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AFE4300: Measuring Impedance NOT using the on-board simulate load cell

user4816734
Prodigy 30 points
Part Number: AFE4300

Hello,

I am having an issue trying to measure the impedance across my tissue sample. I am trying to send a current through the BCM terminal J36and through my tissue, while recording the voltage it returns in the tissue using the BCM terminal J10. It is working fine when I am making normal tissue measurements. However, when I cut the tissue in half, the circuit appears to be open, and I should be seeing voltage values of 0V, but I am seeing values closer to 1.7V, which is the reference voltage. I am assuming this could be a feedback issue, because the circuit is theoretically open, yet I am seeing a voltage through the tissue.

This is frustrating because I am not seeing what I should be. I might be doing something wrong in the setup, but I cannot find any instructions of how to set this type of circuit up, without using the on board load cell. I am also unsure of which way the alternating current flows through the IOUT terminals. Would a diode ensure it flows one way? The only data sheets and guides are for BCM measurements using external devices like hand-to-hand meters, and that's not what I am doing. I am simply trying to measure the impedance across a tissue via the voltage received, and the output current sent through the tissue.

Thanks for your time!

-Alex

  • 0
    TI__Genius 12555 points
    Hi Alex,

    Welcome to e2e forum.
    When you cut the tissue in half, as you said the circuit will be open and with the VSENSE pins floating the output will be undefined.
    One workaround for this situation is to add a large resistor (~100KOhm) between the VSENSE pins. This large resistors gives feedback to the sense amp when the VSENSE pins are open.

    You may also add same resistors in the calibration path to make calculation symmetry.

    The direction of excitation current can be configured using ISW_MUX ( 0x0A) register. For e,g, if ISW_MUX = 0x0804 , this means IOUT1 is connected to output of OPAMP1 and IOUT0 is connected to negative input of OPAMP1. As current flows from input to output of OPAMP1, the excitation current flows as IOUT0 -> Body -> IOUT1.

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 30 points

    Prabin,

    Thank you for your quick reply! I want to elaborate on the first section of your response.

    When the VSENSE should be reading 0V, it is not, and I believe that is due to the feedback as you said. However, I do not want to be reading this feedback voltage. When the tissue is cut it should be an open circuit. Simply adding a resistor to the circuit between the voltage sensors would just force the current through that path, instead of getting stuck in the tissue and reading a weird amount of voltage.

    Additionally, I did already add a resistor in the current path in order to act as a "voltage drop" so I am not getting voltage values above 1.7V across large resistances.

    Thanks for explaining the direction of current flow!

    -Alex