Datasheet

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 V
OL
. This will be
determined by the output that is operating at the highest pro-
grammed output current since V
OL
= VTTO – (I
OUT
¥ 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. 1.50/ TOP MICROSTRIP WIDTH = 8.0mils
2. 0.50/GND
3. 0.50/VTTO
4. 0.50/VCC
5. 0.50/VEE
6. 0.50/VTTI
7. 0.50/GND
8. 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