AD9886SLASHPCB datasheet(31/32 Pages) AD

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AD9886

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PCB LAYOUT RECOMMENDATIONS

The AD9886 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. The following is a guide

for designing a board using the AD9886.

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 AD9886 as close as possible to

the graphics VGA connector. Long input trace lengths are unde-

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

other external sources.

Place the 75

Ω termination resistors as close to the AD9886

chip as possible. Any additional trace length between the termi-

nation resistors and the input of the AD9886 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 chance of reflections.

The AD9886 has very high input bandwidth (330 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 AD9886, 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 # 2508051217Z0 from Fair-Rite, but each

application may work best with a different bead value. Alternately,

placing a 100

Ω to 120 Ω resistor between the 75 Ω termination

resistor and the input coupling capacitor can also benefit.

Digital Interface Inputs

Many of the same techniques that are recommended for the

analog interface inputs should also be used for the digital inter-

face inputs. Most important is to minimize trace lengths, and

then to make the input traces impedances match the input ter-

mination (typically 50

Ω).

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 group-

ings 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 AD9886,

as 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 => capacitor => 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 PVD (the clock generator supply). Abrupt changes in

PVD 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 cir-

cuitry groups (VD and PVD).

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 supply voltage. This can be mitigated by regu-

lating the analog supply, or at least PVD, from a different, cleaner

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

It is also recommend 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 because each sepa-

rate 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 at least place a single ground

plane under the AD9886. 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 => AD9886 =>

digital output trace => digital data receiver => digital ground

plane => 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% tolerances

or less.

Outputs (Both Data and Clocks)

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

drive. Longer traces have higher capacitance, which require

more current that causes more internal digital noise.

Shorter traces reduce the possibility of reflections.

Adding a series resistor of value 50

Ω–200 Ω can suppress reflec-

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

AD9886. If series resistors are used, place them as close to the

AD9886 pins as possible (try not to add 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 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 AD9886 creating

more digital noise on its power supplies.

Digital Inputs

The digital inputs on the AD9886 were designed to work with

3.3 V signals.

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.

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