AM7968 datasheet(75/127 Pages) AMD

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AMD

TAXIchip Integrated Circuits Technical Manual

Sample Values

Using IBM Type 1 STP 150

Ω shielded twisted pair cable, a successful TAXI link was

established using the following component values:

R1 = 101

Ω

R2 = 291

Ω

R3 = 300

Ω

= 0.1

µF

6.0 BOARD LAYOUT CONSIDERATIONS

While the TAXIchip devices are digital in application, they are essentially analog parts,

containing high frequency analog Phase Locked Loops. Reliable operation in a high

frequency analog and digital environment requires that some simple board layout rules

be followed.

For example, most TAXI applications which are laid out on a wire wrap board will not

work reliably. Because they have at most one power and ground plane, most wirewrap

cards have insufficient separation between small signal current and digital switching

current. Digital switching noise can couple into the analog PLL, causing phase errors

and loss of synchronization. The preferred realization of a TAXI application is on a

printed circuit board, where the user can control the layout of power and ground planes.

6.1 Printed Circuit Board Layout

6.1.1 Rules for Layout

The following rules should be followed to ensure minimal noise coupling:

1. Use a PC board with separate GND and VCC planes.

2. Use two capacitors which differ by at least a factor of ten in value to decouple the

devices. The reactance of large capacitors has a significant inductive component at

high frequencies. Because of this inductive component, a single large capacitor is not

very effective against high frequency noise. Two capacitors, one typically of 1

µF and

one of 0.1

µF are more efficient at decoupling than a single large capacitor of 1.1 µF.

The recommended layout is as shown in Figure 6-1.

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