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ADS1299 - AC lead off detection - reference human body impedance

Ashutosh Kulkarni
Expert 1005 points
Other Parts Discussed in Thread: ADS1299

Hi,

I am working on AC lead off detection in ADS1299.

I am simulating skin electrode contact model , combination of 51K resistor and 47nF capacitor to check AC lead off detection. With this I am able to see difference is measured magnitude of excitation signal when leads are OFF.
 We are currently setting a threshold to declare this lead-off, but this is with respect to the skin electrode contact model.

I want to know how this threshold of impedance / magnitude can be set to declare lead-off or leads are not properly connected.

When leads are OFF, the magnitude will approach near Vref, but what if sensors are not properly connected? In that case I need to set some threshold.

Is there any documentation / study available which lets know about the average human impedance, as I assume the 51K resistor used in skin electrode contact model is the human impedance based on some study at TI.

Thanks,

Ashutosh.

  • 0
    TI__Guru 66186 points
    Hello Ashutosh,

    Thank you for your question.

    While AC lead-off is intended to be used just like you describe, TI really does not have the expertise to tell you what impedance thresholds to set in your application. We have not conducted any private studies ourselves to model the human body or skin-electrode impedance.

    I can tell you that from what I've read, the internal impedance of the human body is in the range 500 to 1k ohms. Lead impedance measurements generally do not care much about this - what's more important is the skin-electrode impedance, which can vary widely from 1k to 1M ohms or more. The wide range of skin-electrode impedance stems from the differences at the skin surface (i.e. moisture, hair, electrode surface area, etc.). Over time, even wet electrodes that use a special gel can dry out. Clinicians will usually set a threshold for their specific system (i.e. 100k ohms) beyond which they deem the electrode contact quality to be insufficient.

    I hope this helps!

    Best Regards,