AD621 gain problem

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

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
    1,954
    219
    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?.
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,684
    2,747
    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
    5,800
    1,104
    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
    1,954
    219
    Thanks. I'll try those.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    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
     
  6. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
    1,954
    219
    Dang. Now I gotta go back and see what I did wrong. :)
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,800
    1,104
    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
    1,954
    219
    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.
     
Loading...