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ADS1298: Getting low impedance values for floating channels (not connected to the body)

Milad Faizollah
Prodigy 30 points
Part Number: ADS1298

Hi, I have a question around impedance measurement feature of ADS1298. We're using ac sources to measure the impedance of the channels. In our system, we shorted all the negative inputs of ADS together and used it as the reference electrode. The sample rate is set to 500Sps. In the impedance measurement mode, when electrodes are floating (not connected to the body), including the reference electrode, the amplitude of the signals received from some of the channels (random) are very small, which is in the range of e-15V. This results in measuring very low impedances (less than 1K) for the floating electrodes rather than having high impedance values for those channels! As soon as we connect the reference electrode to the body, the issue goes away and the measured impedance between the electrodes and the reference becomes correct. Does anyone have any explanation/idea on why we get very small values for the impedance of the channels when both the channel's positive input and the reference (channel's negative input) are not connected to the body?

Thanks!

Milad

  • 0
    TI__Guru 55152 points

    Hi Milad,

    Please see the calculating gain for RLD post in the BIOFAQ: https://e2e.ti.com/support/data-converters/f/73/p/772058/2855202

    You may not want short all of the negative inputs together, I would suggest trying the same experiment with fewer electrodes to derive the signal. The impedance measurement mode is not meant to measure impedance of a floating connection, that is what lead-off detection is for. The impedance that you are measuring would be from noise coupling onto the end of the electrode, and the resulting voltage drop across the lead.