Hello,
M24SR family has a input tuning capacitance of 27.5pF. As a résult, target inductance of antenna for a resonant frequency of 14-14.4MHz (recommended for ISO14443 product, see AN4409 Using the same antenna for M24LRXXE-R and M24SRXX-Y Dual Interface EEPROM families).
between In Nucleo-NFC01A schematic, there are capacitors (C2, C6,C7, C8 and C9):
- C2 is an additional tuning cap used to decrease the resonant frequency of antenna in case it is two high; indeed, an antenna design strategy can be to design a spiral which inductance is intentionally lower than ideal value and adjust tuning frequency by use of C2.
- C6,C7 and C8, C9 are implemented for improve robustness during CEM Immunity test. They are not mandatory. C6 and C8 are in series and are equivalelent to a 47pF/2=23.5pF capacitor in parallel to AC0-AC1. The total equivalent tuning capacitance to take into account in spiral coil design becomes 27.5pF (M24SR tuning capacitance) + 23.5pF = 51pF. The corresponding antenna inductance value is 2.5µH to achieve a tuning frequency around 14MHz. The antenna is a double layer spiral coil (each layer is 31mm/30mm/4 turns/0.6mm/0.6mm spiral coil, 870µH each)
A double layer antenna as well as using trimming caps between AC0 (respect. AC1) ang GND are not necessary. for your specific need, you can design a signle layer coil 100x20mm with 4.7µH max inductance.
You could target 4.5µH for instance and adjust tuning frequency with parallel tuning cap as C2. Based on Edesign Suite, it could be:
Feel to tell us if you need more,
best regards,
Henry Crane
STMicroelectronics RFID/NFC support
