MLX90109CDC-AAA-000-TU(2012) 데이터 시트보기 (PDF) - Melexis Microelectronic Systems
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MLX90109CDC-AAA-000-TU Datasheet PDF : 16 Pages
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MLX90109
125kHz RFID Transceiver
(3)
I ant = Qant ⋅ I DDant
A typical coil quality factor (Qant) 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
=
Vant
I DDant
The antenna impedance can also be calculated as:
(5)
Z ant = Qant ⋅ ωres ⋅ Lant
with ωres = 2π*Fres
From (4) and (5):
Qant
⋅ ω res
⋅ Lant
=
Vant
I DDant
=>
Qant
⋅ I DDant
= Vant
ωres ⋅ Lant
Finally in comparison with the formula (2):
(6)
I ant
= Vant
ωres ⋅ Lant
From the formula above, it is clear that Qant has no influence on Iant. Increasing Qant is equivalent to reduce
the antenna supply current IDDant, 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 Nant the number of turns of the
antenna coil) :
N ant
⋅ I ant
=
N ant
⋅ Vant
ωres ⋅ Lant
1
(7)
N ant ⋅ I ant ~
Lant
with
Lant ~
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 (Zant > 1kΩ) and the Q that one can achieve for such an antenna:
(8)
Lmin
=
Z min
Qant ⋅ ωres
3901090109
Rev 011
Page 9 of 16
Data Sheet
Jun/12
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