations: One for Vout = -3V, V1 = 0.5V and V2 = 5V, and another one for Vout = 3V, V1 = 4.5V and V2 = 5V. Solve this system with two unknowns, R2/R1 and R4/R3. Choose R1 and R3 and calculate R2 and R4. The result is R1 = 2k, R2 = 12K, R3 = 2k, R4 = 1.5k.
     
    Last edited: Dec 6, 2009
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