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DC Restorator for an ECG circuit, please help

paul miller44746
Intellectual 540 points
Other Parts Discussed in Thread: LM324, INA126

All,

I am attempting to make an ECG circuit, and I am following the TI application note for the most part.  The amplifiers I am using are somewhat different.

 

I am using an INA126 and an LM324 for intrumentation amp and op-amps.  

I am having several problems the first and most troublesome of which is the DC restorator not functioning. I thought it might require more current than the LM324 could supply, so I tested using a power supply on the Vref pin of my INA126 to cancel out the offset caused by my inputs, and it was only pulling mircoamps to do this, so that ruled that out. I then tested the current flowing through the 3.2Mohm resistor, and current is flowing, but the capacitor is not charging at all. 

I then pulled the DC restorator out of the ECG circuit and attempted to test it with just it there.  I set it up with a 1V DC input into a 3Mohm resistor to the inverting input, with a 1 uF capacitor in feedback, and the non-inverting pin grounded.  I checked the output, with the expectation that I would see it gradually moving toward saturation.  However, the output simply goes to around 1.3V and stays there.  (this was with +/- 10V supplies).  At this point, I checked if there was still current flowing, and there was around .333uA flowing, (this corresponds to what could be expected for the voltage drop on the resistor), but the capcitor was not charging.  Am I doing something incorrectly to test this?

Please assist.

Paul. 

 

  • TI__Expert 7185 points

    Paul,

    I know that you said your are using a TI application note for this, but to be able to help you we really need you to post an exact diagram of your ECG system and annotate on the diagram the node voltages and currents that you are measuring.  Also, please show all power supply connections and detail the input source (if any) that you are using to test this circuit.

    Thanks,

    Matt

  • in reply to Matthew Hann
    Prodigy 10 points

    ECG a is technique for recording bioelectrical waves of the heart. This helps clinicians assess and diagnose the stability of a patient’s heart. This has a digital approach that tend to gives the infinite impulsive response,

                Moreover these tools can be also used in other biomedical signal processing application such as MRI Magnetic Resonance Imaging and EEG Electroencephalography

     

  • Prodigy 50 points

    The values of the components such as feedback capacitor should be calculated careful. The range of errors needs to be estimated. It seems something wrong with the calculation.