MLX90109EDC-AAA-000-RE View Datasheet(PDF) - Melexis Microelectronic Systems
Part Name
Description
MFG CO.
'MLX90109EDC-AAA-000-RE' PDF : 17 Pages View PDF
MLX90109
125kHz RFID Transceiver
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 Qr 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 VDD. Since they directly determine Vant, 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 VDD, together with R1 and R2 yields a high pass filter with a cut-off frequency at:
Fcu t o ff
2
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=100k, R2=19k (to set VMODU), 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 4μs, it should allow a margin of 8 or 12μs.
Remark that when the MLX90109 picks up a Manchester-encoded signal whereas the MODE pin is strapped to VSS (= 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 Zant. If the
current (Idriver) driven by the antenna internal driver FET goes higher than 14mA, the antenna voltage Vant 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.
REVISION 011 - JUNE 19, 2017
3901090109
Page 12 of 17
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