AD9522-5SLASHPCBZ datasheet(74/76 Pages) AD

74 / 76 page

AD9522-5

Rev. 0 | Page 74 of 76

CMOS CLOCK DISTRIBUTION

The output drivers of the AD9522 can be configured as CMOS

drivers. When selected as a CMOS driver, each output becomes

a pair of CMOS outputs, each of which can be individually

turned on or off and set as inverting or noninverting. These

outputs are 3.3 V CMOS compatible.

When single-ended CMOS clocking is used, some of the

following guidelines apply.

Point-to-point connections should be designed such that each

driver has only one receiver, if possible. Connecting outputs in

this manner allows for simple termination schemes and minimizes

ringing due to possible mismatched impedances on the output

trace. Series termination at the source is generally required to

provide transmission line matching and/or to reduce current

transients at the driver.

The value of the resistor is dependent on the board design and

timing requirements (typically 10 Ω to 100 Ω is used). CMOS

outputs are also limited in terms of the capacitive load or trace

length that they can drive. Typically, trace lengths less than

3 inches are recommended to preserve signal rise/fall times and

signal integrity.

CMOS

10Ω

60.4Ω

(1.0 INCH)

MICROSTRIP

Figure 59. Series Termination of CMOS Output

Termination at the far end of the PCB trace is a second option.

The CMOS outputs of the AD9522 do not supply enough current

to provide a full voltage swing with a low impedance resistive, far-

end termination, as shown in Figure 60. The far-end termination

network should match the PCB trace impedance and provide the

desired switching point. The reduced signal swing may still meet

receiver input requirements in some applications. This can be

useful when driving long trace lengths on less critical nets.

CMOS

10Ω

50Ω

100Ω

Figure 60. CMOS Output with Far-End Termination

Because of the limitations of single-ended CMOS clocking,

consider using differential outputs when driving high speed

signals over long traces. The AD9522 offers LVDS outputs that

are better suited for driving long traces where the inherent noise

immunity of differential signaling provides superior performance

for clocking converters.

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