NFC trace impedance

[Apica]

Hi, I'm working with nRF52840 with NFC. I don't have yet the external antenna to connect to (now I'm using uFL), but I'm not sure about the trace width that goes from the chip through uFL. Should it be 50 ohm?

In nRF52840 Development Kit NFC pins and BLE pins have differen width so I'm not sure the two have the same 50 ohm impedance (but they have an specific width).

Thank you

[ketiljo]

The NFC antenna is different from the BLE antenna as it's differential and not 50 ohm. Also, the frequency is very low so the exact impedance doesn't matter as much. 

The inductance in the antenna is what matters for the NFC antenna. Since the antenna is differential, it's better to use a two pin connection instead of U:FL which is a single ended connection. 

The inductance of the antenna together with the NFC antenna tuning capacitors forms a resonant circuit that is tuned to the NFC frequency, 13.56 MHz. Any change in the connection wires to the antenna can be compensated with the tuning caps. 

So just lay out the tracks as a differential pair.