AD8151AST Analog Devices Inc, AD8151AST Datasheet - Page 23

• Manufacturer Part Number: AD8151AST
• Description: IC CROSSPOINT SWIT 33X17 184LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD8151AST

• Rohs Status: RoHS non-compliant
• Function: Crosspoint Switch
• Circuit: 1 x 33:17
• Voltage Supply Source: Dual Supply
• Voltage - Supply, Single/dual (±): ±3 V ~ 5.25 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 184-LQFP
• Number Of Arrays: 1
• Differential Data Transmission: Yes
• Operating Supply Voltage (typ): Not RequiredV
• Mounting: Surface Mount
• Operating Temperature (max): 85C
• Cascading Capability: No
• Line Code: NRZ
• On-chip Buffers: Yes
• On-chip Mux/demux: No
• Operating Supply Voltage (max): Not RequiredV
• Operating Supply Voltage (min): Not RequiredV
• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS
INPUT AND OUTPUT BUSING
Although the AD8151 is a digital part, in any application that
runs at high speed, analog design details have to be given very
careful consideration. At high data rates, the design of the signal
channels have a strong influence on data integrity and its
associated jitter and ultimately bit error rate (BER).
While it might be considered very helpful to have a suggested
circuit board layout for any particular system configuration, this
is not something that can be practically realized. Systems come
in all shapes, sizes, speeds, performance criteria, and cost con-
straints. Therefore, some general design guidelines are pre-
sented that can be used for all systems and judiciously modified
where appropriate.
High speed signals travel best, that is, they maintain their
integrity when they are carried by a uniform transmission line
that is properly terminated at either end. Any abrupt mis-
matches in impedance or improper termination creates
reflections that add to or subtract from parts of the desired
signal. Small amounts of this effect are unavoidable, but too
much distorts the signal to the point that the channel
BER increases. It is difficult to fully quantify these effects
because they are influenced by many factors in the overall
system design.
A constant-impedance transmission line is characterized by
having a uniform cross-section profile over its entire length. In
particular, there should be no stubs, which are branches that
intersect the main run of the transmission line. These can have
an electrical appearance that is approximated by a lumped
element, such as a capacitor, or if long enough, by another
transmission line. If stubs are unavoidable in a design, their
effect can be minimized by making them as short as possible
and as high an impedance as possible.
Figure 37 shows a differential transmission line that connects
two differential outputs from the AD8151 to a generic receiver.
A more generalized system can have more outputs bused and
more receivers on the same bus, but the same concepts apply.
The inputs of the AD8151 can also be considered as a receiver.
The transmission lines that bus the devices together are shown
with terminations at each end.
The individual outputs of the AD8151 are stubs that intersect
the main transmission line. Ideally, their current source outputs
would be infinite impedance, and they would have no effect on
signals that propagate along the transmission line. In reality,
each external pin of the AD8151 projects into the package and
has a bond wire connected to the chip inside. On-chip wiring
then connects to the collectors of the output transistors and to
ESD protection diodes.
Rev. B | Page 23 of 40
Unlike some other high speed digital components, the AD8151
does not have on-chip terminations. While this location would
be closer to the actual end of the transmission line for some
architectures, this concept can limit system design options. In
particular, it is not possible to bus more than two inputs or
outputs on the same transmission line and it is also not possible
to change the value of these terminations to use for different
impedance transmission lines. The AD8151, with the added
ability to disable its outputs, is much more versatile in these
types of architectures.
If the external traces are kept to a bare minimum, then the
output presents a mostly lumped capacitive load of about 2 pF.
A single stub of 2 pF does not adversely affect signal integrity to
a large extent for most transmission lines, but the more of these
stubs, the greater their adverse influence.
One way to mitigate this effect is to locally reduce the capa-
citance of the main transmission line near the point of stub
intersection. Some practical means for doing this are to narrow
the PC board traces in the region of the stub and/or to remove
some of the ground plane(s) near this intersection. The effect of
these techniques is to locally lower the capacitance of the main
transmission line at these points, while the added capacitance of
the AD8151 outputs compensate for this reduction in capaci-
tance. The overall intent is to create as uniform a transmission
line as possible.
In selecting the location of the termination resistors, it is
important to keep in mind that, as their name implies, they
should be placed at either end of the line. There should be
minimal or no projection of the transmission line beyond the
point where it connects to the termination resistors.
EVALUATION BOARD
An evaluation board has been designed and is available to
rapidly test the main features of the AD8151. This board allows
the user to analyze the analog performance of the AD8151
channels and easily control the configuration of the board with
a PC. The board has limited numbers of differential input/
output pairs. Each differential pair of microstrips is connected
to either top mount or side launch SMA connectors. The top
mount SMA connectors are drilled and stubbed for superior
performance. The FR4 type board contains a total of nine
outputs (all even numbered outputs) and 20 inputs (0, 2, 4, 6, 8,
10, 12, 13, 14, 15, 16, 17, 18, 20, 22, 24, 26, 28, 30, 32). It is
important to note that the shells of the SMA connectors are
attached to V
possible during testing.
CC
. This makes only ECL or negative level swings
AD8151