Instrumentation Amplifier output

Discussion in 'General Electronics Chat' started by Dritech, Nov 28, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,
    I did a basic circuit on simula ion, where I used the AD621 and provided a common input.
    Why is the output of the IA not measuring 0V? I also tried another IA and I got similar results.
    Any help would he highly appreciated.
    [​IMG]
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Try it with +- split supplies. The data sheet doesn' t say anything about its rail-to-rail operation with a single supply. In fact, the data sheet implies that split supplies are required.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. I want to use the IA with a bridge. With reference to the schematic below, since all resistors are equal, shouldn't the output of the IA be 0V?
    [​IMG]
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    That is almost right out of the data sheet. Yes, it quotes only split supplies but an example in the data sheet shows single supply operation in a similar circuit. Have you tried modeling it with another (simpler) IC that says it is designed for single supply operation?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    :confused: I think there is something wrong with the simulator or with the AD621 model: the amp output at pin 6 is apparently -3.49V with a single-polarity supply !
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
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    When I connected pin 4 to -12V using the circuit shown in post #4, the meter measures -15V :confused:
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Was pin 5 still grounded when you used split supplies? It's the reference pin.
     
  8. JohnInTX

    Moderator

    Jun 26, 2012
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    One problem is that Vref pin 5 is strapped to what the amp considers the negative PS rail (Vs). The specs say this pin should be 1.6v above Vs. Try setting it to Vcc/2 for single supply operation.
     
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  9. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks all for the replies. I did as JohnlnTX suggested (pin 5 set to 6V) and it worked.
    What method should I use to make the output go to 0V when both inputs are equal?
    I tried using a dual supply (12V and -12V) and setting the reference pin to ground, but it did not work.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    Okay, assuming the simulator is wrong ... what do you get if you actually breadboard the design?
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    re: returning Ref to V/2 ... Super! That agrees with the example in the data sheet.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I've no idea. I've never had/used an AD621.
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
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    Very pricey, about $10 a piece. Bipolar input. Well less than 1 MHz frequency range. I would need a good reason to justify it. Low noise, low CMRR, precise gain?
    Is it really needed in this design?
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Only you know what precision, linearity and stability you need.
     
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