AD9985ASLASHPCB datasheet(30/32 Pages) AD

30 / 32 page

AD9985A

Rev. 0 | Page 30 of 32

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 AD9985A. 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 reduces reflections. If series resistors are used, place them

as close as possible to the AD9985A pins (but 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 increases

the current transients inside of the AD9985A, creating more

digital noise on its power supplies.

DIGITAL INPUTS

The digital inputs on the AD9985A 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 adds 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 the voltage reference with a 0.1 μF capacitor. Place it as

close as possible to the AD9985A pin. Make the ground

connection as short as possible.

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