ST75C520 datasheet(43/45 Pages) STMICROELECTRONICS

43 / 45 page

XII - ELECTRICAL SCHEMATICS (continued)

RING

TIP

220pF

40k

Ω

320

Ω

TL072

22k

Ω

22k

Ω

680pF

13.2k

Ω

13.2k

Ω

22k

Ω

220pF

22k

Ω

220

Ω

40k

Ω

20k

Ω

20k

Ω

TL072

15k

Ω

15k

Ω

82k

Ω

1.2k

Ω

1.2k

Ω

2.2nF

100 nF

µF

100nF

µF

Ω

µF

AGND

TRANSFORMER

Ω

RXA 2

TXA1

TXA2

RXA 1

VREFP

VCM

VREFN

Figure 14

XIII - PCB DESIGN GUIDELINES

Performances of the FAX modem depends on the

ST75C520 intrinsic performances and on the

proper PC board layout. All aspects of the proper

engineering practices, for PC board design, are

beyond the scope of this paragraph.

We recommend the following points :

- in a 4-layer PC board :

Separated digital ground and analog ground,

connected together at one point, as close as

possible to the ST75C520,

- in a 2-layer PC board :

Provide a ground grid in all space around and

under components on both sides of the band and

connect to avoid small islands,

- both AGNDR and AGNDT must be connected

with very low impedance to a single point, (see

Chapter I.7, Power Supply),

- the two 2.2nF capacitorsconnectedto the RXA1 and

RXA2 Pins must be as close as possible to them,

- thetwo 100nFcapacitorsconnectedto the VREFPand

VREFN pins must be as close as possible to them,

- analog and digital supplies must be connected

together, at a single point, as close as possible to

the chip (see Chapter I.7, Power Supply).

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