Discussion in 'General Electronics Chat' started by hp1729, Nov 29, 2015.

1. ### hp1729 Thread Starter Well-Known Member

Nov 23, 2015
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The AD621 is an instrumentation amplifier with two pins where you can put a jumper or resistor to set the gain. Open we get a gain of 10. Shorted we get a gain of 100. Or we can insert a resistor to get a gain between 10 and 100. Two formulas on the data sheet are suggested. One calculated a resistor value for a desired gain. The other calculates a gain for a given resistor. The problem is I can't get these two formulas to agree. Data sheet is attached. It won't let me attach a spreadsheet. Has anybody got these to work?.

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2. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Have you attempted to work the algebra to confirm the two formulas are equivalent?

If they are, then the problem is in your calculations, not in the datasheet.

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
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The first equation on p14 of the datasheet is wrong by my reckoning. '9' should read '10'.

Edit: I don't agree with the second equation either! I make it
Rx = (61,666 - G*0.555)/(G-11)

Re-edit: Ignore the above. I now agree with the datasheet equations.

Last edited: Nov 30, 2015
4. ### hp1729 Thread Starter Well-Known Member

Nov 23, 2015
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Thanks. I'll try those.

5. ### MikeML AAC Fanatic!

Oct 2, 2009
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I think the equations are correct as published in the data sheet.

Play with this:

Simulated gain of actual circuit = V(o)/100m
Computed gain is V(gain); they line up

The independent variable of the simulation is Rx, converted to V(v)
The computed resistance is V(x); they line up

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6. ### hp1729 Thread Starter Well-Known Member

Nov 23, 2015
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Dang. Now I gotta go back and see what I did wrong.

7. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Oops! I've re-done the maths and now agree with the datasheet equations. Post #3 edited.

8. ### hp1729 Thread Starter Well-Known Member

Nov 23, 2015
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Thanks for the time. I re-did my excel and broke it down step by step. Yes it works correctly as stated in the data sheet. I guess I grouped something wrong in the other spreadsheet.