AD9882 Analog Devices, AD9882 Datasheet - Page 35

• Manufacturer Part Number: AD9882
• Description: Dual Interface for Flat Panel Displays
• Manufacturer: Analog Devices

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 PV
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.
REV. A
D
, from a different,
–35–
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