AN4409 datasheet(3/8 Pages) STMICROELECTRONICS

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AN4409

Tuning capacitance dependency for using the same antenna design

Tuning capacitance dependency for using the same

antenna design

Figure 1 here after shows the equivalent circuit of an M24LRXXE-R or an M24SRXX-R

mounted on a loop antenna in the presence of a sinusoidal magnetic field.

Figure 1. M24LRXXE-R or M24SRXX-R tag equivalent Circuit

VOC represents the open circuit voltage available from the antenna. It depends on the

magnetic field strength, the antenna size and number of turns.

The tag antenna impedance is given by ZA = RA + j.LA.ω, where LA is the antenna

inductance.

The M24LRXXE-R or M24SRXX-Y impedance is given by ZS = RS + j.1/CS. ω where RS

represents the power consumption of the chip, and CS represents the serial equivalent

tuning capacitance.

The tuning frequency of the tag described in Figure 1 is defined by the following Equation 1:

Equation 1

Let’s consider now, the tuning frequency of an M24LRXXE-R based tag (Ftun_M24LR) and

the tuning frequency of an M24SRXX-Y based tag (Ftun_M24SR) when using the same

antenna. They are respectively given by Equation 2 and Equation 3:

ftun

Π LA CS

⋅

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