Design NFC Antenna for NFC/RFID Tag

[StefanoSperandio_ST]

Hello everyone!

I’m developing a device that will include an NFC Tag, and I’ve chosen the ST25DV64KC. This Tag needs to be mounted on a case and communicate with the "mainboard" of my project via I²C. Therefore, I’m working on a small circuit that will allow the Tag to be placed on the case and connected to my main circuit (which features an ST microcontroller and additional components).

I’ve already designed the circuit for the ST25DV64KC, but now I need to develop the PCB antenna. For its dimensions, I’m referencing the application notes AN2972 and AN5605, and I’m also using your NFC Inductance tool: https://eds.st.com/antenna/#/.

Now, I have a few doubts:

1. The tool provides the antenna’s inductance value at 13.56 MHz based on the dimensions I input. However, I assume that to achieve resonance at this frequency, I need to introduce a capacitor between the two antenna electrodes (AN0 and AN1), calculated to make the LC circuit resonate at 13.56 MHz, correct?

2. Another doubt concerns placing the designed circuit in the center of the antenna. From the application note AN2972, I’ve noticed that it’s recommended to avoid having ground planes above or below the antenna or placing signal traces near the antenna. So, I’m wondering if there are any precautions I should take to implement this kind of design (antenna surrounding the circuit), or if it’s generally better to design the antenna separately from the rest of the circuit.
   I’m asking this because I’ve noticed that some of your evaluation boards use a similar design, with the circuit entirely within the area enclosed by the antenna’s loops.

Thank you in advance for your time and support!

[Henry Crane]

Hello Stéphano,

1) all our NFC chip embed an internal tuning capacitance and you just need to design the appropriate antenna inductance to get resonance at 13.56MHz. For ST25DV family, internal tuning capacitance to calculate antenna is 28.5pF. Resonance at 13.56MHz leads to 4.8µH. A resonance frequency up to 14MHz is acceptable depending on expected performance.

By default, there is no need for an external capacitance as said above. However, it is recommanded to add some y the external capacitance to adjust the resonance frequency without redesigning the PCB to adapt the antenna:

- a parallel capacitance between AC0-AC1 allows to decrease the resonance frequency 

- two series capacitance (for a better balance on AC0 path and AC1 path) between ST25 and antennas allow to increase the resonance frequency.

Depending on your nneds, you can combine both for resonance frequency increase/decrease capability.

a good strategy is to calculate antenna for 14MHz, and add some small parallel capacitance in parallel only during PCB RF performance validation.

2) Compared to a spiral coil in free space, a spiral coil antenna with some surrounding conductive plane or some plane in the middle, has a lower self inductance and less ability to get energy from magnetic field generated by the reader in vicinity. this is the reason why our app notes recommend to leave the inner space free and leave maximum gap wrst to surrounding electronics.

It is however possible to lay some circuits inside the coil. The actual antenna inductance will be lower than the calculated one with our online tool, and a parallel external tuning capacitor will be mandatory.

feel free to contact us if you need more information.

Best Reagdrs,

Henry Crane NFC/RFID technical support.