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Homework Help: Testing instrumenation amplifier

  1. Dec 2, 2017 #1
    1. The problem statement, all variables and given/known data
    I just have some very simple questions to ask.

    I'm testing a basic 3 OP instrumentation amplifier

    INA.png

    I have to short V2 to ground and test V1 from -2V to 2V, in 0.25 increments.

    I then have to short V1 to ground and test V2 from -2 to 2V in 0.25 increments.

    the results i have for V1 are for example:

    2 vout = ~12.2
    1.75 vout = ~11.4
    1.5 vout = ~ 10.8
    1.25 vout = ~ 9.2
    .
    .
    ...

    i then have this equation to find the gain:

    ∂vout/
    ∂vin1


    when i'm doing this equation, i'm not undestanding the ∂. do i add up all the results and then divide it by all the vin1's added up like:

    12.2+11.4+10.8.../2+1.75+1.25...

    or will i just take one result like 11.4/1.75.

    i know this is stupid question, i just wanna know if i'm doing it right. either way i do it, i get like ~6.

    when i do both the tests, each gain is ~6. do both the gain of each of the buffer amplifier add up 6+6=12 and 12 is the overall gain for the INA?
     
  2. jcsd
  3. Dec 2, 2017 #2

    Baluncore

    User Avatar
    Science Advisor

    Make an accurate table of your outputs against inputs.
    Then for each two rows of the table, compute the step change of input = dVi, and the step change of output = dVo.
    The gain over that small range of input will be dVo / dVi.
    You may find gain varies over the input voltage range and between inputs.
    You may find limits to input voltages that produce sensible outputs.
    INA is a difference amplifier so if gain of + side is 6 and - side is 6, differential gain is 6.
     
    Last edited: Dec 2, 2017
  4. Dec 2, 2017 #3
    I don't understand what u mean when you say "compute the step change of input..."

    say my results are this:

    results.PNG

    how do i get the gain for these 2 result tables? they're very similar, but the negative and positives are opposite for each set of results.
    can you explain a bit more simply how to get the gain using ∂vout/∂vin1?

    thanks
     
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