One antenna used by two NFC devices - ST25R3916-AQWT and ST25DV64KC-JF8D3

[hxtec32]

Hi ST,

One of my clients has asked me to design a PCB which will require the ST25R3916 NFC reader IC, and the ST25DV64KC, to share the same antenna, in order to prevent the tag from interfering with the reader.

I have thought about using an RF switch, such as the pSemi PE42359, however, I am unsure how this will work with differential antennas such as the antennas used in NFC communication. Additionally, it seems that the dynamic tag IC requires different matching circuits than the reader IC, with the dynamic tag IC listing an internal tuning capacitor of 28.5pF, and the reader mentioning automatic antenna tuning with a variable capacitor.

Another thing I was wondering about, is, does the ST25R3916 perform the exact (or better) functions than the ST25DV64KC, when it comes to acting as an NFC tag? Because, I saw it is capable of 'card emulation', which I believe is also the case for the ST25DV64KC. If I only need to use the ST25R3916 to perform both the dynamic tag and reading functions, that would be great.

Also, it would be good to have a little more details about the intricate matching circuits for the ST25R3916, as I was unable to find much information about this online, especially in the case of non-PCB antennas (my client's PCB is in a very space-constrained environment). I was planning to use the W3102 antenna from Pulse Electronics, however, the NFC Tuning Circuit in ST's eDesignTools requires 'parallel resistance', which it seems is not contained in the W3102's datasheet. It mentions a matching circuit though, of 8.2pF, which is, from my understanding, in order to shift the resonant frequency, resulting in better RF performance. All help, information, datasheets, application notes, etc, would be much appreciated.

Kind regards,

Haase Industries.

[Ulysses HERNIOSUS · Accepted solution]

Hi,

as software engineer I cannot comment on the matching topics but as I understand you are going where nobody has gone before with switching an antenna to be used alternatingly from a tag and a reader chip.

Another option might be to have two antennas, one dedicated to the tag and one dedicated for the reader - maybe putting the inactive one to high-ohmic but that is not really my domain.

To your other point: Yes, ST25R3916 supports card emulation in NFC-A (106kbps) and NFC-F (212 and 424kbps). ST25DV64KC supports NFC-V (typically 26kbps). So if NFC-V with potentially superior range is not a hard requirement you might also just use ST25R3916 alternating between Reader and Tag emulation mode.

Regards, Ulysses