How Do You Calculate Vout in an Instrumentation Amplifier Circuit?

[nothing909]

Homework Statement

I'm trying to calculate Vout.

Homework Equations

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VDIFF = VIN2 - VIN1 = VDIFF1 + VDIFF2

VOUT=(VIN2–VIN1)(1+2R1/RG)(R6/R5)

The Attempt at a Solution

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VOUT=(VIN2–VIN1)(1+2R1/RG)(R6/R5)
VOUT=(V2IN-VIN1)(1+2(10x10^3)/2x10^3)(10x10^3/10x10^3)

What are the values of V2IN and VIN1? I was just looking at the circuit and it's 0.1Vpk for both so 0.1-0.1=0 but obviously that's not right. what am i missing?

 

[berkeman]

You would need to use the real-world datasheet specs for the LM324 to calculate Vout, I would think. If you treat them as ideal opamps, of course you will get infinite CM rejection.

Also, it's good to get in the habit of connecting up any unused opamps (like U1C) in a benign configuration, to keep them from oscillating and interfering with the operation of the circuit (same thing goes for unused gates in a multi-gate logic part). In this case you could just show it connected as a unity-gain voltage follower with ground at its + input.

 

[nothing909]

It asks me to calculate the differential gain (which is 11 in the answers). The equation is Ad = VOUT/VDIFF

If I do VOUT=(VDIFF1+VDIFF2)(1+2R1/RG)(R6/R5) = 2.2

Ad = 2.2/0.2 = 11

This corresponds with the answer. Is this the correct way I've done it?

 

[nothing909]

Also if you look at this equation CMRR = 20log(Ad/|Acm|)

if my differential gain is 11 and my common mode gain is 20μ, is it CMRR= 20log(11/20x10^-6) = 114.8

what does the lines mean |Acm|, does this change the of 20μ in the equation?

 

[berkeman]

If I do VOUT=(VDIFF1+VDIFF2)(1+2R1/RG)(R6/R5) = 2.2

Ad = 2.2/0.2 = 11

This corresponds with the answer. Is this the correct way I've done it?

I believe so.

if my differential gain is 11 and my common mode gain is 20μ, is it CMRR= 20log(11/20x10^-6) = 114.8

Where did the Acm = 20uV/V come from? Sorry if I'm missing something obvious.

 

[nothing909]

you're not missing something. I'm doing transient analysis in multisim and i get the Vout for the cm off the graph and then just Acm = Vout/Vcm