ICS426 datasheet(3/6 Pages) ICST

3 / 6 page

Serial ATA/Fibre Channel Clock Synthesizer

MDS 426 B

Revision 091102

Integrated Circuit Systems, Inc. q 525 Race Street, San Jose, CA 95126 q tel (408) 295-9800 q

www.icst.com

ICS426

External Components

Decoupling Capacitor

As with any high performance mixed-signal IC, the

ICS426 must be isolated from system power supply

noise to perform optimally.

A decoupling capacitor of 0.01µF must be connected

between each VDD and the PCB ground plane.

Series Termination Resistor

Clock output traces over one inch should use series

termination. To series terminate a 50

Ω trace (a

commonly used trace impedance), place a 33

Ω resistor

in series with the clock line, as close to the clock output

pin as possible. The nominal impedance of the clock

output is 20

Ω.

Crystal Load Capacitors

The device crystal connections should include pads for

small capacitors from X1 to ground and from X2 to

ground. These capacitors are used to adjust the stray

capacitance of the board to match the nominally

required crystal load capacitance. Because load

capacitance can only be increased in this trimming

process, it is important to keep stray capacitance to a

minimum by using very short PCB traces (and no vias)

between the crystal and device. Crystal capacitors, if

needed, must be connected from each of the pins X1

and X2 to ground.

The value (in pF) of these crystal caps should equal

(CL -18pF)*2. In this equation, CL= crystal load

capacitance in pF. Example: For a crystal with a 20 pF

load capacitance, each crystal capacitor would be 4 pF

[(20-18) x 2] = 20.

PCB Layout Recommendations

For optimum device performance and lowest output

phase noise, the following guidelines should be

observed.

1) The 0.01µF decoupling capacitors should be

mounted on the component side of the board as close

to the VDD pins as possible. No vias should be used

between the decoupling capacitors and VDD pins. The

PCB trace to VDD pins should be kept as short as

possible, as should the PCB trace to the ground via.

2) The external crystal should be mounted just next to

the device with short traces. The X1 and X2 traces

should not be routed next to each other with minimum

spaces, instead they should be separated and away

from other traces.

3) To minimize EMI the 33

Ω series termination resistor,

if needed, should be placed close to the clock output.

4) An optimum layout is one with all components on the

same side of the board, minimizing vias through other

signal layers. Other signal traces should be routed

away from the ICS426. This includes signal traces just

underneath the device, or on layers adjacent to the

ground plane layer used by the device.

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