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ICS308

SERIAL PROGRAMMABLE QUAD PLL VERSACLOCK SYNTHESIZER

Integrated Circuit Systems 

ICS308

SERIAL PROGRAMMABLE QUAD PLL VERSACLOCK SYNTHESIZER

External Components

Series Termination Resistor

Clock output traces over one inch should use series

termination. To series terminate a 50Ω trace (a

commonly used trace impedance), place a 33Ω resistor

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

pin as possible. The nominal impedance of the clock

output is 20Ω.

STROBE Pull-up Resistor

In order for the device to start up in the default state, a

250 kOhm pull-up resistor is required.

Decoupling Capacitors

As with any high-performance mixed-signal IC, the

ICS308 must be isolated from system power supply

noise to perform optimally.

Decoupling capacitors of 0.01µF must be connected

between each VDD and the PCB ground plane.

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)

been the crystal and device. Crystal capacitors must be

connected from each of the pins X1 and X2 to ground.

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

(CL -6 pF)*2. In this equation, CL= crystal load

capacitance in pF. Example: For a crystal with a 16 pF

load capacitance, each crystal capacitor would be 20

pF [(16-6) x 2] = 20.

PCB Layout Recommendations

For optimum device performance and lowest output

phase noise, the following guidelines should be

observed.

1) Each 0.01µF decoupling capacitor should be

mounted on the component side of the board as close

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

between decoupling capacitor and VDD pin. The PCB

trace to VDD pin 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 33Ω series termination resistor

(if needed) should be placed close to each clock

output.

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

same side of the board, minimizing vias through other

signal layers.

ICS308 Configuration Capabilities

The architecture of the ICS308 allows the user to easily

configure the device to a wide range of output

frequencies, for a given input reference frequency.

The frequency multiplier PLL provides a high degree of

precision. The M/N values (the multiplier/divide values

available to generate the target VCO frequency) can be

set within the range of M = 1 to 2048 and N = 1 to 1024.

The ICS308 also provides separate output divide

values, from 2 through 20, to allow the two output clock

banks to support widely differing frequency values from

the same PLL.

Each output frequency can be represented as:

Output Freq. = (Ref. Freq)*(M/N)/Output Divide

MDS 308 F

4

Revision 090704

Integrated Circuit Systems ● 525 Race Street, San Jose, CA 95126 ● tel (408) 297-1201 ● www.icst.com

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