nRF52832 - Initial values for capacitors for NFC coil

[Abomin3v3l]

Hi all.

I did a layout with NFC antenna with a nRF52832 chip.

In the schematic (suggesten on datasheet) I have antenna coil with 2 capacitors to ground.

Drawing of NFC coil on layout:

There is two round holes at the start and end of the coil, the total lenght of coil is 455mm. It follows this design of ST's online tool eDesignSuite.

It is to have 1.08uH and 1.56 ohms.

Resistance was calculated with an  online tool.

Considering these informations, what values of capacitors (C16 and C17) I could use as a starting point for my PCB prototype?

Regards,

Jeferson.

[Kaja]

Hi Jeferson,

you can estimate the values or you can tune the antenna, see our white paper. 

We can help you with antenna tuning, you should remove the ground arm under the antenna. 

Best regards,
Kaja

[Abomin3v3l]

Hi Kaja.
I really apreciate your help.

There is no ground arm nor other signals on the antenna area.

I did added more two capacitors, one on each NFC pin, on each end of the antenna.

The estimated inductance is 1.08uH, so here goes my calculation.

This part of equation:

((2*pi*13.56*1000000)^2) * 1.08*10^-6
Result = 7.839*10^9

Now doing substituition on the complete equation:

Ctune = (2/(7.839*10^9)) - 0 - 4*10^-12
Ctune = 2.511*10^-10
Ctune = 251pF
Ctune = 220pF + 30pF (commercial values)

Does this seem correct for you?

Regards,
Jeferson.

[Kaja]

Hi Jeferson, 


it looks correct! Just add Cp ~1 pF as well, but that gives same result. 

FIY: you should still adjust values on the physical board, as it might deviate a bit from the theoretical value. 

Best regards,
Kaja

[HAP]

Hi Kaja

Is Cp  parasitic capacitance of PCB Wires antenna?

Best regards
HAP

[Kaja]

Hi,

Cp ~ represent the parasitic capacitances between the PCB pads of the crystal, the tracks in the crystal circuitry and, the pads of the nRF device to ground.

Best regards,
Kaja