MLX90109_06 Datasheet, PDF (8/15 Pages) Melexis Microelectronic Systems – 125kHz RFID Transceiver

MLX90109
125kHz RFID Transceiver
A typical coil quality factor (Q_ant) value is 23, resulting in antenna currents of about 100mA
This current resonance of the parallel antenna allows to build very low power reader base stations, contrary
to serial antenna based versions. Readers using a serial antenna can leverage their voltage resonance to
drive bigger antenna’s for long distance reading up to 1m, whereas the MLX90109 is designed to drive
antennas to obtain a reading distance of 1cm up to 15cm (6”) (depending on efficiency and dimensions).
8.3 Antenna Impedance
The antenna impedance is an important system design parameter for the MLX90109.
(4)
Z _ant
=
V_ant
I _DDant
The antenna impedance can also be calculated as:
(5)
Z _ant = Q_ant ⋅ω_res ⋅ L_ant
with
res ^{= 2π*Fres}
From (4) and (5):
Q_ant
⋅ ω _res
⋅ L_ant
=
V_ant
I _DDant
=>
Q_ant
⋅ I _DDant
=
V_ant
ω_res ⋅ L_ant
Finally in comparison with the formula (2):
(6)
I _ant
= ^Vant
ω_res ⋅ L_ant
From the formula above, it is clear that Q_ant has no influence on I_ant. Increasing Q_ant is equivalent to reduce
the antenna supply current I_DDant, hence it reduces the overall current consumption.
Using the previous formula (6), it is possible to define the proportionality between the total number of ampere-
turns, generating the magnetic field and the inductance of the antenna (With N_ant the number of turns of the
antenna coil) :
N _ant
⋅ I_ant
=
N _ant
⋅ ^Vant
ω_res ⋅ L_ant
(7)
N ⋅ I _{ant ant} ¡
1
L_ant
with
L_ant ¡
N2
ant
Hence, to generate a strong field, it is better to choose a low antenna inductance. Limitation to this is given by
the minimal antenna impedance (Z_ant > 1kΩ) and the Q that one can achieve for such an antenna:
(8)
L_min
=
Z _min
Q_ant ⋅ω_res
Remarks:
Note for equation (4): Mind that in reality the strong coupling with the tag may drastically reduce the antenna
impedance.
3901090109
Rev 008
Page 8 of 15
Data Sheet
Jun/06