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AFE4300: how to do Z and θ calibration ?

PYTsai
Genius 11635 points
Part Number: AFE4300

Hi 

In our BCM application, we need to put a 3 meter cable in between human body and AFE4300. The Z  and θ reading value was impacted by this long cable. 

How to do  Z  and θ calibration for 3 meter cable? 

Thanks!

PY Tsai

  • 0
    TI__Mastermind 49946 points
    Hello Tsai,

    We will get back to you before the end of this week.
  • 0 in reply to Praveen Aroul
    TI__Prodigy 215 points
    Here are some considerations and guidelines:
    1. I am assuming that a 4-point measurement is being used - 2 electrodes for injecting the excitation and 2 separate electrodes for sensing the voltage. If that is not the case, then such a method is highly recommended in order to get a proper accuracy.
    2. High contact impedances in the path of the electrodes can affect the measurement accuracy on both the excitation and sense sides. Ensure that at the excitation frequencies of interest, the impedance of the cable is very small compared to the measured value of impedance.
    3. To check the impact of the cable on the excitation side, probe the waveforms on both sides of the cable and check if they are proper - no oscillation, no clipping, etc. Refer Applications section in the datasheet for solutions to possible oscillation issues that can get caused by excessive loading.
    4. To check the impact on the sense side, probe the waveforms on either side of the cable and check if they are OK.
    Since the effects of the loading from the cable are currently unknown, the key step is to determine all of the above prior to any attempt to performing the measurements.
  • 0 in reply to Anand Udupa16
    TI__Genius 11635 points

    Hi Anand, Praveen,

    The cable we used is RG316 it is a low impedance cable. However when the RG316 cable goes into a connector to connect to the PCB. Impedance shift will happen at the connector. It is challenge for us to remove this impedance shift. It would be easier for us if there is a method for us to do calibration. 

    Is there any way to do calibration? 

    Thanks!

    PY Tsai

     

  • 0 in reply to PYTsai
    Genius 3030 points

    Hi PY Tsai,

    I want to know, what element do you suspect of this impedance shifting?
    1- RG316 cable,
    2-SMA onboard connector,
    3-PCB layout.

    There are some Phase Adjusters which help you to ensure the stability of the calibrated phase adjustment.
    On the other hand, in order to measure the plane impedance, you need to have a VNA (Vector Network Analyser)

    Let me know if you have further question about choosing appropriate Phase Adjusters matched to your cable and connector.


    Regards,
    Hossein

  • 0 in reply to Hossein Forouzani
    TI__Genius 11635 points

    Hi Hossein,

    Due to we do not have Vector Network Analyser and not able to know exactly where the impedance shift come from. 

    We are thinking of using a 500 ohm high precision resistor to do calibration. By comparing how the measuring result shift from 500 ohm to do calibration. 

    Is this idea looks reasonable? 

    Regards,

    PY Tsai 

  • 0 in reply to PYTsai
    TI__Prodigy 215 points

    Hi,

     I am not sure how accurate a calibration you will get by using just a 500 Ohm calibration resistor. This is because the effect of the cable on the phase shift of the signal from this calibration resistor could be different from its effect on the actual impedance being measured. It is worth a try. Additionally you could look at using an R-C calibration impedance that is a closer representative of the body impedance's R-C model. 

    Regards,

    Anand.