MLX90121

13.56MHz RFID Transceiver

3901090121

Page 7 of 26

Data Sheet

Rev. 007

Jan-2008

8.

Applications Information

This schematic has been optimized to drive a fifty ohms resistive antenna, using a five volts power supply.

Functional description

The transmitter output TX is connected to the supply by means of a choke L3. C3 is added to avoid a high

dV/dt at the TX output in case of a sudden interruption of the current in the choke. C3 is chosen high

enough to protect the chip, but low enough to keep the resonance of L3-C3 well above 13.56 MHz.

The transmitter signal is coupled with DC blocking capacitor C2 to the antenna matching network, which is

a T network made up by L2, CV1 and L1. CV1 allows a proper matching between the 50Ohm antenna

and the output impedance of the transmitter stage.

The receiver part of the chip gets its signal directly from the antenna by means of R1. It limits the voltage

swing at the RX pin to a level in between the supplies.

One should take care to properly decouple the power supplies of the chip. Especially the Vdd1 supply

which is used for the transmitter output. Any amplitude noise on that supply is AM modulated on the

carrier and will hence be perceived as noise by the receiver part. The same holds for any phase noise that

gets introduced into the quartz oscillator.

For the signal that goes to- and from the microcontroller: one should take care to keep them as far as

possible from the analog parts and the quartz oscillator. To do a first evaluation, it is highly recommended

to use the MLX90121 evaluation board that can be ordered from Melexis. The clock for the microcontroller

can also be derived from the XBUF pin. This pin provides a 13.56MHz buffered clock or 13.56MHz divided

by 2.