AD8108AST datasheet(24/28 Pages) AD

24 / 28 page

AD8108/AD8109

–24–

REV. 0

Figure 51. Board Layout (Bottom Layer)

Optimized for video applications, all signal inputs and outputs

are terminated with 75

Ω resistors. Stripline techniques are used

to achieve a characteristic impedance on the signal input and

output lines also of 75

Ω. Figure 52 shows a cross-section of one

of the input or output tracks along with the arrangement of the

PCB layers. It should be noted that unused regions of the four

layers are filled up with ground planes. As a result, the input

and output traces, in addition to having controlled impedances,

are well shielded.

w = 0.008"

(0.2mm)

a = 0.008"

(0.2mm)

b = 0.024"

(0.6mm)

h = 0.011325"

(0.288mm)

t = 0.00135" (0.0343mm)

TOP LAYER

SIGNAL LAYER

POWER LAYER

BOTTOM LAYER

Figure 52. Cross Section of Input and Output Traces

The board has 16 BNC type connectors: eight inputs and eight

outputs. The connectors are arranged in two crescents around

the device. As can be seen from Figure 49, this results in all

eight input signal traces and all eight signal output traces having

the same length. This is useful in tests such as All-Hostile

Crosstalk where the phase relationship and delay between sig-

nals needs to be maintained from input to output.

The three power supply pins AVCC, DVCC and AVEE should

be connected to good quality, low noise,

±5 V supplies. Where

the same

±5 V power supplies are used for analog and digital,

separate cables should be run for the power supply to the evalu-

ation board’s analog and digital power supply pins.

As a general rule, each power supply pin (or group of adjacent

power supply pins) should be locally decoupled with a 0.01

µF

capacitor. If there is a space constraint, it is more important to

decouple analog power supply pins before digital power supply

pins. A 0.1

µF capacitor, located reasonably close to the pins,

can be used to decouple a number of power supply pins. Finally

a 10

µF capacitor should be used to decouple power supplies as

they come on to the board.

Controlling the Evaluation Board from a PC

The evaluation board include Windows-based control software

and a custom cable that connects the board’s digital interface to

the printer port of the PC. The wiring of this cable is shown in

Figure 53. The software requires Windows 3.1 or later to oper-

ate. To install the software, insert the disk labeled “Disk #1 of

2” in the PC and run the file called SETUP.EXE. Additional

installation instructions will be given on-screen. Before begin-

ning installation, it is important to terminate any other Windows

applications that are running.

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