• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 48 subscribers
  • Views 779 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1292R: Computing Patient Impedance

Daniel F
Prodigy 120 points

Part Number: ADS1292R

Hello,

Using the ADS1292R I am trying to measure patient impedance. Using the respiration circuit I see how to get the change in voltage to compute respiration rate using the derivatives. However I don't see how to compute the actual R(b) baseline patient impedance. Is it done in a different mode by sending in a different signal? 

Thanks,

Dan

  • 0
    TI__Guru 61332 points

    Hi Dan,

    The baseline impedance is the part of the impedance that doesn't change - the cable, electrode, and thoracic cavity. When the patient inhales, this creates a variable impedance and the total impedance is equal to the baseline impedance + the variable impedance. 

    If you haven't read this yet, it should help: http://www.ti.com/lit/an/sbaa181/sbaa181.pdf

  • 0 in reply to Alexander Smith
    Prodigy 120 points

    Thanks Alex. I did read that report. How do we actually compute the baseline impedance though? I think this may be a simple question, just one I'm not sure how to do with the respiration circuitry. I could turn on a test DC voltage and measure from there for a resistance, but I doubt that's the "correct" way to measure baseline impedance. Is there some approach using the voltage drop between the electrodes either on Ch1 or Ch2?

    Thanks for you help!

    Dan

  • 0 in reply to Daniel F
    TI__Guru 61332 points

    Hi Dan,

    So using equation 1 from the end of the app note, you can solve for the baseline impedance. 

    DC_V = (Rbody/(Rbody+Rcable+Retc))* (VREFP VREFM)

    Run the respiration measurement and take the midpoint which will be DC_V. For example, if your measurement was from figure 12 it would be around 0.01371. Input your reference voltage and cable impedance + Retc, (any other impedance in the signal path.) then solve for Rbody. This sum of the resistances in the signal path that do not change is your baseline impedance. 

    We typically recommend to keep this baseline impedance below 10k in order to limit the amount of noise in the respiration measurement, however for systems such as defibrillators, the key care-abouts change. 

    Another example of this can be seen in this article: https://www.planetanalog.com/author.asp?section_id=483&doc_id=565158

    This is doing a similar measurement, but with lead-off instead of respiration. Of course, it's still a current being driven though an impedance, so the theory is the same.