NFC Antenna Design for ST25R3916B-Based Payment Reader – Matching Network & Display Impact

[steave_bhim]

Hi,

I am designing an NFC reader based on the ST25R3916B for a payment interface, ensuring compliance with EMVCo standards. For hardware development, I am using the ST25R3916B-EMVCO reference design as my baseline.

Queries:

1. Custom NFC Antenna Design:

   • I plan to use the same NFC antenna dimensions (51mm × 27mm) and trace length as the reference board but will design it myself.

   • How should I approach designing the coil to ensure proper inductance and matching network?

   • If I change the coil shape but maintain the same number of turns and total trace length, will the matching network also need to be adjusted, or can I use the same network as the reference board?

2. Impact of Display on NFC Performance:

   • My design includes a display on the surface of the reader. Will the display impact NFC coil performance?

   • Should I place the NFC antenna under the display, or is it better to design a ring-shaped PCB antenna around the display to avoid interference?

3. Using a Third-Party NFC Antenna:

   • Since EMVCo certification is crucial, I want to keep the design as close to the reference as possible.

   • If I use an off-the-shelf NFC antenna (e.g., from Toeglass or another manufacturer), will it affect the matching network significantly?

I would appreciate any guidance on these aspects to ensure compliance while optimizing performance.

Thanks in advance!

[Ulysses HERNIOSUS]

Hi,

please review the various additional ANs available on https://www.st.com/en/nfc/st25r3916b.html#documentation , especially AN5276, AN5806, ANF5240, AN5584.

Changes are high that you need to define an own antenna geometry. 

BR, Ulysses

[steave_bhim]

Hi,

is 50 ohm impedance matching is required in antenna coil ?

as i am targeting to make 2 layer PCB which can not have impedance matching capability by JLCPCB.

if its important then any recommended layer stack up of 4 layer PCB for NFC card reader.

Thanks 

[Travis Palmer]

Hi,

The antenna dimension mostly depends on the available space. Anyhow I would not recommend to choose a smaller size than 51mm × 27mm. If possible the size should be bigger.

When adding a display, the Q-factor will be influenced. So you might design your antenna with even wider tracer. 

Most displays are shielded on the backside.  if the antenna is fully covered by the shield, the resulting magnetic field can be reduced to almost zero. In this case it is required to have the NFC antenna bigger than the display. 

After having the antenna size fixed and prototypes available you need to measure the antenna parameter and calculate the matching network. this is described in AN5276 and should be straight forward. I would not recommend to go for a 50Ohm antenna. You should stick to the conventional differential antenna.

The most critical tests are the power transfer and sensitivity tests. 

please let us know, if you have additional questions.

br Travis