MLX90109
125kHz RFID Transceiver
3901090109 Page 11 of 16 Data Sheet
Rev 011 Jun/12
VDD
C
D
=100nF
1 8
7
6
5
2
3
4
100k
L, C
Microcontroller
DATA
CLOCK
100k19k
MODE
SPEED
VDD
MLX
90109
C1
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:
121
)//(2
1
CRR
F
offcut
⋅⋅⋅
=
−
π
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
Ω
, R2=19k
Ω
(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
