REV. B

AD9884A

–22–

PCB LAYOUT RECOMMENDATIONS

The AD9884A is a high precision, high speed analog device. As

such, to get the maximum performance out of the part it is

important to have a well laid-out board.

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 AD9884A as close as possible to

the input connector. Long input trace lengths are undesirable

because they will pick up more noise from the board and other

external sources.

Place the 75

Ω termination resistors as close to the AD9884A as

possible. Any additional trace length between the termination

resistors and the input of the AD9884A increases the magnitude

of reflections, which will corrupt the graphics signal.

Use 75

Ω matched impedance traces. Trace impedances other

than 75

Ω will also increase the magnitude of reflections.

The AD9884A has very high input bandwidth (500 MHz). While

this is desirable for acquiring a high resolution PC graphics

signal with fast edges, it means that it will also capture 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 AD9884A, sometimes low-

pass filtering the analog inputs can help to reduce noise. (For

many applications, filtering is unnecessary.) Our experiments

have shown that placing a series ferrite bead prior to the 75

Ω

termination resistor is helpful in filtering out excess noise. Spe-

cifically, we used the Part #2508051217Z0 from Fair-Rite, but

each application may work best with a different bead value.

Power Supply Bypassing

We recommend you bypass each power supply pin with a 0.1

µF

capacitor. The exception is in the case where two or more sup-

ply pins are adjacent. 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 AD9884A, as that interposes resistive

vias in the path.

The bypass capacitors should be connected between the power

plane and the power pin. Current should flow from the power

plane ¡ capacitor ¡ power pin. Do not make the power connec-

tion 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

phase and frequency. This can be avoided by careful attention

to regulation, filtering, and bypassing. It is highly desirable to

provide a separately regulated supply for the analog circuitry

Some graphic controllers use substantially different levels of

power when active (during active picture time) and when 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 voltage. This can be mitigated by regulating the analog

(for example, from a +12 V supply).

We also recommend that you use a single ground plane for the

entire board. Experience has repeatedly shown that the noise

performance is better, or at least the same, 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.

P6

RGBIN

P0 P1 P2 P3 P4 P5

HSYNC

PXCK

HS

ADCCK

DATACK

DOUTA

HSOUT

DOUTB

D3

D7

D1

D5

6.5 PIPE DELAY

P7

Figure 26. Dual Channel Mode, Parallel Outputs, Alternate Pixel Sampling (Odd Pixels)