AD9985SLASHPCB datasheet(30/32 Pages) AD

30 / 32 page

AD9985

Rev. 0 | Page 30 of 32

PLL

Place the PLL loop filter components as close to the FILT pin as

possible.

Do not place any digital or other high frequency traces near

these components.

Use the values suggested in the data sheet with 10% tolerances

or less.

OUTPUTS (BOTH DATA AND CLOCKS)

Try to minimize the trace length that the digital outputs have to

drive. Longer traces have higher capacitance, which requires

more current, which causes more internal digital noise.

Shorter traces reduce the possibility of reflections.

Adding a series resistor of value 22 Ω to 100 Ω can suppress

reflections, reduce EMI, and reduce the current spikes inside of

the AD9985. However, if 50 Ω traces are used on the PCB, the

data outputs should not need resistors. A 22 Ω resistor on the

DATACK output should provide good impedance matching

that will reduce reflections. If series resistors are used, place

them as close to the AD9985 pins as possible (although try not

to add vias or extra length to the output trace in order to get the

resistors closer).

If possible, limit the capacitance that each of the digital outputs

drives to less than 10 pF. This can easily be accomplished by

keeping traces short and by connecting the outputs to only one

device. Loading the outputs with excessive capacitance will

increase the current transients inside of the AD9985, creating

more digital noise on its power supplies.

DIGITAL INPUTS

The digital inputs on the AD9985 were designed to work with

3.3 V signals, but are tolerant of 5.0 V signals. Therefore, no

extra components need to be added if using 5.0 V logic.

Any noise that gets onto the Hsync input trace will add jitter to

the system. Therefore, minimize the trace length and do not run

any digital or other high frequency traces near it.

VOLTAGE REFERENCE

Bypass with a 0.1 µF capacitor. Place as close to the AD9985 pin

as possible. Make the ground connection as short as possible.

AD9985/PCB Scheda tecnica(PDF) 30 Page - Analog Devices