EVB90109_12 Datasheet, PDF (4/10 Pages) Melexis Microelectronic Systems – MLX90109 Evaluation Board

EVB90109
MLX90109 Evaluation Board
2.2 Parallel antenna
The parallel antenna of the EVB90109 consists of the inductance (L) of the reader coil and the tune capacitor
(C4), which form a resonant system at 125 kHz with a total (inductance and capacitance) antenna impedance
of about 1.3kOhms. The MLX90109 is specified to drive a minimal antenna impedance of 1kOhm, which can
be theoretically calculated using the following formula.
Z_ant = Q_ant ⋅ω_res ⋅ L_r
Care should be taken that the presence of a transponder may reduce the theoretical antenna impedance to
be smaller that the minimum specified. Therefore it may be of interest to take some extra margin in the
antenna design by taking a slightly higher quality factor to increase the antenna impedance.
A tune capacitance with a poor quality factor may also reduce the global performances of the antenna, as
shown in the following formula.
Q_ant = Q_L // Q_C
Therefore, the tune capacitance should be taken with a high quality factor or low ESR (plastic or mica) to not
reduce the total impedance of the antenna and moreover, to not annihilate all the efforts made in the design
of the antenna to work with a high quality factor and then to get the maximum of operating distance.
2.3 Modulation network
The modulation network is used to set the voltage on the MODU pin of the MLX90109. As shown in the
following formula, the voltage on the antenna can be adjusted according to this voltage. This feature can be
used to reduce the power consumption of the chip by setting a lower voltage on the antenna. Under no
circumstances it can be used to make proportionnal modulaiton, due to internal feedback loop which doesn’t
allow this use.
V_ant = V_DD − V_MODU − V_overshoot
The EVB90109 is provided with two resistors R1 and R2 as the modulation netwok controlled through the pin
MODUR2 to set the voltage on the antenna respectively On (Vmodu to 0.8 Volts) and Off (Vmodu to 5 Volts).
Others footprint C1, C3 and R3 and an extra pin MODUR3 are available on the board and can be used for
specific applications.
2.4 Antenna voltage in read operation
The resistors R1 and R2 set VMODU to 0.8V (1V for full temperature range [–40, 85] C) which is the lowest
possible voltage to guarantee that the antenna driver of the MLX90109 stays out of its linear operating zone.
The field is set On (VMODU to 0.8V) and Off (VMODU to VDD) when applying 0V and VDD on pin MODUR2.
390129010901
Rev 006
Page 4 of 10
Evaluation Board
Aug-2012