AD9880
Rev. 0 | Page 58 of 64
PCB LAYOUT RECOMMENDATIONS
The AD9880 is a high-precision, high-speed analog device. To
achieve the maximum performance from the part, it is impor-
tant to have a well laid-out board. The following is a guide for
designing a board using the AD9880.
Analog Interface Inputs
Using the following layout techniques on the graphics inputs is
extremely important:
• Minimize the trace length running into the graphics
inputs. This is accomplished by placing the AD9880 as
close as possible to the graphics VGA connector. Long
input trace lengths are undesirable, because they pick up
more noise from the board and other external sources.
• Place the 75 Ω termination resistors (see Figure 3) as close
to the AD9880 chip as possible. Any additional trace length
between the termination resistors and the input of the
AD9880 increases the magnitude of reflections, which
corrupts the graphics signal.
• Use 75 Ω matched impedance traces. Trace impedances
other than 75 Ω also increase the chance of reflections.
The AD9880 has very high input bandwidth (300 MHz). While
this is desirable for acquiring a high resolution PC graphics
signal with fast edges, it means that it also captures any high
frequency noise present. Therefore, it is important to reduce the
amount of noise that gets coupled to the inputs. Avoid running
any digital traces near the analog inputs.
Due to the high bandwidth of the AD9880, sometimes low-pass
filtering the analog inputs can help to reduce noise. For many
applications, filtering is unnecessary. Experiments have shown
that placing a series ferrite bead prior to the 75 Ω termination
resistor is helpful in filtering out excess noise. Specifically, the
part used was the Fair-Rite 2508051217Z0, but each application
may work best with a different bead value. Alternatively, placing
a 100 Ω to 120 Ω resistor between the 75 Ω termination resistor
and the input coupling capacitor can also be beneficial.
Power Supply Bypassing
It is recommended to bypass each power supply pin with a
0.1 µF capacitor. The exception is in the case where two or more
supply pins are adjacent to each other. For these groupings of
powers/grounds, it is only necessary to have one bypass
capacitor. The fundamental idea is to have a bypass capacitor
within about 0.5 cm of each power pin. Also, avoid placing the
capacitor on the opposite side of the PC board from the
AD9880, since that interposes resistive vias in the path.
The bypass capacitors should be physically located between the
power plane and the power pin. Current should flow from the
power plane to the capacitor to the power pin. Do not make the
power connection between the capacitor and the power pin.
Placing a via underneath the capacitor pads down to the power
plane is generally the best approach.
It is particularly important to maintain low noise and good
stability of PV
DD
(the clock generator supply). Abrupt changes
in PV
DD
can result in similarly abrupt changes in sampling clock
phase and frequency. This can be avoided by careful attention to
regulation, filtering, and bypassing. It is highly desirable to
provide separate regulated supplies for each of the analog
circuitry groups (V
D
and PV
DD
).
Some graphic controllers use substantially different levels of
power when active (during active picture time) and when idle
(during Hsync and Vsync 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
DD
, from a different, cleaner, power
source (for example, from a 12 V supply).
It is recommended to use a single ground plane for the entire
board. Experience has repeatedly shown that the noise perfor-
mance is the same or better with a single ground plane. Using
multiple ground planes can be detrimental since each separate
ground plane is smaller and long ground loops can result.
In some cases, using separate ground planes is unavoidable. For
those cases, it is recommend to place a single ground plane
under the AD9880. The location of the split should be at the
receiver of the digital outputs. In this case it is even more
important to place components wisely because the current
loops are much longer, (current takes the path of least
resistance). An example of a current loop is power plane to
AD9880 to digital output trace to digital data receiver to digital
ground plane to analog ground plane .
PLL
Place the PLL loop filter components as close as possible to the
FILT pin.
Do not place any digital or other high frequency traces near
these components.
Use the values suggested in the datasheet with 10% tolerances
or less.
