Help with ECG circuit

Discussion in 'The Projects Forum' started by monty_mm, Nov 23, 2008.

  1. monty_mm

    Thread Starter New Member

    Nov 4, 2008
    Hello there,
    i'm trying to do this ECG circuit (hobbying around).

    What i have so far is the input fed to an optocoupler(it is amplified before the opto), and then i try to amplify it, but for some reason the opamp would not amplify large enough for me.

    The output i get is hardly 0.1V p-p and i'm trying to get a larger voltage to feed to a PIC18. i tried changing the resistor values on the opamp but i just couldnt get a higher voltage! Can someone help and see if i've gone wrong somewhere? i've attached a portion of my circuit. Any help is appreciated. Thanks.
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Why the optocoupler? Are you certain that the output is linear?

    Your op amp has a gain of about 1/2. Change the feedback resistor to 100K and use 1K or so to ground to get the gain up.
  3. monty_mm

    Thread Starter New Member

    Nov 4, 2008
    Thanks for your reply.

    I used the optocoupler because i want to feed the output to a PC via USB (using a PIC18).

    i tried changing the resistor values as per your suggestion but it seems there is no effect, the output is still the same as before (about 0.1p-p). i dont understand why its doing this.
  4. mik3

    Senior Member

    Feb 4, 2008
    Increase the gain as beenthere suggested but to make it work connect a 1K resistor between ground and the non inverting input (pin 3, according to the diagram). To be able to vary the gain make the 100K resistor variable.
  5. Audioguru


    Dec 20, 2007
    I think i answered this thread on another website.

    1k is far too low for the tiny current from a photo-diode. Use 100k or 1M.
    Get rid of Q2 and R9.