M24SR Capacitor frequency antenna tuning

[GLeva]

Hello,

I would like to implement the M24SR02 in my design, with an NFC antenna.

The problem is that, in order to have a resonant frequency of 14.2MHz, the inductance of the NFC antenna needs to be around 5µH in order to resonate with the 25pF capacitance inside the M24SR02, but this value of inductance is difficult to reach with a small size antenna (<10mm).

Is it possible to add a capacitor between AC0 and AC1 pin, or two capacitors between the ACx pins and the ground, in order to make the capacitance higher, which will make the needed inductance of the antenna lower, and thus allows me to use a smaller antenna ?

Best regards,

Guillaume

[JP Miller]

Dear customer,

It is indeed possible to add a capacitor between AC0 and AC1 pins so that at an equivalent resonant frequency, the inductance of the NFC antenna is smaller. On the other hand, due to the quality factor of the external capacitor, the overall quality factor of the tag will increase. For a resonant frequency of 14.2Mhz, less energy might be available to the tag, and the overall communication distance between the reader and the tag might get worse.

An alternative solution to consider if possible in your application is to design a 2 layers PCB antenna instead of a single layer. We provide the antenna design tool which you might find useful.

https://my.st.com/analogsimulator/flex_app/bin/eds_antenna_design.html?app=analogsimulator&msg.action=new_design&msg.data.application=antenna&msg.data.specs.length=10&msg.data.specs.width=10&referrerPage=%2Fcontent%2Fst_com%2Fen%2Fproducts%2Fnfc

With a 15 turn antenna on 2 layers, with 150 µm conductor spacing and 150 µm width, one can reach ideally a 4.8µH, or 3.6µH more typically , which can then be adjusted with a smaller capacitance to meet the 14.2 Mhz resonant frequency.

Have a good day

Regards