# AD621 gain problem

Thread Starter

#### hp1729

Joined Nov 23, 2015
2,304
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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#### joeyd999

Joined Jun 6, 2011
4,477
The problem is I can't get these two formulas to agree...
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.

#### Alec_t

Joined Sep 17, 2013
12,068
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:
Thread Starter

#### hp1729

Joined Nov 23, 2015
2,304
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)
Thanks. I'll try those.

#### MikeML

Joined Oct 2, 2009
5,444

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Thread Starter

#### hp1729

Joined Nov 23, 2015
2,304
I think the equations are correct as published in the data sheet.

Play with this:

View attachment 95564

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
Dang. Now I gotta go back and see what I did wrong. #### Alec_t

Joined Sep 17, 2013
12,068
Oops! I've re-done the maths and now agree with the datasheet equations. Post #3 edited.

Thread Starter

#### hp1729

Joined Nov 23, 2015
2,304
Oops! I've re-done the maths and now agree with the datasheet equations. Post #3 edited.
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.

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