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NFC port spec. Coil inductance

I wish to design a Laird BL652 Bluetooth with an NFC port. Unfortunately I have not been able to discover the coil inductance. They use a Nordic nRF52832 chipset.

My hope is that you may have an application circuit that will indicate what is required. For example I am using a long range NFC using M24LRxx ST in another product with a 4u7H PA6512 CoilCraft NFC aerial.

The application is small and we plan to use a CR2477 lithium button primary cell. This to be connected to VCC in series with a protective resistor. The NFC circuit and the debugging interface must not charge the battery

  • Hello Andrew

    What do you mean you have not been able to discover the coil inductance?

    You can find information on tuning the NFC antenna in this whitepaper. For the NFC to function properly its antenna must be tuned to resonate at 13.56MHz, which is done by combining the antenna coil inductance with tuning capacitors. As is stated in the white-paper, the total capacitance is a combination of Cp which is the PCB parasitics, and Cint which is the internal capacitance plus pad parasitics.

    So typically you would choose/design the antenna with a given inductance, then find the PCB parasitics due to your board layout, and then calculate the tuning capacitors necessary to have the antenna resonate at 13.56MHz.

    Best regards

    Jørn Frøysa

  • Thanks Jorn,

    If the Laid BL652 module data sheet advised me what the loading capacitance and parasitic capacitance was on each of the two NFC port pins, I could work out the inductance. Alternatively just advise me what inductance coil to use? I have asked this question and they are not sure but it might be 100nH?

    A note says the loading is 300pF which could mean 500uH or it could mean 250uH dependingly on the meaning of Laird's note. For example another IC NFC we are already using has this information documented.

    So it might give me a clue if Nordic have an application circuit although of cause it is guess work as to whether the BL652 is based on such an application note. I would really prefer to wait until these things have been resolved and the maker is in a position to answer this question. But I am not being given that information.

    Sorry my question was not on calculating a tuned circuit but that I have not found a Nordic application or evaluation board circuit and I am asking you for that.

  • The closest thing we have would be our nRF52 Devkit (pca10040). All nRF52 devkits come with a NFC tag, the layout for the tag is included in the kits hardware files. It should have an inductance of around 720nH. The tuning capacitors of the Devkit are 300 pF, however that would change depending on your layout. There is not one specific inductance value to use, it depends on what you're interfacing with or design requirements. I'm not an NFC expert, but from what I've found the most important features of the NFC antennas are that transmitter and receiver geometries are similar, as this increases the coupling coefficient, so you should design the antenna to roughly match the antenna you're interfacing with, and then choose capacitors to ensure it resonates at 13.56MHz.

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