Adjusting the resonant frequency of ST25DV-I2C

[KT_25]

I would like to connect a 1.4uH antenna to ST25DV-I2C.
The datasheet says that there is a 28.5pF capacitor inside the IC for RF interface tuning.
However, when using a 1.4uH antenna, the resonant frequency shifts from 13.56MHz.
How should I adjust the resonant frequency?

Also, are there any application notes that describe how to adjust the resonant frequency?

[JL. Lebon · Accepted solution]

Hello, 

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

If you do the math as explained in this application note, you can see that you need an antenna that has an inductance around 4.8uH. So, your antenna inductance is too low: with 1.4uH, you will have a resonant frequency at 25MHz. You can decrease the resonant frequency by adding a parallel capacitor between AC0/AC1.

Best regards.

[KT_25]

Thank you for your reply.

When using a 1.4uH antenna, am I correct in understanding that the resonant frequency can be set to 13.56MHz by connecting a 69.9pF capacitor to AC0 and AC1?

Also, is there anything I should be careful of when selecting the capacitor?

[JL. Lebon]

Hello, 

Yes, that's right.

For optimal performance, I also recommend tuning the resonant frequency a little above 13.56MHz (13.7Mhz is a good value).

I also recommend to you to measure the resonant frequency after you add the parallel capacitor to ensure the theorical and practical values match.

Best regards.

[KT_25]

Thank you for the detailed explanation. I understand now that it can be adjusted.