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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Alec_t

Joined Sep 17, 2013
10,727
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:

MikeML

Joined Oct 2, 2009
5,444
I think the equations are correct as published in the data sheet.

Play with this:

150.gif

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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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. :)
 

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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