AD8152 datasheet(21/32 Pages) AD

21 / 32 page

REV. A

AD8152

–21–

First, the output current can be programmed to the smallest amount

required to maintain BER performance. If an output circuit

always has a short length and the receiver has good sensitivity,

then a lower output current can be used.

It is also possible to lower the voltage on VTTO to lower the

power dissipation. The amount that VTTO can be lowered is

dependent on the lowest of all the output’s VOL. This will be

determined by the output that is operating at the highest pro-

grammed output current since VOL = VTTO – (IOUT

¥ 25 W).

EVALUATION BOARD AND PCB LAYOUT HINTS

The AD8152 evaluation board was designed to allow the user to

analyze signal integrity in many configurations, as controlled by

a standard PC.

The FR4 board comes equipped with a full complement of

136 SMA connectors to support the complete 34

34 matrix of

points. Each differential pair of microstrip is connected to either

top mount or side-launch SMA connectors. The mounting area of

the short center pin top-mount SMA connectors are drilled (seven

holes) and stubbed for greatly improved performance. In the

area surrounding SMA top-mount center pin and drill holes, all

internal planes are relieved or cleared out (see Figure 10 for layout).

ALL TOP-MOUNT SMAs SIT ON PCB TOP LEVEL

TOP VIEW OF TOP LEVEL TRACE

MICROSTRIP

SMA CENTER PIN

PLANE RELIEF

DRILL HOLES

(7 EACH)

BOTTOM VIEW OF BOTTOM LEVEL TRACE

Figure 10. Top-Mount SMA PCB Layout, Two Views

The FR4 PC board is eight layers with a thickness of 62 mils

(1.57 mm). The two outer most metal layers hold the high speed

microstrip routing lines. The two outer most dielectric layers are

5 mils thick and must be controlled impedance (50

W) layers. These

are the only two layers that require controlled impedance. The

next two inner metal layers are ground (reference) planes for the

microstrip and are the shell for the SMA connectors. The remain-

ing four inner metal layers are for the four AD8152 supply and

digital control signal routing. From top to bottom the four supply

layers are VTTO, VCC, VEE, and VTTI. Because all four supply

PCB metal layers float, positive, negative, and even dual-supply

configurations are possible. The variety of supply configurations

ease the connection of test equipment. The four inner supply

layers also provide an interlayer capacitance, which has better

impedance versus frequency than standard chip capacitors.

DIELECTRIC

THICKNESS

SILKSCREEN

COPPER

LAYER

NUMBER

1.50/ TOP MICROSTRIP WIDTH = 8.0mils

0.50/GND

0.50/VTTO

0.50/VCC

0.50/VEE

0.50/VTTI

0.50/GND

1.50/BOTTOM MICROSTRIP WIDTH = 8.0mils

0.5mils

5.0mils

4.0mils

16.0mils

4.0mils

16.0mils

4.0mils

5.0mils

0.5mils

SILKSCREEN

THICKNESS/DESIGNATION

(IN OUNCES)

Figure 11. Evaluation Board Stack-Up

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