EVB90109
MLX90109 Evaluation Board Manual
Page 4 of 10
REVISION 006 - JUNE 20, 2017
390129010901
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.
r
res
ant
ant
L
Q
Z
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.
C
L
ant
Q
Q
Q
//
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.
overshoot
MODU
DD
ant
V
V
V
V
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.
