Need help to understand ecg circuit .. Really urgent

Discussion in 'The Projects Forum' started by albert, Jun 22, 2009.

  1. albert

    Thread Starter New Member

    Jun 22, 2009
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    Hi every one
    i need help with circuit that attached to this message , i built this one and it workig very good but i cant undersatand the circuit , specially the roll of "ic2a" opamp... I realy need advice asap. Thanks in advance.
    Albert
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    How do you expect us to figure out what it's supposed to be doing without giving us the purpose of the circuit?

    Why don't you post a link to the site where you found the circuit?
     
  3. albert

    Thread Starter New Member

    Jun 22, 2009
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  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Was this written by the patient or the person who built the circuit?
     
  5. albert

    Thread Starter New Member

    Jun 22, 2009
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    person who built the circuit
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Did you ask for his theory of operation?
     
  7. dummhuman

    New Member

    May 11, 2009
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    I could be way off base here, but it looks to me like that opamp is acting as a kind of noise reducer; I believe it is there to reduce certain (high?) frequency common mode AC signals (noise?). If you look at how the opamp is connected, it is comparing the average of the outputs of two other opamps with a DC reference, and the feedback is AC coupled, it is also notable how the average being compared is coupled to ground through a higher value capacitor than is on the feedback loop, thus reducing IC2A's response to low frequncies. I hope this is of some help, but I am just an engineering student, so this could be far off base.

    Good Luck,
     
  8. albert

    Thread Starter New Member

    Jun 22, 2009
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    yes .. Thanks
     
  9. albert

    Thread Starter New Member

    Jun 22, 2009
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    thank you !! I think you write here ?
    I understand that c2 is noice reduce , but why c1 .. Its act like integrator ? What do you think ?
     
  10. dummhuman

    New Member

    May 11, 2009
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    After looking at this circuit for a while longer, there are two possibilities, of which I now believe the second to be true, because of the capacitor C2, which appears to provide a short to ground for common mode AC voltages.

    *WAG*1) that C1 is there to provide AC feedback to IC2A, as well as to (maybe) isolate the output from VDD; if this is the case, IC2A is helping to get rid of common mode AC.

    *likely*2) C1 and R9 are an integrator used for getting rid of common mode (low frequency or DC) voltages. (They makeup an RC filter with corner frequency 1.6HZ).

    If I were you, I would build the circuit, without IC2A or any associated circuitry, observe the output of the instrumentation amplifier, then put in IC2A and observe the output, this should show you whether it is helping with DC or AC common mode rejection. Alternatively, you could do it in SPICE, and observe the results, and hope it's accurate, because SPICE can lie (if the models are not accurate for your application).

    ***Upon re-reading my post before putting it up, I think only the second guess makes sense, IC2A is used for DC common mode voltage elimination.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    Could it be that IC2A is used to monitor DC drift of the input amplifiers and compensate for it. Or even to compensate for any DC offset generated by body chemicals reacting to the "electrodes" unevenly.
     
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