ICS84321 datasheet(12/18 Pages) ICST

12 / 18 page

84321AY

www.icst.com/products/hiperclocks.html

REV. B JUNE 9, 2005

Integrated

Circuit

Systems, Inc.

ICS84321

260MHZ, CRYSTAL-TO-3.3V DIFFERENTIAL

LVPECL FREQUENCY SYNTHESIZER

FIGURE 5B. PCB BOARD LAYOUT FOR ICS84321

The following component footprints are used in this layout

example:

All the resistors and capacitors are size 0603.

POWER AND GROUNDING

Place the decoupling capacitors C14 and C15, as close as pos-

sible to the power pins. If space allows, placement of the

decoupling capacitor on the component side is preferred. This

can reduce unwanted inductance between the decoupling ca-

pacitor and the power pin caused by the via.

Maximize the power and ground pad sizes and number of vias

capacitors. This can reduce the inductance between the power

and ground planes and the component power and ground pins.

The RC filter consisting of R7, C11, and C16 should be placed

as close to the V

CCA pin as possible.

CLOCK TRACES AND TERMINATION

Poor signal integrity can degrade the system performance or

cause system failure. In synchronous high-speed digital systems,

the clock signal is less tolerant to poor signal integrity than other

signals. Any ringing on the rising or falling edge or excessive ring

back can cause system failure. The shape of the trace and the

trace delay might be restricted by the available space on the board

and the component location. While routing the traces, the clock

signal traces should be routed first and should be locked prior to

routing other signal traces.

• The differential 50

Ω output traces should have the

same length.

• Avoid sharp angles on the clock trace. Sharp angle

turns cause the characteristic impedance to change on

the transmission lines.

• Keep the clock traces on the same layer. Whenever pos-

sible, avoid placing vias on the clock traces. Placement

of vias on the traces can affect the trace characteristic

impedance and hence degrade signal integrity.

• To prevent cross talk, avoid routing other signal traces in

parallel with the clock traces. If running parallel traces is

unavoidable, allow a separation of at least three trace

widths between the differential clock trace and the other

signal trace.

• Make sure no other signal traces are routed between the

clock trace pair.

• The matching termination resistors should be located as

close to the receiver input pins as possible.

CRYSTAL

The crystal X1 should be located as close as possible to the pins

25 (XTAL_IN) and 24 (XTAL_OUT).The trace length between the

X1 and U1 should be kept to a minimum to avoid unwanted

parasitic inductance and capacitance. Other signal traces should

not be routed near the crystal traces.

TL1, TL21N are 50 Ohm

traces and equal length

TL1

GND

C16

TL1N

VCC

Close to the input

pins of the

receiver

C11

C14

VIA

C15

VCCA

PIN 1

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