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Different AFE4300 impedance measurments

Nissim Halabi
Prodigy 20 points

Other Parts Discussed in Thread: AFE4300

Hello,

I'm using the AFE4300-EVM for 4-wire impedance measurements, and getting different reading results with I/Q Modulation (and with AC rectification as well) on same sensing loads with different current load. As far as I understood changing the load on the current source should not change the read result as long as the cuurent source is not "saturated". Am I missing something?

For example:

a------R1---b----R2----c----R3--------d

Assume the values R1=R3=400ohm and R2=30ohm for the resistors. Points "a" and "d" are connected to IOUTs, and points "b" and "c" are connected to VSENSEs.

When changing R1 to 500ohm, I get different readings between the VSENSEs (points "b" and "c").

Thanks for your help.

  • 0
    TI__Guru 50036 points

    Hello,

    Can you confirm if you are using R1 and R3 to emulate electrode impedance? The Electrode impedance is not supposed to change between runs. You need to calibrate under the specific electrode impedance condition with the reference resistor.

    Though theoretically the voltage across Vsense is supposed to be insensitive to the change in R1 & R3, the sense circuitry has a signal sensitive input impedance (50k typical) that can affect the read result.  Hence it is necessary to calibrate.

  • 0 in reply to Praveen Aroul
    Prodigy 20 points

    Indeed R1 and R3 are used to emulate electrode impedance.

    When applying a 4-wire impedance measurement on the human body, the "electrodes' impedance"  does change.  In the case of what the AFE4300 sees, I consider the  "electrode impedance" as the impedance between the current electrodes (Iout) to the pickup electrode (Vsense), incl. contact with the skin. That is what a bio-impedance measurement reflects. Indeed, theoretically the voltage across Vsense is suppose to be insensitive to changes in R1 and R3. However, it seems that is does happen.

    Can I somehow calibrate the circuitry with respect to the above interpretation of  input impedance so that impedance measurements between Vsenses by the AFE4300 be consistent?

  • 0 in reply to Nissim Halabi
    TI__Guru 50036 points

    I have attached the document that details the calibration procedure and impedance measurement using AFE4300.

    8311.SBAA202-17Jan.pdf

    What is your application of bio-impedance? In your application, how frequent does the electrode impedance change? Does it change over time?

    Send me your register settings and measurement results for further analysis.