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.

 

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.

 

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.

 

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.

 

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).

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.