Antenna to be used along with ST25DV16K and PCB design guidelines

[sidmohan]

Hello,

can you please suggest on what type of antennas do you suggest to be used along with ST25DV16K ? I was hoping to use NFC_ANTENNA_25X15 between the pins AC0 & AC1.

Also can you please suggest if it will be a good idea to have the antenna on bottom layer and all the other electronic components on the top layer in the two layer board ? Or will it be better to increase the PCB size laterally and have the antenna on the same top layer as the electronic components ?

I also plan to use a 2.4Ghz wireless communication chip antenna so what precautions should be taken to avoid interaction between the chip antenna and NFC antenna ?

regards

Siddharth

[sidmohan]

It will be great to receive some inputs please. 

thanks 

[Henry Crane · Accepted solution]

Hello Siddharth,

in case you did not, I suggest you the reading of our Application note AN2972, How to design an antenna for dynamic NFC tags that deals with antenna impedance and implementation.

you van use any spiral coil antenna which series equivalent inductance is between 4.5and 4.7µH at 13.56MHz. Our online design tool eDesign Suite (https://www.st.com/content/st_com/en/support/resources/edesign.html) offer a module to calculate your own antenna. For instance a 25x15mm antenna with 15 turns (200µm width and spacing) could feet. 

As recommended in AN2972, the spiral coil shall not be masked by any metallic surface (component or ground/power plane), for an optimized performance. Placing components inside the spiral coil is possible only there is few mm free space between the inner turn of the coil and the area where components are located. this is particularly recommended in case you shield the components areas by pouring space with ground planes (which is recommended to isolate components from 13.56MHz or 2.45GHz RF signals). See our SMARTAG1 and SMARTAG2 eval boards for implementation example using an antenna surrounding components on a PCB. 

With a 25x15mm, it is likely that spiral coil will be almost masked by component area and performance be almost null. I recommend you to extend the PCB to place components (distributed on top and bottom to limit PCB extension). Bluetooth antenna should be far as possible from NFC antenna, at least, not close or on top of spiral coils wires.

In case you absolutely need to have a total PCB area of 25x15mm (the antenna surface), you may consider to stack two PCBs (one for antenna and one for components) separated by 5mm spacer or a ferrite sheet piece.

feel free to contact us  

 Best Regards,

Henry Crane, NFC technical support team.

[sidmohan]

thanks for the reply, in my case the antenna spiral is on the bottom layer and all the components on the top layer with standard two layer pcb dielectric thickness between the two layers. Attached is the image for your kind reference. 

[sidmohan]

Does the above configuration of single layer PCB look fine to you ? or do you have any suggestions on the same?

[Henry Crane]

Hello,

this kind of design is OK for NFC communications however digital components are exposed to 13.56MHz.

Try to minimize the surface of signal loop for components that may be sensitive to 13.56MHz. Unless component placement is a constraint (for instance connectors that have to be on the border of the PCB), try to compact the layout in order to reduce the length for tracks and then reduce signal loops.

I can see that there is some free space between the PCB boarder and the antenna: you may consider to enlarge the antenna dimension to the border of the PCB and reduce the spiral track width and spacing in order to enlarge the inner area. 

A real life validation of your PCB prototype will be necessary to verify the impact on your application of magnetic field generated by the NFC/RFID reader you selected. More than a functional validation, you'll have to measure the noise generated by the magnetic field in the system to assess its robustness.

Best regards,

Henry.