Posted on March 01, 2018 at 16:20Hi Danyon,
The ST25R3911B is capable of driving one differential antenna (using RFO1 and RFO2) or switching between two single ended antennas (one connected between RFO1 and GND the other to RFO2 and GND).
Which RFO/RFI is used by the chip can then be selected by flipping the 'rfo2' register.
These antennas could either be normal antennas (like the differential one) or 50Ohm antennas.
50Ohm antennas are already foreseen with damping resistor, series and parallel capacitor to be tuned to 50Ohm at 13.56MHz. They can then be plugged using a 50Ohm coax cable to the NFC reader unit. In addition the NFC Reader unit requires a matching circuit to match the IC's load impedance seen on RFO1/2 to 50Ohm.
The minimum load impedance for differential use is Zload = 8Ohm. (Datasheet Table 100) Therefore the minimum load impedance for single ended use would be Zload = 4Ohm.
Since the target (50Ohm) is fixed, the matching values do not have to be changed when changing the antenna:
For example:
4Ohm to 50Ohm matching network:
LEMC = 240nH
CEMC = 330pF
Cseries = 680pF
Cpara = 270pF
8Ohm to 50Ohm matching network:
LEMC = 330nH
CEMC = 220pF
Cseries = 470pF
Cpara = 180pF
For conventional antennas (not matched to 50Ohm) the approach of finding the proper matching network is similar to what is described in the AN4974.
You have to first measure the antenna parameter.
Measured Antenna Parameters:
Inductance @ 1MHz: 929nH
DC resistance @ 1MHz: 329mOhm
Parallel resistance @ SRF: 1500Ohm
SRF: 62.667MHz
To get the proper values for the single ended antennas you have to input twice the Antenna Inductance, DC resistance and Parallel resistance. The Self Resonance Frequency stays the same.
The inputs into the 'ST25R Antenna Matching Tool' would then be:
Inductance @ 1MHz: 1858nH
DC resistance @ 1MHz: 658mOhm
Parallel resistance @ SRF: 3000Ohm
SRF: 62.667MHz
Since the tool will calculate the circuitry for a differential matching network, we will have to input also twice the target impedance.
The 'Matching inputs' would look like this:
EMC Inductor: 270nH
DC Resistance: 500mOhm
EMC filter: 11750kHz
Target matching Z: 16Ohm (this will give us 8Ohm for single ended use)
Target antenna Q: 17
After hitting 'calculate matching values' you will get reasonable results.
Of course the matching inputs are subject to change - according to the AN4974 -
Antenna
matching
for
ST25R3911B/ST25R391x
devices
.
BR Travis
