How to make ST25DV work

[uveuvenouve]

I drew a pcb with antenna and put st25dv on it
but i get read data error on the st app

is that st25dv need to program first?or just my soldering is bad? or antenna isnt standard?
the antenna is designed as this

P.S. I can read the tag by other app call NFC tools(error read and write memory still), but still error with st official app
and it is not very sensitive actually , this design should be around 1.1uH in the st nfc inductance simulate. is this good?
how to improve?

[JL. Lebon · Accepted solution]

Hello, 

If the antenna is 1.1uH then this is probably your problem.

Each family of tag have different characteristic impedance, and each require a specific antenna tunned for this impedance. There is no "standard" NFC antenna.

The ST25DV has a tuning capacitance of 28.5pF. The antenna must be shaped in order to have a resonant frequency of 13.56MHz for the couple ST25DV + antenna.

In order to achieve this, the antenna inductance should be around 4.8uH for the ST25DV.

I recommend you read the following application note that explains this in detail: https://www.st.com/resource/en/application_note/an2972-how-to-design-an-antenna-for-dynamic-nfc-tags-stmicroelectronics.pdf

This application note also explains how to measure the resonant frequency. You can measure it on your current antenna to confirm that it is 13.56MHz.

Best regards.

[uveuvenouve]

Hi ,

thanks for your reply
and i have few more questions
1.how is 4.8uH calculated?
2.how to get as much as power in energy harvesting? i saw some information that said the antenna size should as close as the reader antenna size. but i want to use on different phones. it is quite hard to match all the smartphone antenna size. is there any other way to increase the energy harvesting?

Best Regards,

[JL. Lebon]

Hello, 

The calculation of the 4.8uH is described in the application note: La = 1/Cs*(2*pi*fcc)^2, with fcc as the resonant frequency (13.56MHz) and Cs the tuning capacitance (28.5pF for the ST25DV).

For the energy harvesting, yes, it is better to match the reader antenna size, but as you said, every smartphone has different antennas, so it is difficult to achieve.
Another recommendation would be to try to minimize the number of antenna turns.
And last recommendation is to have the tuning frequency a little above 13.56MHz (13.7MHz would be a good target).

Best regards.

[uveuvenouve]

Thank you