AN3249 STMICROELECTRONICS | Alldatasheet
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Technical content
M24LR series internal capacitance considerations for antenna tuning
1 Introduction
The datasheet of the M24LR series devices(a) specifies a typical internal tuning capacitance value of 27.5 pF measured at 0.5 VPEAK. While this value is correct in production test conditions, more considerations have to be taken into account for tuning the antenna in an real application environment. This document describes the M24LR series tuning capacitance variation as a function of input voltage, explains the M24LR series tuning capacitance specification and gives a robust tuning capacitance target for the inductive loop antenna design. a. I 2C/ISO15693 Dual Interface EEPROM IC Table 1: Applicable products Type Applicable products Dual Interface EEPROM M24LR series(1) 1. While the considerations introduced in this docum ent applies to the entire M24LR series, the M24LR64-R has been used hereafter for specific measurements purpose.
2 Tuning capacitance variation as a function of input
and 2 VPEAK at 13.56 MHz using a network analyzer. Figure 1. M24LR64-R serial equivalent capacitance CS measurement example best measurement repeatability.
3 M24LR series capacitance for antenna tuning
3.1 M24LR series eq uivalent schematic
presence of a magnetic field.
Figure 2. M24LR equivalent circuit (VOC) can be determined using the following equation. impedance. RS and CS represent the serial equivalent model of the M24LR impedance. determined using the following equation. CS as a function of VAC0-AC1 is shown at low power by the curve in Figure 1.
the correct tuning capacitance. reference for antenna design must be defined. interface I2C/RFID devices” for more details on antenna tuning criteria and methodology.
3.2 Correct M24LR series capacit ance value for antenna tuning
magnetic field amplitude flowing into the antenna. M24LR antenna and the reader antenna decreases (because the magnetic field increases). Figure 3. M24LR RF voltage As an example, the Q value for the ANT1 reference design is between 30 and 35. AC0-AC1 ≥ 1.9 VPEAK at the largest distance from reader.
antennas tuned respectively for CS1 < CS2, CS2 = 29 pF and CS3 > CS2. Figure 4. M24LR RF input voltage different antenna inductance antenna inductance LA designed for CS = 29 pF.
4 Conclusion
in the datasheet which corresponds to production test conditions.
5 Revision history
Table 2. Document revision history 23-Jul-2010 1 Initial release.