AD624 Maximum Gain

Discussion in 'General Electronics Chat' started by Dritech, Dec 1, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    5
    Hi all,

    The AD624 has a maximum pin programmable gain of 1000, but is this the maximum gain even if an external resistance is used?
    On simulation I managed to get a gain of around 6000 by using an external resistance of 15ohms. Will this work in prectice?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Not sure, but why do you want to have so much gain in a signle stage? All the offset errors and input currents will be exagerrated, and the bandwidth will be very small. I would suggest lower gain and multiple stages.
     
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  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Did you read the datasheet?
    There is a table and a formula for the gain:

    AD624_gain_specs.png

    Bertus
     
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  4. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the replies. @bertus , but the datasheet states that 1000 is the maximum for pin programmable gains and the formula does not state the maximum gain that can be obtained when using an external resistance.
    So the best option would be to use the pin programmable gain of 1000 and then use another op-amp for ampllfying the remaining 5000?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Perhaps the limit of 1000 is mentioned to garantee the stability of the circuit.
    Using the gain formula your 15 Ohms resistor would give a gain of 2667 times.

    Bertus
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Sorry my mistake, I meant to say around 2650. Thanks for your suggentions. I will set the IA with a gain of 1000 and then amplify the rest using a precision op-amp.
     
  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again.
    Most of the instrumentation amplifiers schmatics I see online have their inputs sourced from the same object (ex: bridge circuit, electrodes etc.)
    Is it a prectical to connect one input of the instrumentation amplifier to a fixed reference voltage and the other input connected to a sensor? (see attached image)

    [​IMG]
     
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