Instrumentation Amplifier keeps dying out

Discussion in 'The Projects Forum' started by msrav1, Sep 26, 2008.

  1. msrav1

    Thread Starter Member

    May 26, 2008
    I am a student and I am trying to build a heart rate monitoring system.

    I am having problems with an INA118 instrumentation amplifier, I work on it for a few hours and then when i come back to the next day the amplifier does not work anymore, it keeps going to the rails.

    It requires negative and positve supplies and since it can take up +-18V, i am feeding it with 2 9V batteries connected together so that i get +9 and -9V. Does it require a regulated voltage?

    When I give my amplifier to my friends who is working on a different project but using the same amplifier, his system does not work, so the amplifier has definetily died out. I am not sure what I am doing that could be killing it, because my circuit is a simple circuit to test it, i feed sine wave signal of 20mvp-peak at 10hz, and set the gain to 20.
    I am not sure what the problem is, I am going to use a 5V regulator to supply it with regulated voltage.
    Help would be greatly appreciated, Thanks
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Can you post a schematic of your circuit?

    It would also be helpful if you could sketch the way you are connecting the function generator to the INA during your test.

  3. msrav1

    Thread Starter Member

    May 26, 2008
    I put 3.3mega ohm resistors in series with the signal generator because when I eventually measure ECG from my body, I will use these resistors in series with the ECG electrodes. The circuit after the INA118 is just a band pass filter.

    THank you for your time.
  4. Audioguru


    Dec 20, 2007
    The inputs of the instrumentation amplifier must have a DC voltage that is within the allowed input common-mode range. The DC voltages of your inputs could be any voltage since the signal generator (or the patient) is not connected to the IC's supply ground.

    You have its gain set to 10,000 or more.

    It is difficult for an instrumentation amplifier IC to drive a resistance as low as 1k. Use 10k with a 0.1uF input capacitor.
  5. jpanhalt


    Jan 18, 2008
    Check out the Analog Devices design manual here. (Click on the links to the Designer's Guide, 3rd edition.) You don't show how your reference pin is connected. That manual has an entire section on various ways to do that.

  6. msrav1

    Thread Starter Member

    May 26, 2008
    I connected Vref to the common voltage of my 2 battery connected +9V to -9V supply.

    So the problem is that the common mode DC voltage inputs is too high?
    So would I need to connect the return cable/-ve cable to the Vref input as well as the -ve input to the instrumentation amplifier? I'm a bit confused,

    OK thank you for the link, it is obvious I need to do some reading.
    Thank You.