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ADS1292R for 4-electrode system?

Katariina Tuohimaki
Prodigy 100 points
Other Parts Discussed in Thread: ADS1292R

Dear all,

It is possible to use ADS1292R to measure ECG and bioimpedance with 4 electrodes where 2 would be for current injection and another 2 for voltage detecting? I think I found some reference to this but I am not sure. Datasheet gives examples just for 2-electrode systems.


So, is 4-electrode system possible? Is there any example existing of using this chip with 4 electrodes?


Thank you!

  • 0
    TI__Mastermind 38745 points
    Hey Katariina,

    You could use 4 electrodes if you wanted to for the respiration impedance measurement. The only change from what is given in the datasheet is instead of connecting the RESP_MOD lines to the measurement lines, just feed them to the patient as their own electrodes. If you want to better understand for the respiration measurement on the ADS1292R works, check out this app note: www.ti.com/.../sbaa181.pdf

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 100 points

    Dear Brian,

    thank you for your quick answer and helpful website. I studied two examples given in datasheet and ended solution show in PDF (IN1 for bioimpedance, IN2 for ECG). Would you have any furter change recommeds for this? I was not sure if AVDDs for respiratory inputs were needed. Maybe some capasitors are also unneeded.

    I also took a look to evaluation board designed for ADS1292R. Is it true, that I am not able to use this kit for 4-electrode system, when it is specially designed for 2 electrodes? There are not extra connectros for user´s own applications, and specific ECG cable is required?

    Finally, I am just making sure that I am able to measure ECG and bioimpedance (respiratory rate) at the same time with this chip. Kit´s user guide gave option either ECG or bioimpedance. Was this just for display settings?

    It would be very helful to have answers to these follow-up questions.

    Thank you!TI, proposal settings for 4-electrode system.pdf

  • 0 in reply to Katariina Tuohimaki
    TI__Mastermind 38745 points

    Hey Kateriina,

    Katariina Tuohimaki said:
    Would you have any furter change recommeds for this?

    You'll need to make a few changes. I attached a diagram of what I had in mind below. Note that instead of ac coupling the channel 2 inputs, you could use an RL electrode connected to the ADS1292R's integrated RLD buffer. Let me know if something in the diagram doesn't make sense.

    Katariina Tuohimaki said:
    I also took a look to evaluation board designed for ADS1292R. Is it true, that I am not able to use this kit for 4-electrode system, when it is specially designed for 2 electrodes? There are not extra connectros for user´s own applications, and specific ECG cable is required?

    Yes that is true. The EVM was designed to measure respiration impedance from the RA to LA electrodes. You can use any electrode you want, but keep in mind that adding impedance from the calbe will make it more difficult to resolve breathing impedance.

    Katariina Tuohimaki said:
    Finally, I am just making sure that I am able to measure ECG and bioimpedance (respiratory rate) at the same time with this chip. Kit´s user guide gave option either ECG or bioimpedance. Was this just for display settings?

    Yes you just measure them individually using the two simultaneous sampling channels.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    TI__Mastermind 38745 points

    ForumDrawing.pdfSorry the diagram didn't attach. It's attached to this post.

  • 0 in reply to Brian Pisani
    Prodigy 100 points

    Thank you for your answer.

    Your drawing was very clear and helpful at some points. The only problem is, that I am really willing to avoid polarization and extra impedance problem in bioimpedance measurement. That is why, it would be very important for me to have current injection separated from voltage detection. So my goal would be to have four electrode system, where 2 are for current injection and another 2 for impednace and ECG measurement (even if they would after that go to different inputs). I would like to avoid impedance to be measured from same electrodes which current is injected. Is this possible? I am sorry about my unclear figure.

    If your opinion is that my idead would not work, I think that maybe 2-electrode system is then better. ECG should not be so affected by extra impedance - just respiration, and I am willing to avoid any extra wires. This is also why I am not going to apply RLD or any other drivers at this point.

    I really appreciate you quick answers even for larger questions!

    BR,

    Katariina

  • 0 in reply to Katariina Tuohimaki
    TI__Mastermind 38745 points
    Hey Kateriina,

    They really need to be on the same wire. A great way to think about it is that the channel 1 inputs measure the voltage of a voltage divider created with the RESP_MOD outputs as the source.

    I agree that a 2-lead system would be optimal. I've personally been able to take respiration and ECG data simultaneously using this feature before.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 100 points

    Dear Brian,

    I am a little bit confused. Please, only for this last time, give me small conclusion.

    From your first answer, I understood that it is possible to separate current injection for its own electrodes (and measure ecg and impedance from another pair): "The only change from what is given in the datasheet is instead of connecting the RESP_MOD lines to the measurement lines, just feed them to the patient as their own electrodes." Also app note added in the first answer succest 4-electrode system as one possibility.

    From your the lastest answer and measurement setting picture you sent, I understood that (in case of impedance) current injection and voltage detection cannot be separated: "They really need to be on the same wire."

    So which conclusion is the right one, and why? Because of my english, I would ask you to be as clear as possible :)

    Thank you for your answers!

    BR, Katariina

  • 0 in reply to Katariina Tuohimaki
    TI__Mastermind 38745 points
    Hey Kateriina,

    Injection and measurement need to happen on the same line for impedance measurement, but the ECG measurement line can be completely different. Note that in the diagram I created, the injection and measurement for impedance appear on the same electrode but the ECG measurement electrode is different. That I what I meant when I said they could be on different electrodes. The impedance measurement force and sense, however, MUST be on the same electrode.

    However, you could have all three on the same pair of electrodes if you wanted. That is to say, the impedance force and sense lines and the ECG Lead I lines could be connected to the same pair of electrodes. The ECG one is the only one that may be separated.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 100 points

    Very much thank you Brian! This was very helpful and now I know my options.

    BR,

    Katariina

  • 0 in reply to Katariina Tuohimaki
    Prodigy 100 points
    Dear all,

    I am happy to inform you that tetrapolar measurement is possible with ADS1292R, and my first proposal with little modifications (suggested by my friendly collegues) works nicely. Also demonstration kit can be modified for four electrodes.

    BR,
    Katariina