SAA7372 View Datasheet(PDF) - Philips Electronics

Part Name

Description

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SAA7372

Digital servo processor and Compact Disc decoder (CD7)

Philips Electronics 

Philips Semiconductors

Digital servo processor and Compact Disc

decoder (CD7)

Product specification

SAA7372

MGA362 - 2

G

f4

f3

BW

f

Fig.18 Motor servo mode diagram.

7.14 Servo part

7.14.1 DIODE SIGNAL PROCESSING

The photo detector in conventional two-stage three-beam

compact disc systems normally contains six discrete

diodes. Four of these diodes (three for single foucault

systems) carry the central aperture signal (CA) while the

other two diodes (satellite diodes) carry the radial tracking

information. The CA signal is processed into an HF signal

(for the decoder function) and LF signal (information for the

focus servo loop) before it is supplied to the SAA7372.

The analog signals from the central and satellite diodes

are converted into a digital representation using

analog-to-digital converters (ADCs). The ADCs are

designed to convert unipolar currents into a digital code.

The dynamic range of the input currents is adjustable

within a given range, which is dependent on the value of

external resistor connected to pin IrefT. The maximum

current for the central diodes and satellite diodes is given

in the following formulae;

Iin (max, central)

=







2----.-R-4----I×-r--e--1-f-T--0---6-







µA

Iin (max, satellite)

=







1----.-R-2----I×-r--e--1-f-T--0---6-







µA

The VRH voltage is internally generated by control circuitry

which ensures that the VRH voltage is adjusted depending

on the spread of internal capacitors, using the reference

current generated by the external resistor on IrefT. In the

application VRL is connected to VSSA1. The maximum input

currents for a range of resistors is given Table 10.

Table 10 Maximum current input

RIrefT (kΩ)

220

240

270

300

330

360

390

430

470

510

560

620

DIODE INPUT CURRENT RANGE

D1 TO D4 (µA) R1 AND R2 (µA)

10.909

10.000

8.889

8.000

7.273

6.667

6.154

5.581

5.106

4.706

4.286

3.871

5.455

5.000

4.444

4.000

3.636

3.333

3.077

2.791

2.553

2.353

2.143

1.935

This mode of VRH automatic adjustment can be selected

by the preset latch command.

Alternatively, the dynamic range of the input currents can

be made dependent on the ADC reference voltages VRL

and VRH. The maximum current for the central diodes and

satellite diodes is given in the following formulae;

1998 Feb 26

24

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