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ADS1292R: Measurement of human impedance is not accurate

XiaoMing Wang
Prodigy 40 points
Part Number: ADS1292R
Other Parts Discussed in Thread: ADS1291, ADS1292, ADS1298R

I need to measure human impedance when using ADS1292RIPBS chip recently. I use resistors and capacitors in parallel to simulate human impedance.The results show that different capacitance values have an effect on the measurement results.As it turns out, the capacitance changes the phase of the breath demodulation control signal I originally set.The reading is correct by changing the phase offset value in the RESP1 register. But the actual situation does not allow to modify the register, could you please ask the relevant engineer of your company to answer it?Thanks.

  • 0
    TI__Mastermind 19815 points

    Hi,

    Highly appreciate for using TI's products.

    This ticket has been received/viewed, and I will get back to you between 3~5 days.

    May I understand your question more clearly -

    Are you trying to change/write value into RESP1 register in ADS1292R, but the ADS1292R doesn't allow you to do so?


    According to "ADS129x Low-Power, 2-Channel, 24-Bit Analog Front-End for Biopotential Measureme ads1292r", 
    This(RESP1) register applies to the ADS1292R version only. For the ADS1291 and ADS1292 devices, 02h must be written to the RESP1 register.

    Would you please double check which Product/Part number you are using?

    If possible, wondering if you could provide a bit more detail what error/warning or response you encountered when you try to modify/write/change the RESP1 register?

    And, in the meantime, let me circle back with our engineer team.

    Appreciate.

  • 0 in reply to ChienChun Yang
    Prodigy 40 points

    Our company is going to measure human impedance through the CHIP ADS1292.The impedance of the human body is the impedance value of the parallel connection between the resistor and the capacitor and the capacitance value changes.I have tested and found that different capacitance values will change the phase of the modulated signal.It will lead to different impedance values measured by ADS1292 under different capacitance values.Is there any way to solve this problem?Thanks.

  • 0 in reply to XiaoMing Wang
    TI__Mastermind 19815 points

    Hi,

      Would you mind sharing a bit/portion of your schematic, indication which capacitors you are referring to, and the phase difference you observed? 

    Or, if the similar circuit(e.g. capacitors) you are working on is showing in the datasheet, would you please point out?

    www.ti.com/.../ads1292r.pdf

  • 0 in reply to ChienChun Yang
    Prodigy 40 points

    As shown in the figure, RT is the resistance and CT is the capacitance. I used ADS1292 to measure the impedance of RT in parallel with CT.For example, when RT=100 ohms, CT=1nF,ADS1292 reads 100 ohms,But when I changed the capacitance of the CT to 47nF, I read 9 ohms.Because the ADS1292 measures impedance using a square wave modulated a sinusoidal wave method.The phase difference between square wave and sine wave will affect the measurement result.Different capacitance values measured will result in different phase differences.The ADS1292 can only initially set the phase difference to a fixed value through the RSP1 register.You can't calibrate the phase difference by yourself. Is there any way to solve this problem?Thanks.

  • 0 in reply to XiaoMing Wang
    TI__Mastermind 19815 points

    Hi,

    Do you have/use a eval kit/board?

    May I ask which frequency is selected? And, Internal or external frequency?

    The respiration function may not suitable for such low/small impedance measurement as human body impedance is much higher.

    Could you try some pure resistance(e.g. pure 2k or 3k ohm or larger resistance) first, and then try some parallel RC combination that can yields larger impedance with respect to the selected frequency, e.g. impedance 2k to 15k ohm, as mentioned in the page 63 in https://www.ti.com/lit/ds/symlink/ads1292r.pdf?ts=1649786937114&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS1292R

    "Standard requirements are respiration impedance values ranging from 2 kΩ to 15 kΩ, modulation clock frequencies of 32 kHz or 64 kHz, and noise levels of less than 10 µV"

    Also, page 63, https://www.ti.com/lit/ds/symlink/ads1292r.pdf?ts=1649786937114&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS1292R

    "When the modulation frequency is 32 kHz, gains of 3 and 4 and phase of 112.5° and 135° are recommended. When the modulation frequency is 64 kHz, gains of 2 and 3 and phase of 135° and 157° are recommended for best performance."

    Thanks

  • 0 in reply to ChienChun Yang
    Prodigy 40 points

    Hi,

    I am currently using the internal part frequency with a frequency value of 512K, an impedance sampling frequency of 32kHz, and a phase of 112.5°.After testing, different capacitance values of capacitance CT will still change the measurement results. Because the human body's capacitance changes, we can't measure the correct result.Our product is used in AED products, and the range of human impedance to be measured is 10Ω ~200 Ω. Do you have a better plan?Thanks.

  • 0 in reply to XiaoMing Wang
    TI__Mastermind 19815 points

    Hi,

    The device was designed for respiration impedance, letter R stands for Respiration; not for general human bioimpedance.

    Could you try the following to see if the measurement work properly in the range specified in the datasheet -

    Could you try

    some pure resistance(e.g. pure 2k or 3k ohm or larger resistance, potentiometer) first, and then try

    some parallel RC combination that can yields larger impedance with respect to the selected frequency, e.g. impedance 2k to 15k ohm, as mentioned in the page 63 in https://www.ti.com/lit/ds/symlink/ads1292r.pdf?ts=1649786937114&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS1292R

    "Standard requirements are respiration impedance values ranging from 2 kΩ to 15 kΩ, modulation clock frequencies of 32 kHz or 64 kHz, and noise levels of less than 10 µV"

    Also, small impedance may incur large current that's out of the spec that could damage the device.

    Side note, for good general bioimpedance measurement, you may want to try tetrapolar bioimpedance measurement.

    You may also take a look of ADS1298R to see if it fits your need.

    Hi ,

     Do you have/use an ADS1292R EVM?

    Thanks.

  • 0 in reply to ChienChun Yang
    TI__Mastermind 19815 points

    Hi,

    This ticket/thread has been pending for more than 1 week.

    I am closing this ticket/thread.

    Please open another ticket/thread if there is question.

    Thanks