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ADS1298R impedance measurement?

ozyusa
Prodigy 40 points
Other Parts Discussed in Thread: ADS1298R

Hi, 

I have some questions about the operation  principles of ADS1298R. 

1) What is the impedance measurement range?

2) Can we use it with four electrode method? If yes, how is it measued?

3) Is it available?

 4) Can we measure real and the imaginary part of the impedance? 

  • 0
    TI__Mastermind 18285 points
    Hello,

    I am moving your post to the correct forum.
  • 0
    TI__Guru 66466 points

    Hello Ozyusa,

    Welcome to our forum.

    The ADS1298R uses an AC excitation signal to detect the change in impedance across one's chest during respiration. It is not just a DC 4-wire impedance measurement with paths for "force" and "sense." This app note can better explain how this technique works: www.ti.com/lit/an/sbaa181/sbaa181.pdf.

    Can you tell us more about your application? What is the impedance that you are trying to measure?

  • 0 in reply to Ryan Andrews
    Prodigy 40 points
    We are trying to measure respiration by looking at the impedance change of the thorax. We are going to use four electrode method by injecting AC current and measuring voltage. Theoretically, skin contact resistance plus internal resistance of the thorax is about 2K so that we have to measure an impedance around 2K. We also would lile to look at the real and the imaginary parts of the impedance. Can ADS1298R is useful for these purposes. Thanks.
  • 0 in reply to ozyusa
    TI__Guru 66466 points

    In that case, yes, the ADS1298R would be a great fit to measure thorax impedance. To calculate the real and imaginary parts of the impedance, I believe you would have to determine the phase between the excitation signal (RESPMOD) and the voltage that you measure. The ADS1298R does not do this automatically, so we would have to come up with a way to accomplish that. My initial thought was to dedicate one channel to measure the RESPMOD output directly. Does that seem feasible? How accurately do you need to be able to measure the imaginary part of the impedance?

    To address your other questions, the ADS1298R uses the reference voltage VREF to generate the modulation square wave. The example application we provide in our datasheet uses 40k resistors in series with the RESPMOD outputs. Assuming a baseline body impedance of 2k and a 2.4-V VREF, the expected current in the respiration path would be 2.4V / 82k = 29.3uA, which was enough to measure impedance changes ranging from 0.1 ohms to 1 ohm. A 4-electrode method can be used for the impedance measurement if space and cost allow. Also, know that the same 2 electrodes used for the respiration measurement can still be used for ECG simultaneously.

    Best Regards,