MLX90109_06 Datasheet, PDF (10/15 Pages) Melexis Microelectronic Systems – 125kHz RFID Transceiver

MLX90109
125kHz RFID Transceiver
9.3 Power consumption
If the power consumption is not critical and the reader does not have to be put in power down, the MODU
voltage can be strapped to the required level (between 0.8V and V_pd). However, the power consumption can
be reduced by controlling the voltage on V_MODU pin (e.g. with an IO port of a microcontroller).
9.4 Noise cancellation
The read performance of a reader is linked with its robustness versus noise. The IC design has been
optimized to get a high signal-to-noise ratio (SNR). The resonant antenna is a natural band-pass filter, which
becomes more effective as its quality factor Q_r increases.
Noise rejection could also be improved by a careful PCB design, and by adding decoupling capacitor(s) on
the supply lines. The most sensitive pins to noise injection are MODU and V_DD. Since they directly determine
V_ant, the noise could be considered as an amplitude modulation (AM) data from a transponder.
If the noise on both pins were identical, it would cancel out, giving a very noise-insensitive reader. Adding a
capacitor C1 between MODU and V_DD, together with R1 and R2 yields a high pass filter with a cut-off
frequency at:
F_{cut −off}
=
2⋅π
1
⋅ (R₁ // R₂ ) ⋅ C₁
Typically, such a filter should short all noise in the data spectrum, but for many cases, it might be beneficial to
set it to less than the net frequencies (50Hz, 60Hz). For example: R1=100kΩ, R2=19kΩ (to set V_MODU), and
C1=220nF gives a cut off frequency of 45Hz.
9.5 Integrated decoding
The MLX90109 provides the option to have a decoded output. This significantly reduces the complexity of the
microcontroller software.
The data is available when the output clock signal is high. The clock signal has a 50% duty cycle when the
data is valid. When the noise level is stronger than the signal level, for instance when no tag is present in the
reader field, the duty cycle will be random. The microcontroller can use this feature to detect the presence of
a tag: in that case, it must allow some asymmetry on the clock. As the sampling error may be 4 s, it should
allow a margin of 8 or 12 s.
Remark that when the MLX90109 picks up a Manchester-encoded signal whereas the MODE pin is strapped
to V_SS (= Biphase decoding), the clock will also be asymmetric.
9.6 Close coupling
For very short operating distances, a strong coupling with a tag may drastically reduce the antenna
impedance Z_ant. If the current (I_driver) driven by the antenna internal driver FET goes higher than 14mA, the
antenna voltage V_ant may be reduced and the MLX90109 may be unable to read the transponder.
Coupling effect is application-dependent and must be evaluated case by case.
3901090109
Rev 008
Page 10 of 15
Data Sheet
Jun/06