AN3249 datasheet(4/7 Pages) STMICROELECTRONICS

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M24LR series capacitance for antenna tuning

AN3249

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DocID017760 Rev 2

Optimizing antenna tuning consists in designing an antenna with the correct impedance for

the correct tuning capacitance.

Considering variations of CS according to VAC0-AC1, the correct capacitance value used as

reference for antenna design must be defined.

Please refer to AN2972 “Designing an antenna for the M24LRxx-R and M24LRxxE-R dual

interface I2C/RFID devices” for more details on antenna tuning criteria and methodology.

3.2

Correct M24LR series capacitance value for antenna tuning

According to Equation 1, VOC depends on the antenna dimensions, number of turns and the

magnetic field amplitude flowing into the antenna.

As a consequence, for a given antenna, VOC increases when the distance between the

M24LR antenna and the reader antenna decreases (because the magnetic field increases).

Figure 3 represents the M24LR input voltage variation as a function of VOC for an antenna

tuned on a M24LR serial capacitance value based on the curve shown in Figure 1. As

explained above, this case corresponds to a decreasing distance between M24LR antenna

and reader antenna; the longer the distance from the reader antenna, the closer the value of

VOC gets to 0 V .

Figure 3. M24LR RF voltage

As VOC increases, a sharp increase of VAC0-AC1 occurs when the serial capacitance of the

M24LR reaches a value that satisfies the resonance condition

LA • CS • ω2 = 1. At resonance, VAC0-AC1 = Q • VOC where Q = RS/(RA + RS) • 1/(RS • CS •

ω).

As an example, the Q value for the ANT1 reference design is between 30 and 35.

The maximum operating distance from the reader is defined by the maximum distance for

which the M24LR input RF voltage VAC0-AC1 remains above 1.9 VPEAK which is the

minimum voltage for the M24LR to be properly energized and perform read and write

operations. Optimizing an antenna for an operating range consists in designing an antenna

allowing VAC0-AC1 ≥ 1.9 VPEAK at the largest distance from reader.

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