MC33690 Standalone Tag Reader Circuit, Rev. 5

Read Function

Freescale Semiconductor

6

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

D

C

Q

+

-

1/32 counter

1/2

RD

CEXT

AGND

Buffer

VDD

500ns

S/H

Buffer

Comparator

100KW

8MHz

Clock 8 MHz

Data out

Interface

125 kHz

TD2

11.25

°, 22.5°, 33.75°, 45°, 56.25°, 67.5°, 78.75°, 90°

AM Data

4MHz

125 kHz

LVR

Shutdown

TD1

10nF

+0

°, -11.25°, -22.5°, -33.75°, -45°, -56.25°, -67.5°, -78.75°

CA

LA

R1

R2

CEXT

RA

VDD

+

-

Setup and Preload

Self synchronous

sample and hold

500mA