MLX90109 Datasheet, PDF (7/9 Pages) List of Unclassifed Manufacturers – 125kHz RFID Integrated Transceiver

Applications Information
uctrl
Read/
Write
VDD
C2=68nF
C1=47nF
39k
MLX90109
125kHz RFID Integrated Transceiver
Rpar
Rr, Lr, Cr
VDD
C_D=100nF
1
8
2
7
3
6
DataIN
ClockIN
4
5 ^MODE
SPEED
uctrl
MLX90109 Read/Write demokit:
Above schematic is used in a demokit which
allows to
• read and write
• the MLX90111 with 2cm diameter aircoils
• at 2 and 4kbaud, using Manchester and
Biphase encoding
• up to 7cm
whilst consuming only 5mA, inlcuding antenna
current.
An Atmel microcontroller (AT90S8515) was used
to have enough ports to drive a ledbar.
Antenna parameters:
- Lr= 89uH, Cr= 18nF
- Qr=30
- Rpar= Optional parallel resistance
Coil design considerations
Let's start from an operation frequency of 120
kHz. In order for the antenna not to get detuned
by stray capacitance to ground, it is good not to
have a high L value. In this way C is big and the
detuning is minimal.
The field of the coil is proportional to the number
of windings and to the current flowing through it:
H ~ N.I = N.V/(ω.L)
with V the ac voltage across the coil, and ω.L
the impedance of the coil.
Furthermore the inductance of the coil is
proportional to the square of the number of
windings.
L = Lo*N²
With Lo the inductance of 1 turn.
So
H ~ V/(ω.Lo.N)
Reducing the number of turns on the coil is good
for improving the field strength as well as for
making the construction of the coil easier.
Antenna impedance specification is determined
by the current that the antenna driver can
supply. Outside the specification, the feedback
loop will not operate as designed for, and
functionality will deteriorate drastically.
Zant = Zr // Rpar with Zr = Lr/(Rl*Cr)
MLX90109 Parallel Resonant transceiver
Page 7 of 9
Rev 1.4 19-Dec-00