MC33690 データシートの表示（PDF） - Freescale Semiconductor

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MC33690

Standalone Tag Reader Circuit

Freescale Semiconductor 

Read Function

RA

TD1

LA

1/32 counter

125 kHz

1/2

4 MHz

8 MHz

125 kHz

Self synchronous

Setup and Preload

CA

sample and hold

TD2

R1

RD

R2

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°

VDD

Buffer

500ns

S/H

+

-

Buffer

Comparator

+

100KW

-

VDD

DQ

C

500mA

AM Data

Clock 8 MHz

Shutdown

LVR

Interface

Data out

AGND

CEXT

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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