NFC Capacitor tuning

[pjreijiri]

Hello everyone, I want to integrate NFC to one of my projects and I am having a problem with the value of some of the capacitors. I am looking into using the nRF52832.

I need to figure out the value of the capacitors Ctune1 and Ctune2. Even though there are equations on how to pick the capacitors, I need some help since this is not my expertise.

I have some questions about the formula in the datasheet. I am not really sure what is Cp1 and Cp2. Are they related to the antenna? How to determine their value? Also what is Cint1 and Cint2? Are they Cpad_NFC? if so, then they should be 4pF, right?

One more silly question and please forgive the ignorance. For the capacitor and inductor values, what are the magnitude of units that needs to be used? is it pF, F, uH, H, ...?

It would be really appreciate if you can help me with these questions.

[Jørn]

Hello pjreijiri

See our whitepaper on NFC antenna tuning.

Cp1 and Cp2 are capacitances present due to the layout of the PCB traces, which lead out to the antenna. These are typically dependent on your layout, substrate and substrate thickness. Cint1 and Cint2 are the internal parasitic capacitances of the chip as well as the pad capacitances. The pad capacitances dominates these values, and they are therefore approximately 4pF (see page 7 of the link above). The capacitors will typically be in the pF (picofarad) range, and the inductor is typically in the uH range.

There are descriptions of two methods for tuning your antenna in the white-paper mentioned above.

EDIT

Cp is caused by parasitics in your layout. So to find their value you will have to calculate based on, or preferably measure, your physical layout.

The capacitances mentioned for NXP and Broadcom are there to tune the antenna to 15.4MHz and 20.6 MHz respectively (we use 13.56MHz). As it states in the datasheet, all values there are only for reference, and will differ based on your layout.

If you look at page 423 of the nRF52832 product specification it states that for an antenna inductance of 2uH, you will get tuning capacitors in the range of 130pF (with 4pF Cint as stated in the whitepaper). This corresponds to a Cp of roughly 3.76pF. However this is only an estimate, and the actual value will depend on your physical layout. If you look at the nRF52 DK they have tuning capacitors of 300pF on each line, however they have long lines, connector P23 and the pads of C42/43 and R25/26 along the lines. All of this affects the inductive and capacitive parasitics present along the lines, which in turn affects the value you need for the tuning capacitors. This is why tuning is typically done using lab-equipment as described in the white-paper I linked above.

Best regards

Jørn Frøysa

[pjreijiri]

Thank you Jørn for your help.

If I am to use this antenna for testing: www.mouser.com/.../ANFCA-3225-A02-473370.pdf

Which value for Cp would be recommended? It mentions NXP and Broadcom but nothing concerning Nordic. Could you shed some light please?

Regards. Patrick

[Jørn]

I have updated my answer with some additional comments.

[pjreijiri]

Thank you Jørn, I don't have access to lab equipment. So I will see what I can do with theoretical studies and calculations.

Thank you for your time.