ADPD4100 Respiration Rate Monitoring

The ADPD4100 is listed as a featured product for Respiration Rate Monitoring on the following Analog Devices website:

https://www.analog.com/en/applications/markets/healthcare-pavilion-home/vital-signs-measurement/respiration-measurement.html

However, I cannot find much other information about how to perform respiration monitoring with the ADPD4100 on the Analog Devices website or elsewhere. 

Can I please get some guidance?

Are components besides the ADPD4100 needed for respiration rate monitoring? Is the AD5933 (which is also listed as a featured product) needed?

Are there any examples or figures for the circuits / programming needed available?

-Brian

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  • +1
    •  Analog Employees 
    on Jun 18, 2021 12:31 PM

    Hi Brian,

    The ADPD4100 respiration measurement is based on impedance change of the chest during the respiration cycles.  A reference circuit can be found in Figure 5(a) in this article Multiparameter Vital Signs Monitoring Is Easier Than Ever Before | Analog Devices. In this circuit, the excitation is from GPIO2 and excitation current is applied to the body through the RC circuit (GPIO2-10k-1nF-electrodes/body-1nF-10k-GND). IN7 and IN8 form a differential pair to measure the voltage across the two electrodes (body). 

    The components needed for this design are the resistors and capacitors to form a path from the GPIO to ground for applying the excitation to the body.

    Regards,

    Glen B.

  • Thanks for the information, Glen!

    Follow up question - there are many options to select from for GPIO pin output (snippet of options for GPIOOUT1, ADPD4100 datasheet pg 87, shown below). Which of them would be used for respiration rate?

  • This is perfect - thanks, Glen! This really helped.

  • Hello Glen, do you mind explaining the theory of operation a bit here? From what I can tell, the excitation current waveform is a periodic pulse with a width of 3us and period of 20us. How should impedance be accurately extracted from the measured voltage response? Do you have any equations for this? Some background information and an explanation of the theory of operation would be much appreciated!

  • 0
    •  Analog Employees 
    on Jun 25, 2021 4:10 PM in reply to MX_bwuerstle

    Hi Brian,

    The RC circuit (GPIO2-10k-1nF-electrodes/body-1nF-10k-GND) serves as a voltage divider. The 1nF coupling capacitors are for sharing electrodes with ECG. If you measure only the respiration, the 1bF capacitors are not needed.

    During respiration cycle, the body impedance changes and so does the voltage across the body/electrodes. This voltage signal is measured by the ADPD4100 where the current limiting resistors are optimized to allow appropriate current input to the TIA.  

    The exact waveform of the differential voltage can be complicated due to variations in human subject and the skin-electrode interface, although it can be simulated with simulator such as LTSpice to incorporate the decoupling capacitors, body impedance and the skin-electrode interface model. 

    The first order of approximation of the body impedance can be calculated from the ADC output as below:

     

    Regards,

    Glen B.

  • Thanks! However, it is still not clear to me why a pulse is used as the excitation waveform. All the literature that I've found on pulse-based impedance measurements are related to wide-band impedance spectroscopy which requires FFT analysis. Is this true for the example you've provided, Glen?

  • 0
    •  Analog Employees 
    on Jun 28, 2021 6:11 PM in reply to MX_bwuerstle

    Hi Brian,

    Subcutaneous fat is typically modeled as a capacitor (or a capacitor in parallel with a big resistor). Depending of the electrodes of choice, the skin-electrode interface has both resistive and capacitive components. To be able to stimulate the deep tissue, pulse as excitation waveform is easy to implement. For the impedance spectroscopy technology, the FFT analysis gives complete information -  the amplitude and the phase information of the data. Hence you can separate the resistive component and capacitive component. For FFT analysis, you would need an ASIC with the FFT engine (such as the AD5940 from ADI) or run the FFT as data processing.

    The example we provided doesn't require FFT. It can achieve decent respiration measurement though.

    Regards,

    Glen B.

Reply
  • 0
    •  Analog Employees 
    on Jun 28, 2021 6:11 PM in reply to MX_bwuerstle

    Hi Brian,

    Subcutaneous fat is typically modeled as a capacitor (or a capacitor in parallel with a big resistor). Depending of the electrodes of choice, the skin-electrode interface has both resistive and capacitive components. To be able to stimulate the deep tissue, pulse as excitation waveform is easy to implement. For the impedance spectroscopy technology, the FFT analysis gives complete information -  the amplitude and the phase information of the data. Hence you can separate the resistive component and capacitive component. For FFT analysis, you would need an ASIC with the FFT engine (such as the AD5940 from ADI) or run the FFT as data processing.

    The example we provided doesn't require FFT. It can achieve decent respiration measurement though.

    Regards,

    Glen B.

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