MLX90109 데이터 시트보기 (PDF) - Melexis Microelectronic Systems

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MLX90109

125kHz RFID Transceiver

Melexis Microelectronic Systems  

MLX90109

125kHz RFID Transceiver

8.3. Antenna Impedance

The antenna impedance is an important system design parameter for the MLX90109.

(4)

Zant



Vant

I DDant

The antenna impedance can also be calculated as:

(5)

Zant  Qant res  Lant

with ωres = 2*Fres

From (4) and (5):

Qant res

 Lant



Vant

I DDant

=>

Qant

 I DDant



Vant

res  Lant

Finally in comparison with the formula (2):

(6)

I ant



Vant

res  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) :

Nant



Iant



Nant



Vant

res  Lant

1

(7)

Nant  Iant ~

Lant

with

Lant ~

N

2

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 > 1k) and the Q that one can achieve for such an antenna:

(8)

Lmin



Z min

Qant  res

Remarks:

Note for equation (4): Mind that in reality the strong coupling with the tag may drastically reduce the antenna impedance.

Note for equation (5): Mind that the quality factor of the antenna (Qant) result in the quality factor of the coil and the quality

factor of the capacitance as:

(9)

Qant  Qcoil // Qcapacitance

So, a capacitance with a low quality factor may also reduce the antenna impedance.

REVISION 011 - JUNE 19, 2017

3901090109

Page 10 of 17

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