1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Determining voltage gain of instrumentation amplifier

  1. Oct 14, 2013 #1


    User Avatar
    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.


    2. Relevant equations



    [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?

    [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


    User Avatar
    Science Advisor
    Gold Member

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


    User Avatar
    Gold Member

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


    User Avatar

    Staff: Mentor

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


    User Avatar
    Gold Member

    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted