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Determining voltage gain of instrumentation amplifier

  • #1
JJBladester
Gold Member
286
2

Homework Statement



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


Homework 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]

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]
 

Answers and Replies

  • #2
UltrafastPED
Science Advisor
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What does Vcm=225 mV represent?
 
  • #3
JJBladester
Gold Member
286
2
I believe it represents a common-mode voltage (a noise signal or some other common signal on both inputs).
 
  • #4
NascentOxygen
Staff Emeritus
Science Advisor
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[tex]V_{out}=\left ( 1+\frac{2\cdot 100k\Omega }{1.0k\Omega} \right )\left ( 10mV-5mV \right )= ???[/tex]
Work this out again.
 
  • #5
JJBladester
Gold Member
286
2
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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