Read Function
AM Data
R A
L A
TD1
1/32 counter
125 kHz
1/2
4 MHz
8 MHz
Clock 8 MHz
Shutdown
C A
125 kHz
Self synchronous
sample and hold
Setup and Preload
LVR
TD2
Interface
R 1
V DD
11.25 ° , 22.5 ° , 33.75 ° , 45 ° , 56.25 ° , 67.5 ° , 78.75 ° , 90 °
+0 ° , -11.25 ° , -22.5 ° , -33.75 ° , -45 ° , -56.25 ° , -67.5 ° , -78.75 °
Buffer
500ns
+
Comparator
+
RD
S/H
-
Buffer
100KW
-
D
C
Q
Data out
VDD
R 2
500mA
AGND
C EXT
CEXT
10nF
Figure 2. Tag Reader Block Diagram
2
Read Function
When answering to the base station, a transponder generates an absorption modulation of the magnetic field. It results in an
amplitude/phase modulation of the current across the antenna. This information is picked up at the antenna tap point between
the coil and the capacitor. An external resistive ladder down scales this voltage to a level compatible with the demodulator input
voltage range (see Section 15, “Tag Reader” ).
The demodulator (see Figure 2 ) consists of:
?
?
?
?
an input stage (emitter follower)
a sample and hold circuit
a voltage follower
a low offset voltage comparator
The sampling time is automatically set to take into account a phase shift due to the tolerances of the antenna components (L and
C) and of the oscillator. The allowed phase shift measured at the input RD ranges from ? 45 ° to + 45 ° . Assuming that the phase
MC33690 Standalone Tag Reader Circuit, Rev. 5
6
Freescale Semiconductor
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