AD8191AASTZ Analog Devices Inc, AD8191AASTZ Datasheet - Page 24

• Manufacturer Part Number: AD8191AASTZ
• Description: IC,Telecom Switching Circuit,QFP,100PIN,PLASTIC
• Manufacturer: Analog Devices Inc

Specifications of AD8191AASTZ

• Function: Switch
• Circuit: 1 x 4:1
• On-state Resistance: 100 Ohm
• Voltage Supply Source: Single Supply
• Voltage - Supply, Single/dual (±): 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 100-LQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD8191A
Additionally, the p and n of each differential pair must have a
controlled differential impedance of 100 Ω. The characteristic
impedance of a differential pair is a function of several variables
including the trace width, the distance separating the two traces,
the spacing between the traces and the reference plane, and the
dielectric constant of the PCB binder material. Interlayer vias
introduce impedance discontinuities that can cause reflections
and jitter on the signal path; therefore, it is preferable to route
the TMDS lines exclusively on one layer of the board,
particularly for the input traces. In some applications, such as
using multiple AD8191As to construct large input arrays, the
use of interlayer vias becomes unavoidable. In these situations,
the input termination feature of the AD8191A improves system
signal integrity by absorbing reflections. Take care to use vias
minimally and to place vias symmetrically for each side of a
given differential pair. Furthermore, to prevent unwanted signal
coupling and interference, route the TMDS signals away from
other signals and noise sources on the PCB.
Both traces of a given differential pair must be equal in length
to minimize intrapair skew. Maintaining the physical symmetry
of a differential pair is integral to ensuring its signal integrity;
excessive intrapair skew can introduce jitter through duty cycle
distortion (DCD). The p and n of a given differential pair
should always be routed together to establish the required 100 Ω
differential impedance. Enough space should be left between the
differential pairs of a given group so that the n of one pair does
not couple to the p of another pair. For example, one technique is
to make the interpair distance 4 to 10 times wider than the
intrapair spacing.
Any group of four TMDS channels (that is, Input A, Input B,
Input C, Input D, or the output quad group) should have closely
matched trace lengths to minimize interpair skew. Severe
interpair skew can cause the data on the four different channels
of a group to arrive out of alignment with one another. A good
practice is to match the trace lengths for a given group of four
channels to within 0.05 inches on FR4 material.
The length of the TMDS traces should be minimized to reduce
overall signal degradation. Commonly used PCB material, such
as FR4, is lossy at high frequencies; therefore, long traces on the
circuit board increase signal attenuation resulting in decreased
signal swing and increased jitter through intersymbol
interference (ISI).
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Controlling the Characteristic Impedance of a TMDS
Differential Pair
The characteristic impedance of a differential pair depends on a
number of variables including the trace width, the distance
between the two traces, the height of the dielectric material
between the trace and the reference plane below it, and the
dielectric constant of the PCB binder material. To a lesser
extent, the characteristic impedance also depends upon the
trace thickness and the presence of solder mask. There are
many combinations that can produce the correct characteristic
impedance. Generally, working with the PCB fabricator is required
to obtain a set of parameters to produce the desired results.
One consideration is how to guarantee a differential pair with a
differential impedance of 100 Ω over the entire length of the
trace. One technique to accomplish this is to change the width
of the traces in a differential pair based on how closely one trace
is coupled to the other. When the two traces of a differential
pair are close and strongly coupled, they should have a width
that produces a 100 Ω differential impedance. When the traces
split apart, to go into a connector, for example, and are no
longer so strongly coupled, the width of the traces should be
increased to yield a differential impedance of 100 Ω in the new
configuration.
TMDS Terminations
The AD8191A provides internal, 50 Ω single-ended
terminations for all of its high speed inputs and outputs. It is
not necessary to include external termination resistors for the
TMDS differential pairs on the PCB.
The output termination resistors of the AD8191A back-
terminate the output TMDS transmission lines. These back-
terminations act to absorb reflections from impedance
discontinuities on the output traces, improving the signal
integrity of the output traces and adding flexibility to how the
output traces can be routed. For example, interlayer vias can be
used to route the AD8191A TMDS outputs on multiple layers of
the PCB without severely degrading the quality of the output signal.