Arduino ADC, Wheatstone bridge and amp

Discussion in 'Embedded Systems and Microcontrollers' started by ibrahim abukharmeh, Dec 17, 2015.

  1. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
    13
    0
    Hello,
    I have the following circuit, it works with simulator and manged to breadboard it and it works as well on the bread board, note that R6 is Pt100 RTD when prototyping on breadboard, the values of it should Vary from (100 to 138.51) ohm.
    Amp Circuit.png
    , if i calculated the transfer characteristic using Tina, that's what i get
    TF.png
    now, i am trying to read the output of the amp using arduino adc, but its not working, its giving random values, not as the predicted one, although that if i measure the output of the amp and the ground from the circuit previously attached using multimeter, it works. my question is what could affect the arduino internal ADC to cause such problem, although that the adc worked, since i have tried to do voltage divider and input to arduino and it works. will i gain anything if i used external adc, will it solve my issue.

    Many Thanks for help

    Mod Note: Moved to Embedded Systems
     
    Last edited by a moderator: Dec 17, 2015
  2. kyka

    New Member

    Jun 7, 2015
    22
    1
    Do you use the arduino reference voltage pin?
     
    ibrahim abukharmeh likes this.
  3. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
    13
    0
    i tried to use it, but honestly have no previous experience of using it, what my reference voltage should be,
    like should i use 7805 regulator from my 12 volt source and input it to the arduino, or what should i do
    hence what i am gonna gain if i inputted 5 volt to it, since its default 5 v
    or should i use the ground of my circuit which is basically between two 6 volt supply as reference
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    Have you verified the voltage output on U2-6 is what you expect?

    Generally an A2D needs a fixed precision reference or any reading will vary as does whatever you do use as a ref.

    Someone with Arduino experience will need give you details as that device works, but I am sure you can find t,ons of info and tutorials on the interwebs.
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,804
    833
    Which model of the Arduino are you connecting it to? The default reference voltage is the Arduino's supply voltage, which varies from model to model.

    You have to define the reference voltage using the analogReference keyword. Note that an external reference is limited to 0-5VDC.

    How are you connecting your circuit to the Arduino? Hopefully, they have shared common. The + side of VM3 should go to yo0ur analog pin, and the other side should go to ground on the Arduino. Can you share a schematic of that connection?

    Also, it might be helpful if you posted your Arduino code.
     
    ibrahim abukharmeh likes this.
  6. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
    13
    0
    i forget to share the ground, mostly that my mistake, do i need to connect the ground from my source to the gnd pin of my arduino, or do i need to power the whole arduino from the same source ,
    the way i was connecting was just connecting the out from the amp to A0 from arduino and nothing else.
    and my arduino model is UNO R3 Chinese reblica
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    "Ground" can mean many many things so I will not say yes or no without seeing at least a block diagram, but any and every voltage measurement needs to be done across TWO points.

    So if your sensor/op amp runs off one supply and your micro off another supply then you need two wires connecting these two modules. The ground symbol on you above schematic should connect to the similar point (zero volts) on the Arduino board.

    Using two separate supplies is fine as long as the zero volt point (what I will call ground) can be connected safely.
     
    ibrahim abukharmeh likes this.
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,804
    833
    Not connecting ground between your circuit and the Arduino is definitely causing your problem.

    I mentioned in a previous post where the two connections should be made. See post #5
     
    ibrahim abukharmeh likes this.
  9. BramLabs

    Member

    Nov 21, 2013
    98
    5
    Sir Ibrahim Abukhharmeh, i want to ask, why you choose 4.1k ohm and 3.1k ohm as your wheatstone bridge ? Doesn't it make self-heating ?
    Thanks
     
Loading...