AD9882SLASHPCB datasheet(35/36 Pages) AD

35 / 36 page

REV. A

AD9882

–35–

Some graphic controllers use levels of power when active (during

active picture time) that are substantially different from those used

when they are idle (during horizontal and vertical sync periods).

This can result in a measurable change in the voltage supplied to

the analog supply regulator, which can in turn produce changes

in the regulated analog supply voltage. This can be mitigated by

regulating the analog supply, or at least PVD, from a different,

cleaner, power source (for example, from a 12 V supply).

Using a single ground plane for the entire board is also rec-

ommended. Experience has repeatedly shown that the noise

performance is the same or better with a single ground plane.

Using multiple ground planes can be detrimental because each

separate ground plane is smaller than one common ground

plane, and long ground loops can result.

In some cases, using separate ground planes is unavoidable. For

those cases where they must be used, it is recommended that at

least a single ground plane be placed under the AD9882. The loca-

tion of the split should be at the receiver of the digital outputs. For

this case, it is even more important to place components wisely

because the current loops will be much longer (current takes the

path of least resistance). The following is an example of a current

loop: power plane

Æ AD9882 Æ digital output trace Æ digital

data receiver

Æ digital ground plane Æ analog ground plane.

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% or smaller

tolerances.

Outputs (Data and Clocks)

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

drive. Longer traces have higher capacitance and require more

current, which causes more internal digital noise.

Shorter traces reduce the possibility of reflections.

Adding a series resistor with a value of 22

W to 100 W can suppress

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

the AD9882. However, if 50

W traces are used on the PCB, the

data output should not need these resistors.

A 22

W resistor on the DATACK output should provide good

impedance matching that will reduce reflections. If EMI or

current spiking is a concern, use a lower drive strength setting

by adjusting register 14H. If series resistors are used, place them as

close to the AD9882 pins as possible (but avoid adding 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 be accomplished easily 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 the AD9882, creating

more digital noise on its power supplies.

Digital Inputs

The digital inputs on the AD9882 were designed to work

with 3.3 V signals, but are tolerant of 5.0 V signals. No extra

components need to be added if 5.0 V logic is used.

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

mF capacitor. Place as close to the AD9882

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

AD9882/PCB Datasheet(PDF) 35 Page - Analog Devices

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