MLX90109

125kHz RFID Transceiver

3901090109

Page 9 of 16

Data Sheet

Rev 011

Jun/12

(3)

DDant

ant

ant

I

Q

I

⋅

=

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)

DDant

ant

ant

I

V

Z

=

The antenna impedance can also be calculated as:

(5)

ant

res

ant

ant

L

Q

Z

⋅

⋅

=

ω

with

From (4) and (5):

DDant

ant

ant

res

ant

I

V

L

Q

=

⋅

⋅

ω

=>

ant

res

ant

DDant

ant

L

V

I

Q

⋅

=

⋅

ω

Finally in comparison with the formula (2):

(6)

ant

res

ant

ant

L

V

I

⋅

=

ω

Using the previous formula (6), it is possible to define the proportionality between the total number of ampere-

antenna coil) :

ant

res

ant

ant

ant

ant

L

V

N

I

N

⋅

⋅

=

⋅

ω

with

ant

L

~

2

ant

N

(7)

ant

ant

I

N

⋅

~

ant

L

1

Hence, to generate a strong field, it is better to choose a low antenna inductance. Limitation to this is given by

(8)

res

ant

Q

Z

L

ω

⋅

=

min

min