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Problem with TI's ECG front-end

Discussion in 'The Projects Forum' started by filo123, Jun 4, 2012.

  1. filo123

    Thread Starter New Member

    Jun 4, 2012
    I am building a simple 3-lead ECG for a university project and I don't get the desired ECG signal. I started with a reference front-end from TI (paper SLYT226) that looks like:

    My circuit:
    the only not exact components are R4 R5 (40,2k instead of 40k) and R9 (22k instead of 20k).

    The circuit is built on a two sided PCB, all traces kept short, resistors and capacitors 0805 SMD 1%, both ground and power planes, at least one 100n cap at every chip. I tried powering it with 5V both from batteries and USB (I know.... noise).
    That what the layout looks like (I omitted GP&PP for clarity):

    That is my output TP1:
    When I "zoom in" I see only 50Hz spikes.

    That is the signal from INA326 (TP2):
    The lower envelope is my ECG signal.

    And the signal from the integrator output (TP4):

    The ECG leads are about 35cm long (single wire). I use commercial 3M electrodes (I get about 300k chest resistance measured with a multimeter).
    What am I doing wrong? Or is something known wrong with this reference front-end?
    Thank you for your answers.