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I have an NFC tag application that uses an external antenna. The antenna itself has not been properly selected just yet, but my question is with regards to connecting the external antenna to the NFC tag.

NFC Tag is the NXP I2C Plus chip - https://www.nxp.com/docs/en/data-sheet/NT3H2111_2211.pdf

A high-level overview diagram is shown below.

enter image description here

The antenna ports appear to be balanced but I don't know for sure.

Possible options:

Have 2 pads where I can solder some wire to and run up to the external antenna.

  • Twisted pair vs not?

Add a u.fl connector or similar RF connector and use that to connect to the external antenna.

  • Would I need a balun or can I connect it straight?
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  • \$\begingroup\$ Yes, the ports are balanced, and the "antenna" is really just a coil. If you have a long way to run the connection, you may have to use a u.fl connector, but if it's close, it's easier to get an NFC antenna with an FFC (flat-flexible-cable) pigtail and mount the mating FFC connector on your PCB. \$\endgroup\$ Dec 19 '20 at 16:52
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The chip's data sheet says this: -

enter image description here

And, realistically, 13.56 MHz is not particularly problematic with most reasonable types of connection.

The antenna ports appear to be balanced but I don't know for sure.

It's primarily a magnetic field communications method so impedance balancing isn't needed. It would be if it were a dipole antenna (for instance) but, it isn't. It's a near-field magnetic field antenna.

Twisted pair vs not?

I would twist the wires between the PCB and the antenna just to cut down extraneous magnetic fields that would otherwise be lost if twisting is not used.

Would I need a balun or can I connect it straight?

No, a balun isn't required because it's a near-field magnetic field antenna. Balun's are needed when connecting (for example) a balanced dipole to a coaxial feed. They are not needed here; it will work equally effectively with a balanced drive or an unbalanced drive. Not sure what the chip provides but, it's largely irrelevant.

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According to the datasheet you linked, you should consult NXP application note AN11276 "NTAG Antenna Design Guide" at http://www.nxp.com/documents/application_note/AN11276.pdf

This is Ref. 5 in the datasheet.

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