AN3249 datasheet(5/7 Pages) STMICROELECTRONICS

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AN3249

Conclusion

To satisfy this condition, antenna inductance must satisfy the resonance criteria

LA • CS • ω2 = 1, where CS is the M24LR serial capacitance measured at the minimum

operating voltage 1.9 VPEAK (as shown in Figure 1, where CS is close to 29 pF at

1.9 VPEAK).

To demonstrate this, Figure 4 represents the M24LR RF Input voltage VAC0-AC1 as a

function of the RF voltage VOC available from the antenna for different inductive loop

antennas tuned respectively for CS1 < CS2, CS2 = 29 pF and CS3 > CS2.

The comparison graphics shown in Figure 4 assume using the same antenna dimensions

and number of turns.

Figure 4. M24LR RF input voltage different antenna inductance

As shown in Figure 4, the minimum RF voltage VOC for which VAC0-AC1 is above 1.9 VPEAK

(and consequently maximum distance at which the M24LR can operate) is reached for an

antenna inductance LA designed for CS = 29 pF.

Conclusion

Taking into account this information, STMicroelectronics recommends designing antennas

for the M24LR series using CS = 29 pF as a reference value, instead of the value specified

in the datasheet which corresponds to production test conditions.

AN3249 Arkusz danych(PDF) 5 Page - STMicroelectronics