July 2011

Doc ID 17256 Rev 2

1/11

AN3178

Application note

Using a surface-mount inductor

as an M24LRxx antenna

Introduction

The M24LRxx ICs are a family of Dual Interface EEPROMs from STMicroelectronics. Each

EEPROM IC can be accessed as a single memory bank via a wired or RF interface.

The RF interface:

●

Is based on a passive RFID technology supporting the ISO15693 standard.

●

Communicates with ISO15693 RF systems (called RFID readers),

●

Does not require any on-board power to operate. Both energy and data are received

from the RFID reader.

In order for the RF interface of the IC to operate, an inductive antenna is required. This

antenna must be connected to the IC RF port.

There are three different methods for implementing an inductive antenna.

1.

On-board etched antenna: this method consists of designing an on-board inductive

antenna etched on the PCB itself. It allows for a wide range of antenna sizes but has a

cost in terms of PCB real estate. Refer to he application note AN2972 Designing an

antenna for the M24LR64 dual interface I²C/RFID device for guidance. ST also

provides antenna templates in Gerber format.

2.

Off-board antenna module: this method consists of using an external antenna,

connected to the M24LR64, and to the main PCB through an I²C connector. This allows

for a larger antenna, resulting in a greater read range. The off-board antenna can be

made of:

a)

A daughter board using simple PCB technology

b)

A flexible antenna, equipped with an M24LRxx device and a wired I²C connection.

3.

External surface-mount inductor: this consists of connecting an external inductor to

the M24LRxx. This can have a small footprint and requires no design effort, but it has a

reduced read range.

This application note provides guidelines for the third option: using an external surface-

mount inductor as an RF antenna for M24LRxx ICs.

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