Matching network circuitry for NFC in NRF52840-QIAA-T

[Sing Yew Chan]

Hi all, 

I would like to ask whether is there any details description of the matching network circuitry of NFC for NRF52840 in the datasheet.

The only information I could find on the datasheet is only the shunt capacitors used to tune the resonance of the antenna circuit.

Based on the information from other NFC chip such as SLRC61003HNY from NXP, they do provided the info on the matching circuitry and the characteristic impedance.

https://www.nxp.com/docs/en/application-note/AN11019.pdf

Matching circuitry for TX (pg 21)

Characteristic impedance of NFC antenna trace (pg20)

Can you guys help to answer the following questions to clear my doubt, 

Question 1: The matching network provided for the NFC is too simple that the matching circuitry do not contain the EMC filtering and Q factor selection, do we need to have the EMC filter/Rq resistor to determine the Q factor which shown in the matching network for SLRC61003HNY? If EMC filtering needed, what will be the direction  (TX or RX)? 

Question 2: What is the characteristic/target impedance of the NFC antenna traces?

Thank you very much and have a nice day.

Regards,

Sing Yew Chan

[Simonr]

Hi

The only thing that's necesary is the tuning caps, and the rest is built onto the chip. EMC filtering for instance, is only necessary for a NFC reader, which the nRF52 devices don't support.

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.

Best regards,

Simon