mlx90121 Melexis Company, mlx90121 Datasheet - Page 6

• Manufacturer Part Number: mlx90121
• Description: 13.56mhz Rfid Transceiver
• Manufacturer: Melexis Company

7.
Power supply
The 90121 requires a nominal 3 or 5 volts external
power supply. Operation is guaranteed between
2.7 and 5.5 Volts. The current drain depends on
the antenna impedance and the output matching
network configuration. Care must be taken about
the power supply: power supply ripple and noise
will
performance.
Transmitter
The output transistor is a low Ron MOSFET. The
drain is directly accessible on the TX pin. A
recommended application schematic optimized to
drive a resistive fifty ohms antenna with a five
volts power supply is provided as a part of this
specification. A simple resonant circuit or/and a
simpler matching network can be connected to the
output. In that case, the general performance and
harmonic suppression will be reduced.
100 % modulation is achieved by means of gating
the square wave drive of the output transistor.
A variable modulation depth is obtained by means
of switching a resistor in series with the output
transistors’
resistor provides the default modulation depth
setting. Increasing this external resistor will
increase the modulation depth.
Receiver
The receiver input is typically connected to the
antenna
modulation from the tag is then recovered by
means of a diode envelope detector.
FSK and PSK recovery
The demodulated input signal is amplified and
band pass filtered. The signal is then hard limited
by a logarithmic amplifier, and fed to the digital
section. PSK decoded, FSK decoded or a direct
FSK signal can be used for further decoding.
ASK recovery
For ASK recovery, the high pass sections of the
band pass filters are removed, to avoid falling
edge degradation by the filter settling time. The
signal is DC coupled and fed to the input of the
logarithmic amplifier. The logarithmic amplifier
works as a high gain amplifier and at the same
time it generates the envelope of the ASK signal.
The demodulated output from the log amp is then
3901090121
Rev. 007
severely
General Description
through
source
degrade
an
connection.
external
the
overall
resistor.
An
external
system
The
Page 6 of 26
fed to a comparator. To avoid signal degradation,
the time constant of the comparator has to be
switched from fast response during acquisition to
a slow time constant during the tags response.
This is done by switching the CK signal at the
beginning of the response of the tag. The
recovered data stream is fed to the digital section
for further processing.
Majority Voting
Both FSK/PSK or ASK can use the Majority
Voting function that will filter for noise and jitter,
that will correct distorted signals and will hence
improve performance.
Reference clock and internal oscillator
The reference clock may be obtained externally
by applying a suitable clock signal to the XTAL1
pin. A sine wave centered at VCC/2 or a CMOS
logic compatible signal is an acceptable external
system clock. The built-in reference oscillator will
work either with a quartz crystal or a ceramic
resonator. The nominal system clock frequency is
13.56 MHz.
Reset defaults and power management
After a power on reset has been performed, the
device is put in its default configuration. There are
three power modes available. In the transmission
mode, the device is fully powered.
mode, only the reference oscillator is running.
This allows for a fast start up. In the power down
mode, the device
completely switched off, offering essentially a zero
state.
Serial communication interface
The communication interface normally uses 6
wires:
-
-
-
-
-
-
CK: serial clock input
DIN: data input
DOUT: data output
DSYNC: synchronization output for DOUT
MODE:
selection input
RTB:
input.
13.56MHz RFID Transceiver
reception or transmission selection
configuration
internal bias system is
MLX90121
or
communication
Data Sheet
Jan-2008
In the idle