AD9888
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).
Adding a series resistor with a of value 22 Ω to 100 Ω can sup-
press reflections, reduce EMI, and reduce the current spikes
inside the AD9888. However, if 50 Ω traces are used on the
PCB, the data output should not need these resistors.
It is also recommended to use a single ground plane for the
entire board. 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, and long ground loops can result.
A 22 Ω resistor on the DATACK output should provide good
impedance matching that will reduce reflections. If EMI or
current spiking is a concern, we recommend using a lower drive
strength setting by adjusting Register 14H. If series resistors are
used, place them as close to the AD9888 pins as possible (but
avoid adding vias or extra length to the output trace in order to
get the resistors closer).
In some cases, using separate ground planes is unavoidable. For
those cases, it is recommended to at least place a single ground
plane under the AD9888. The location 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).
An example of a current loop: power plane => AD9888 =>
digital output trace => digital data receiver => digital ground
plane => analog ground plane.
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 AD9888 creating
more digital noise on its power supplies.
PLL
Digital Inputs
Place the PLL loop filter components as close to the FILT pin
as possible.
The digital inputs on the AD9888 were designed to work with
3.3 V signals, but are tolerant of 5.0 V signals. So, no extra
components need to be added if using 5.0 V logic.
Do not place any digital or other high frequency traces near
these components.
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.
Use the values suggested in the data sheet with 10% tolerances
or less.
Voltage Reference
Bypass with a 0.1 µF capacitor. Place it as close to the AD9888
pin as possible. Make the ground connection as short as possible.
Outputs (Both Data and Clocks)
Try to minimize the trace length that the digital outputs have to
drive. Longer traces have higher capacitance, requiring more
current and causing more internal digital noise. Shorter traces
reduce the possibility of reflections.
REV. B
–29–
ROCHESTER [ Rochester Electronics ]
