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ECG Amplifier using INA321 and OPA336

Shrenik Vora
Prodigy 130 points
Other Parts Discussed in Thread: INA321

I am trying to make a simple ECG amplifier using INA 321 and OPA 336. I am driving this amplifier using a an ECG simulator(http://www.heinstruments.com/ecg-simulator.htm), hence I do not have a right leg drive circuit.
My schematic is attached. I have made the circuit on a breadboard for now, the problem that I have is that it works for a few seconds and then stops. The problem seems to be with the INA321 and the feedback opamp used for the reference. I have attached a snapshot of the scope that shows the output turning off.

Any ideas on why this might be happening?

ECG circuit issue.zip
  • 0
    Prodigy 130 points

    Does anyone have any ideas on this?

  • 0 in reply to Shrenik Vora
    TI__Genius 12325 points

    Shrenik,

     I don't immediately see anything wrong with your circuit, have you verified that the power supply is not shutting down after a few seconds? It's hard to tell from your oscilloscope screen shot but is the output voltage centered around proper voltage (2.5V)? Also, does this behavior occur if the circuit is not connected to the ECG simulator? 

  • 0 in reply to John Caldwell
    Prodigy 130 points

    Yes, the output is centered about 2.5V and the power supply isn't shutting off. In fact, I am using a split power supply to provide 2.5V and 5V separately instead of using the voltage divider.

    The flatline that you see at the end of the scope screenshot is about 2.46V. This behavior occurs even without the the ECG simulator when the circuit is connected to lets say fingers of right and left hand. In the absence of any input, the output stays at about 2.5V. 

    I have even tried connecting the SD pin to VCC, to ensure that the IA doesn't shutoff. 

  • 0 in reply to Shrenik Vora
    Prodigy 130 points

    I tried connecting the SD pin to VCC again and the circuit seems to work now. I don't know what was wrong the last time I did it.

    Shrenik