# LM324 as instrumentation amplifier

Discussion in 'The Projects Forum' started by JehanKhan, Apr 29, 2015.

1. ### JehanKhan Thread Starter New Member

Oct 24, 2011
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Hello all
I am working on a project and i have been advised to use LM324 IC to develop the instrumentation amplifier. I have calculated the gain with simple op-amp.
But my question is that how to find the gain for instrumentation amplifier? as i have Vin1=0.5Vand Vin2=0.51V and change in output voltage is 1.5V. so how to calculate all the resistances,including the Rgain?
Instrumentation amplifier is shown in the link:
http://en.wikipedia.org/wiki/Instru...dia/File:Op-Amp_Instrumentation_Amplifier.svg

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,600
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hi,
Its Vo= (1 + ((R1+R1)/RG))*Vc

3. ### MikeML AAC Fanatic!

Oct 2, 2009
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1,066
The equation for Vout vs the differential input voltage as a function of all the resistors is shown in the Wiki. Are you incapable of transposing an equation?

4. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
0
If i was capable of doing that, i wont even have ask on this forum.
I only have differential input voltage, the output voltage...i can calculate gain by assuming the resistors values but i need some practical number for this practical project. So if you can at least guide me in the direction would be enough.

5. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
0
Thanks for the quick response.
I need the resistors value. If you can throw me some formula

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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Since this is obviously homework, we don't just do the problem for you.

What gain should the amp have to satisfy the requirements?

7. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
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Gain that i am considering is 500 and this is not a homework its more than that. i am really stuck with it. i dont know what i am missing. i have looked into alot of data sheets and alot of internet surfing i have done.

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Differential input is 0.5V-0.51V = -0.01V

Output voltage is 1.5V

Gain = 1.5V/-0.01V = -1500

Where did you get a gain of 500?

Mar 6, 2009
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10. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
0
Revision:
V1=0.0490V, V2=0.051V, Vid=0.0028V, Vo=1.5V so i get gain about 536.
now how do i select the resistors? specially the Rg (gain resistor)?

11. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,600
394
As you are trying to design a IA, why not study some IA data sheets and see how a commercial IA is designed.?

12. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
0
yes i am trying to design IA but i have to do it with LM324.

13. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,600
394
I know that..
BUT you are asking for guidance on the resistor values, which could be the same as the values used in commercial IA's.
Why not try the the AD623 resistor values.?
Its important that the resistors you use are at least 0.1% tolerance in order to minimise common mode voltage problems.
E

14. ### JehanKhan Thread Starter New Member

Oct 24, 2011
9
0
Thanks everybody for the help.
I have tried some resistors combinations and now implementation phase starts. Lets see what would the results say.
Thanks again