Datasheet

AD8108/AD8109
Rev. B | Page 21 of 32
Using additional crosspoint devices in the design can lower the
number of outputs that must be wire-OR’ed together. Figure 51
shows a block diagram of a system using eight AD8108s and
two AD8109s to create a nonblocking, gain-of-2, 64 × 8
crosspoint that restricts the wire-OR’ing at the output to only
four outputs. The rank 1 wire-OR’ed devices are AD8108s,
which have higher disabled output impedance than the
AD8109.
8
4
4
4
4

4
4
RANK 2
16 × 8 NONBLOCKING
16 × 16 BLOCKING
RANK 1
(64:16)
4
4
8
8
4
4
8
8
8
8
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
AD8109
AD8109
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
IN 00–07
OUT 00–07
NONBLOCKING
ADDITIONAL
8 OUTPUTS
(SUBJECT TO
BLOCKING)
IN 08–15
IN 16–23
IN 24–31
IN 32–39
IN 40–47
IN 48–55
IN 56–63
1kΩ
1kΩ
1kΩ
1kΩ
01068-051
Figure 51. Nonblocking 64 × 8 Array with Gain of 2 (64 × 16 Blocking)
Additionally, by using the lower four outputs from each of the
two rank 2 AD8109s, a blocking 64 × 16 crosspoint array can be
realized. There are, however, some drawbacks to this technique.
The offset voltages of the various cascaded devices will
accumulate, and the bandwidth limitations of the devices will
compound. In addition, the extra devices will consume more
current and take up more board space. Once again, the overall
system design specifications will determine how to make the
various tradeoffs.
MULTICHANNEL VIDEO
The excellent video specifications of the AD8108/AD8109 make
them ideal candidates for creating composite video crosspoint
switches. These can be made quite dense by taking advantage of
the AD8108/AD8109’s high level of integration and the fact that
composite video requires only one crosspoint channel per
system video channel. There are, however, other video formats
that can be routed with the AD8108/AD8109 requiring more
than one crosspoint channel per video channel.
Some systems use twisted-pair wiring to carry video signals.
These systems utilize differential signals and can lower costs
because they use lower cost cables, connectors, and termination
methods. They also have the ability to lower crosstalk and reject
common-mode signals, which can be important for equipment
that operates in noisy environments or where common-mode
voltages are present between transmitting and receiving
equipment.
In such systems, the video signals are differential; there is a
positive and negative (or inverted) version of the signals. These
complementary signals are transmitted onto each of the two
wires of the twisted pair, yielding a first-order zero common-
mode signal. At the receive end, the signals are differentially
received and converted back into a single-ended signal.
When switching these differential signals, two channels are
required in the switching element to handle the two differential
signals that make up the video channel. Thus, one differential
video channel is assigned to a pair of crosspoint channels, both
input and output. For a single AD8108/AD8109, four
differential video channels can be assigned to the eight inputs
and eight outputs. This will effectively form a 4 × 4 differential
crosspoint switch.
Programming such a device will require that inputs and outputs
be programmed in pairs. This information can be deduced by
inspection of the programming format of the AD8108/AD8109
and the requirements of the system.
There are other analog video formats requiring more than one
analog circuit per video channel. One 2-circuit format that is
commonly being used in systems such as satellite TV, digital
cable boxes, and higher quality VCRs is called S-video or Y/C
video. This format carries the brightness (luminance or Y)
portion of the video signal on one channel and the color
(chrominance, chroma, or C) on a second channel.
Since S-video also uses two separate circuits for one video
channel, creating a crosspoint system requires assigning one
video channel to two crosspoint channels, as in the case of a
differential video system. Aside from the nature of the video
format, other aspects of these two systems will be the same.
There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
blue) directly from the image sensors. RGB is also the usual
format used by computers internally for graphics. RGB can be
converted to Y, R-Y, B-Y format, sometimes called YUV format.
These 3-circuit video standards are referred to as component
analog video.
The component video standards require three crosspoint
channels per video channel to handle the switching function. In
a fashion similar to the 2-circuit video formats, the inputs and
outputs are assigned in groups of three, and the appropriate
logic programming is performed to route the video signals.