EVB90109_12 Datasheet, PDF (5/10 Pages) Melexis Microelectronic Systems – MLX90109 Evaluation Board

EVB90109
MLX90109 Evaluation Board
2.5 Noise cancellation in read operation
The resonant antenna is a natural band-pass filter, which becomes more effective as Q_ant increases.
The MLX90109 has an internal first order filtering of the envelope that changes according to the setting of the
SPEED pin to fit to the biphase and Manchester data spectrum:
2kbaud (speed = 1): 400Hz to 3.6kHz
4kbaud (speed = 0): 800Hz to 7.2kHz
Noise that is injected on MODU pin or on VDD will cause Vant to change, and will therefore be considered to
be data by the sampler.
This noise can simply be cancelled by adding a capacitor C1 between MODU and VDD. Together with the
modulation network R1 and R2, it creates a high pass filter with cut off frequency at:
1
2 ⋅π ⋅ (R₁ // R₂ )⋅ C₁
Note: Due to the long time constant introduces and the parasitic oscillations that it may provoke with the
transistor Q1, the capacitor C1 must be removed when using the EVB90109 in On/Off keying modulation
mode.
2.6 Antenna voltage in 100% modulation operation
The modulation network R1 and R2 can be used to set the voltage amplitude on the antenna On and Off by
applying 0 Volts and Vdd on the pin MODUR2.
Switching ON the magnetic field depends on the internal driver of MLX90109 and takes less than 5 carrier
periods. Switching OFF the magnetic field depend on the quality factor (Qant) of the parallel antenna
connected to the reader (about 24 periods with the EVB90109). To reduce this fall time, the “fast decay”
circuit can be used.
390129010901
Rev 006
Page 5 of 10
Evaluation Board
Aug-2012