ST25DV04K PCB design

[Pgonz.1]

Hi,

Before my question, I am new to NFC designs.

I want to test my first PCB design, I want it to work like an NTAG, and I want to be able to read and write on my PCB. For this first project, I don't need anything else.

My idea is to use the ST25DV04K or similar with a chip antenna like the 2450AT18A100E, I would like to know how to connect the pads of these two, do I need any decoupling capacitors?

The characteristics of the PCB would be around 1 or 1.5cm in diameter and 0.8mm or 1.6mm thick.

My other question is, once the PCB is ready can I write straight away, or does it need to be programmed before?

I repeat that for me this is new, if you have any idea where can I start, some course or info?

Thank you.

[JL. Lebon · Accepted solution]

Hello,

NFC is based on magnetic coupling. The magnetic field frequency is 13,56MHz.

The 2450AT18A100E antenna that you propose is designed to work at 2.4GHz and is not meant to work with magnetic coupling. This one is for radio transmission. So it does not fit.

For NFC, what you need is just an inductance. Think of NFC as working like a transformer where the metal between the two inductances is replaced by air.

The easiest way to design an NFC antenna is usually to do it directly with traces on the PCB.

The NFC antenna needs to be large enough so that there is enough coupling with the NFC reader's antenna. Your PCB with 1.5 cm diameter means that if you do a antenna directly with PCB traces, it will be 1.5cm diameter at maximum, which is vey small, probably too small.

If you can't increase your PCB size, then you can use a non-shielded discreet coil with the correct inductance (4.8uH) or a pre-build antenna (search for 13.56MHz RFID antenna at your distributor web site). Be aware that the smallest the antenna is, the smallest the read range is.

You can read this application note to understand how to design an antenna: https://www.st.com/resource/en/application_note/an2972-how-to-design-an-antenna-for-dynamic-nfc-tags-stmicroelectronics.pdf

There is also a free to to help you here https://eds.st.com/antenna/#/

Best regards.

[Pgonz.1]

Hello,

Just seeing this now. Thank you so much for your answer.

A non-shielded discreet coil seems like a good option.

If I find a way to design the antenna with traces, do I need something else or I can just connect the trace to the chip? Also, I guess the trace should be away from the chip ideally. If the antenna is round, could the chip be in the center?

I'll check this, and probably order various versions if so. Thank you again for your help.

Best regards,

Pablo

[JL. Lebon]

Hello,

"If I find a way to design the antenna with traces, do I need something else or I can just connect the trace to the chip? Also, I guess the trace should be away from the chip ideally. If the antenna is round, could the chip be in the center?"

-> you just connect the traces to the chip. You can add a footprint for a small smd capacitor in parallel between the traces close the chip's pins as a provision for an additional tuning capacitor (this allow to decrease a bit the tuning frequency in case it is needed).

-> ideally, it should be away from the chip, but it can work with the chip in the middle. What is important is that there is enough space between the chip and the antenna coils for the magnetic filed to pass through.

Best regards.

[EMsv.1]

Hi.i want to use discreet coil with 4.8uH inductance.which is package is good?

[JL. Lebon]

@EMsv.1​ 

Hello,

I have no exact package name recommendation has it will depend on your design, but what is important is that the magnetic fields must be able to flow out of the inductor. This means:

• the inductor must be unshielded.
• the core of the inductor must be opened so that the magnetic field can escape the core (i.e. toroid inductors will not work as the magnetic field is captive to the toroid core).

Take a package so that the core is as large as possible, this will increase the reading area.

Best regards.