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LDC0851: Custom Coil Design

Kaushal K
Prodigy 90 points
Part Number: LDC0851

Hello Team, 

We have designed the NFC custom coil antenna using Coil Designer Tool.

We are using NFC coil antenna with our custom PCB and we had taken reference of this tool to calculate the inductance value.

As per our mechanical dimensions, we entered the input parameters and we got the resultant inductance value. (Screenshot attached)

With the given values we fabricated the PCBs and we measured the inductance value with LCR meter.

As per calculation from tool, inductance value is 337nH (as per attached screenshot), but if we measure the value using LCR meter, it is around 680nH (which is more than double).


Can anyone please suggest why there is so much difference in both?

Thanks In Advance !

Kaushal K

  • 0
    TI__Mastermind 40295 points

    Kaushal,


    Inductance of real coils will vary over frequency.
    As an example, the plot below shows the measured inductance over frequency of a coil from our LDCCOILEVM.

    Note how the inductance and equivalent parallel resistance increases abruptly as the frequency approaches 10MHz.
    The measured inductance doubles from 5MHz to 11.5MHz.
    This is due to the interaction between the coil's inductance and its parasitic capacitance.

    if the frequency in the plot were to continue to increase past 13MHz, it would eventually the coil's resonant frequency,
    where the inductance and parasitic capacitance of the coil will cancel one another, and the coil will look entirely resistive.
    At frequencies above resonance, the coil will look capacitive, not inductive.

    Why does this matter?
    The design tool assumes a frequency equal to the specified sensor frequency, while the LCR meter most likely uses a different frequency for the measurement.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 90 points

    Thank You John for the response.

    We also tried measuring using the Vector network analyzer kindly refer to the attached image below.

    We are getting Impedance around 49.5 ohm and using XL = 2πfL, we are calculating Inductance value (L) and it is 581nH.

    Kindly have a look and share your views on the same.

    Thanks,

    Kaushal

  • 0 in reply to Kaushal K
    TI__Mastermind 44060 points

    Hi Kaushal,

    John in currently out of office. 

    The network analyzer will provide the most accurate inductance per your frequency of operation, 13.56MHz.  LCR meter uses a fixed frequency(typically 1kHz from what I have seen), so not really a useful measurement when targeting a specific frequency.