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using an nfc tag as an antenna

bharadwaj r
Intellectual 565 points
Other Parts Discussed in Thread: TRF7970A

i assume this is a silly question to ask. but i am in  the process of pcb printing an nfc antenna for trf-7970A, so i was wondering if it would be possible to a use an nfc tag as an antenna buy removing the memory chip. as these behave just like inductors , they should be able to substitute each other. 

how feasible would this experiment be?

  • 0
    TI__Guru* 91086 points

    Bharadwaj -

    Not a silly question - but usually the tag coil is much higher inductance than what can be tuned to 50 Ohms and have correct Q value with ease using standard values.

    for example, if you measure them at 13.56MHz, you might see values between 3uH to 5uH or higher, depending on which tag type and vendor you choose.

    For example, if coil is 3uH @ 13.56MHz, this would yield 1 +j255.47, to get correct Q value for ISO14443A & B applications, you would start with 6.8pF shunt, 1.8kOhm shunt and 40pF in series, to get resonant @ 50 Ohms...then fine tune it. This would be best case...

    if the coil was in the range of 5uH, we can redo the math and see that no shunt cap can be applied and the shunt resistor needed for Q dampening would be a bit high (3.6k versus 3.4k, or lower), then around 28pF to get to 50Ohm resonant. So if there was any variation from sample to sample or the Q dampening resistor, the tuning would suffer.

    so this goes towards the worst case.

    if you could count on the L of the coil being constant from sample to sample and on the lower end, then hopefully you can see that you could get there with these devices, but you should choose carefully the inductance and test plenty of them to satisfy yourself that what you end up with is repeatable.

      

  • 0
    TI__Mastermind 26420 points

    It is possible, since it is just an antenna.  You are going to have to measure the inductance of the antenna and tune it, but you will have to do this with any antenna you design.  

    The problem I think you are going to have, is physically connecting to the tag.  They are very difficult to solder to, without melting all the plastic.    

    I assume you just want to do this for prototyping?  If so, there may be 2 other solutions that are better:

    1. Hand wind an antenna out of copper wire
    2. Buy an off the shelf antenna

    Taoglas offers a very nice NFC antenna that is already tuned to a 50 ohm point.  This means that if you just follow our 50-ohm matching circuit when laying out the TRF7970A signal path, that this is a pre-tuned antenna.  You can purchase this antenna from Digikey.  It uses a standard u.fl connector to snap it onto a board.  

    Hope this helps.

    Thanks,

    JD