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AD8197 Datasheet(PDF) 26 Page - Analog Devices
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AD8197 Datasheet(HTML) 26 Page - Analog Devices
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Preliminary Technical Data
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Gbps, which corresponds to the video format, 1080p with deep
The length of cable that can be used in a typical HDMI/DVI
application depends on a large number of factors, including:
Cable quality: the quality of the cable in terms of conductor
wire gauge and shielding. Thicker conductors have lower
signal degradation per unit length.
Data rate: the data rate being sent over the cable. The signal
degradation of HDMI cables increases with data rate.
Edge rates: the edge rates of the source input. Slower input
edges result in more significant data eye closure at the end
of a cable.
Receiver sensitivity: the sensitivity of the terminating
As such, specific cable types and lengths are not recommended
for use with a particular equalizer setting. In nearly all applica-
tions, the AD8197 equalization level can be set to high, or 12 dB,
for all input cable configurations at all data rates, without
degrading the signal integrity.
PCB LAYOUT GUIDELINES
The AD8197 is used to switch two distinctly different types of
signals, both of which are required for HDMI and DVI video.
These signal groups require different treatment when laying out
a PC board.
The first group of signals carries the audiovisual (AV) data.
HDMI/DVI video signals are differential, unidirectional, and
high speed (up to 2.25 Gbps). The channels that carry the video
data must be controlled impedance, terminated at the receiver,
and capable of operating at the maximum specified system data
rate. It is especially important to note that the differential traces
that carry the TMDS signals should be designed with a
controlled differential impedance of 100 Ω. The AD8197
provides single-ended, 50 Ω terminations on-chip for both its
inputs and outputs, and both the input and output terminations
can be enabled or disabled through the serial interface.
Transmitter termination is not required by the HDMI 1.3 standard
but its inclusion improves the overall system signal integrity.
The audiovisual (AV) data carried on these high speed channels
are encoded by a technique called transmission minimized
differential signaling (TMDS) and in the case of HDMI, is also
encrypted according to the high bandwidth digital copy
protection (HDCP) standard.
The second group of signals consists of low speed auxiliary
control signals used for communication between a source and a
sink. Depending upon the application, these signals can include
the DDC bus (this is an I
C bus used to send EDID information
and HDCP encryption keys between the source and the sink),
the consumer electronics control (CEC) line, and the hot plug
detect (HPD) line. These auxiliary signals are bidirectional, low
speed, and transferred over a single-ended transmission line
that does not need to have controlled impedance. The primary
concern with laying out the auxiliary lines is ensuring that they
conform to the I
C bus standard and do not have excessive
In the HDMI/DVI standard, four differential pairs carry the
TMDS signals. In DVI, three of these pairs are dedicated to
carrying RGB video and sync data. For HDMI, audio data is
interleaved with the video data; the DVI standard does not
incorporate audio information. The fourth high speed differ-
ential pair is used for the AV data-word clock, and runs at
one-tenth the speed of the TMDS data channels.
The four high speed channels of the AD8197 are identical.
No concession was made to lower the bandwidth of the fourth
channel for the pixel clock, so any channel can be used for any
TMDS signal. The user chooses which signal is routed over
which channel. Additionally, the TMDS channels are symmetrical;
therefore, the p and n of a given differential pair are inter-
changeable, provided the inversion is consistent across all inputs
and outputs of the AD8197. However, the routing between
inputs and outputs through the AD8197 is fixed. For example,
in quad mode, Output Channel 0 always switches between
Input A0, Input B0, Input C0, Input D0, and so forth.
The AD8197 buffers the TMDS signals and the input traces can
be considered electrically independent of the output traces. In
most applications, the quality of the signal on the input TMDS
traces are more sensitive to the PCB layout. Regardless of the
data being carried on a specific TMDS channel, or whether the
TMDS line is at the input or the output of the AD8197, all four
high speed signals should be routed on a PCB in accordance
with the same RF layout guidelines.
Layout for the TMDS Signals
The TMDS differential pairs can either be microstrip traces,
routed on the outer layer of a board, or stripline traces, routed
on an internal layer of the board. If microstrip traces are used,
there should be a continuous reference plane on the PCB layer
directly below the traces. If stripline traces are used, they must
be sandwiched between two continuous reference planes in the
PCB stack-up. 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 PC board 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 AD8197s to construct large input arrays, the use
of interlayer vias becomes unavoidable. In these situations, the
input termination feature of the AD8197 improves system signal
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