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Determining voltage gain of instrumentation amplifier

  1. Oct 14, 2013 #1

    JJBladester

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    Gold Member

    1. The problem statement, all variables and given/known data

    The following voltages are applied to the instrumentation amplifier in the figure.
    Vin1=5 mV, Vin2=10 mV, and Vcm=225 mV. Determine the final output voltage.

    8-39.jpg

    2. Relevant equations

    [tex]R_{1}=R_{2}=R[/tex]

    [tex]A_{cl}=1+\frac{2R}{R_{G}}[/tex]

    [tex]V_{out}=\left ( 1+\frac{2R}{R_{G}} \right )\left ( V_{in2}-V_{in1} \right )[/tex]

    3. The attempt at a solution

    This is an odd-numbered problem in the back of my book, so I know the answer should be 1.005 V.

    I already know my initial answer (see below) is wrong because it comes out to 1 V and does not take into account Vcm. Since I'm given Vcm but not Acm, CMRR, or anything else, how do I solve this problem?

    Incorrect:
    [tex]V_{out}=\left ( 1+\frac{2\cdot 100k\Omega }{1.0k\Omega} \right )\left ( 10mV-5mV \right )=1mV[/tex]
     
  2. jcsd
  3. Oct 14, 2013 #2

    UltrafastPED

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    What does Vcm=225 mV represent?
     
  4. Oct 14, 2013 #3

    JJBladester

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    I believe it represents a common-mode voltage (a noise signal or some other common signal on both inputs).
     
  5. Oct 15, 2013 #4

    NascentOxygen

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    Staff: Mentor

    Work this out again.
     
  6. Oct 15, 2013 #5

    JJBladester

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    That's what I get for working 15 hours of overtime this weekend and trying to get homework done.

    Vout = 1.005 V as expected... Amazing what sleep will do for you. Thanks for pointing out the math error!
     
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