NFC

[mej7000]

Do you agree that the following AI response is accurate? If so, I understand it to mean that I need 3 capacitors:  one 68pF in series with each NFC trace and one 430pF in parallel with the NFC traces?

The AANI-CH-0030 NFC chip antenna from Abracon (inductance 0.298 µH, DCR 1.62 Ω, Q-factor 15.7 at 13.56 MHz, size 10 x 4 x 0.8 mm) can be used with the nRF54L15 NFCT (NFC Tag mode) by adding an external matching network to tune the resonant frequency to 13.56 MHz and match the chip's impedance. 

Matching Network Topology The standard topology for Nordic NFCT pins (NFC1, NFC2) is:

• Two series capacitors (C_s1, C_s2) between the NFCT pins and the antenna ends to adjust impedance matching.
• One parallel capacitor (C_p) across the antenna ends to tune the resonant frequency.
• Schematic configuration:
  • NFC1 -- C_s1 -- Antenna end 1
  • NFC2 -- C_s2 -- Antenna end 2
  • C_p between Antenna end 1 and end 2

This is a typical pi-network for NFC tags, where C_s1 = C_s2 = C_s (symmetric), and the effective capacitance C_m = C_s / 2.Recommended Component ValuesAssuming nRF54L15 NFCT internal capacitance C_int ≈ 4 pF (similar to nRF52 series, as specific nRF54L15 C_int is not publicly documented), the network is tuned for resonance at 13.56 MHz.

• C_s1 = C_s2 = 68 pF (standard 0402 or 0603 ceramic, ±5% tolerance, e.g., Murata GRM1555C1H680JA01D)
• C_p = 430 pF (0402 or 0603 ceramic, ±5% tolerance, e.g., Murata GRM1555C1H431JA01D)

These values provide:

• Total effective capacitance C_total ≈ C_p + C_m + C_int ≈ 430 pF + 34 pF + 4 pF ≈ 468 pF (close to calculated 462 pF for resonance, with margin for tolerances).

Calculations and Formulas

• Resonant Frequency Formula: f = 1 / (2π √(L × C_total)), where L = 0.298 µH, f = 13.56 MHz = 13.56 × 10^6 Hz.
• Required C_total: C_total = 1 / (4π² f² L) = 1 / (4 × π² × (13.56e6)² × 0.298e-6) ≈ 462 pF (using omnicalculator.com/physics/resonant-frequency-lc or manual calculation).
• Effective Matching Capacitance (C_m): C_m = C_s / 2 (for C_s1 = C_s2 = C_s).
• Total C: C_total = C_p + C_m + C_int, where C_int ≈ 4 pF (nRF internal capacitance).
• Adjustments: For exact tuning, measure the assembled antenna impedance with a VNA (Vector Network Analyzer) and adjust C_p ±10% to achieve 13.56 MHz resonance and Q-factor ~20–30 for bandwidth. The AANI-CH-0030's low Q (15.7) limits efficiency, so aim for Q = 2πf L / R_a ≈ 15.7 (matches datasheet).

Notes

• The nRF54L15 NFCT is passive tag mode (NFC-A Listen), so the matching focuses on resonance at 13.56 MHz for reader power harvesting.
• No inductors needed in the matching network (the provided values are capacitor-only).
• Test for reading distance (~10–30 mm per Abracon datasheet) and adjust C_s for impedance (target ~50 Ω).
• For Altium, create a custom footprint for AANI-CH-0030 (10 x 4 mm) and add the network components in the schematic. Import BOM via Reports > Bill of Materials > Import.
• If C_int for nRF54L15 differs (check Nordic support or devzone.nordicsemi.com), recalculate C_p = 462 pF - C_int - (C_s / 2).

This setup tunes the AANI-CH-0030 to 13.56 MHz for nRF54L15 NFCT. For precise values, prototype and measure with a VNA.

 

[mej7000]

I think I understand the NFC matching network now... I am guessing that Parasitic Capacitance will require me to drop the Ctune value to something less that 17.2pF, but as a starting point, do you agree that these calculations are correct?  
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using docs.nordicsemi.com/.../nfct.html and docs.nordicsemi.com/.../parameters.elec_spec.html as a reference what value should what are the values of Ctune1 and Ctune2 if Ctune1=Ctune2 and Cint1=Cint2=5pf on the nrf54L15 and Lant = 3.1uH to achieve 13.56Mhz and how should those capacitors be arraigned?

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To achieve resonance at 13.56 MHz with an antenna inductance (Lant) of 3.1 μH on the nRF54L15, and given that the internal capacitance per pin (Cint1 = Cint2 = 5 pF), you need to calculate the value of the external tuning capacitors (Ctune1 = Ctune2) to be connected from NFC1 to ground and NFC2 to ground.

Calculation

The resonance frequency for the parallel LC circuit is:

f = 1 / (2π * sqrt(L * C_total))

Where:

• f = 13.56 MHz
• L = 3.1 μH
• C_total = Cint1 + Ctune1 (for NFC1) and Cint2 + Ctune2 (for NFC2)

Since the circuit is symmetric and Ctune1 = Ctune2, Cint1 = Cint2, the total capacitance across the coil is (Cint1 + Ctune1) + (Cint2 + Ctune2).

Step 1: Calculate total required capacitance

C_total = 1 / ( (2πf)^2 * L )

Plug in the values:

• f = 13,560,000 Hz
• L = 3.1 × 10⁻⁶ H

C_total = 1 / ( (2 * π * 13,560,000)^2 * 3.1e-6 ) ≈ 1 / ( (85,237,000)^2 * 3.1e-6 ) ≈ 1 / (7.269e15 * 3.1e-6) ≈ 1 / (2.254e10) ≈ 44.4 pF

Step 2: Subtract internal capacitance

The total internal capacitance is Cint1 + Cint2 = 5 pF + 5 pF = 10 pF.

C_external_total = C_total - (Cint1 + Cint2) = 44.4 pF - 10 pF = 34.4 pF

Step 3: Divide by two for each external capacitor

Ctune1 = Ctune2 = C_external_total / 2 = 34.4 pF / 2 = 17.2 pF

Arrangement

• Connect one 17 pF capacitor from NFC1 to ground.
• Connect one 17 pF capacitor from NFC2 to ground.

This matches the recommended arrangement in the nRF54L15 documentation, where the antenna coil is connected differentially between NFC1 and NFC2, and each pin has a tuning capacitor to ground to achieve resonance at 13.56 MHz NFCT antenna recommendations – nRF54L15.

Note: Use the closest standard value (e.g., 18 pF if 17 pF is not available) and fine-tune as needed for your specific PCB and antenna.

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Summary:
For Lant = 3.1 μH and Cint1 = Cint2 = 5 pF on nRF54L15, use Ctune1 = Ctune2 ≈ 17 pF (each from NFC1/NFC2 to ground) to achieve 13.56 MHz resonance, as per the official recommendations.

[JONATHAN LL]

mej7000 said:

For Lant = 3.1 μH and Cint1 = Cint2 = 5 pF on nRF54L15, use Ctune1 = Ctune2 ≈ 17 pF (each from NFC1/NFC2 to ground) to achieve 13.56 MHz resonance, as per the official recommendations.

This is sounds reasonable. 

I personally like to start with the antennas recommendations and then work my way up in cap values to see how the resonance moves and adjust accordingly, but this seems ok.  

Regards,
Jonathan

[mej7000]

That's good advice. I will try that approach. I ordered a VNA for testing.

[JONATHAN LL]

Happy to help so feel free to ask some follow questions here if you need. We are here to help!

Regards,
Jonathan

[Wm_Post]

Abricon does not have any reference designs/evaluation boards for their NFC chip antennas.  They already provide the PCB footprint inside their datasheets - this one (https://abracon.com/datasheets/AANI-CH-0030.pdf) IS accurate.  A NFC chip antenna is essentially an inductor, so only the right capacitor (parallel capacitance combining internal chip cap plus external cap) is needed to resonate at 13.56MHz - it’s an LC parallel circuit.