Problem on amplifying signal in INA2126

Discussion in 'The Projects Forum' started by JotaTR, May 6, 2013.

  1. JotaTR

    Thread Starter New Member

    May 6, 2013
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    0
    Hi guys,

    I have bought 5 INA2126 one month ago. I am trying to use them in order to amplify a signal from thermocouples.
    First i am testing the OpAmp and i am getting some results that i dont understand:

    I put a resistance od 1400Ohms in order to amplify the signal from a voltage source (65mV). I feed the opAmp with a power suplly imposing 10V. I have confirmed the connections and they are correct (as it says on the datasheet). The output that i get is alway a litle bit less than the supply power (for 10V i get 9.4 instead of 4.04V that i was supposed to get)

    Vin- (pin 1) and Vin+ (pin 2) are connected do the 65mV voltage source.
    RgA (pin 3) and RgA (pin 4) are connect by the 1.4kOhm resistance.
    RefA (pin 5) is connected to the negative supply voltage
    SenseA (pin 7) is connected to VoA (pin6).
    V- (pin8) and V+ (pin 9) are connected to the negative and positive suply voltages.

    What am i doing wrong? Is there any advise that you can give me?

    Thanks
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I guess that the problem is that refA sholud be tied to ground, not negative supply.
     
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  3. wayneh

    Expert

    Sep 9, 2010
    12,157
    3,064
    Do you mean you are applying a differential voltage to pins 1&2, or that they are both seeing the same +65mV?

    Assuming it's differential, how is the power supply of the op-amp connected to the supply of the thermocouple? I'm guessing there is no common ground?

    The voltages applied to the op-amp input pins need to be referenced to the power supply of the op-amp. They cannot float above or below the power rails.
     
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  4. JotaTR

    Thread Starter New Member

    May 6, 2013
    5
    0
    Thanks for the fast response.
    I never used instrumental differential amplifiers and they seem more complicated than the normal ones.

    I am applying to pins 1&2 a differential voltage i suppose. I put the red wire (+) into the pin 2 and the black wire (-) to pin 1.
    When you say "supply of thermocouple" your are talking about the 65mV imposed voltage right? The common ground shoul come from power source? i have there a 3rd output wich is the ground. Should i connect the ground output to the breadboard and use it as my ground?

    How do i reference the voltage applied to the inputs to the power supply of the op-amp? using the op-amp reference pin and Connecting it to the ground coming from the voltage source (op-amp power supply)?
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Definitely, and as I said before, the refA needs to be going to this ground, not to negative supply.
    You can ground one of the input pins for example.
     
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