AD8108 Analog Devices, AD8108 Datasheet - Page 17

• Manufacturer Part Number: AD8108
• Description: 325 Mhz, 8x8 Buffered Video Crosspoint Switch (Gain=1)
• Manufacturer: Analog Devices

Figure 7 illustrates a 16 × 16 crosspoint array, while a 24 × 24
crosspoint is illustrated in Figure 8. The 16 × 16 crosspoint
requires that each input driver drive two inputs in parallel and
each output be wire-ORed with one other output. The 24 × 24
crosspoint requires driving three inputs in parallel and having
the outputs wire-ORed in groups of three. It is required of the
system programming that only one output of a wired-OR node
be active at a time.
IN 00–07
IN 08–15
IN 16–23
IN 08–15
IN 00–07
×
8
8
8
×
8 INPUTS
×
IN 00–07
8
8
TERMINATION
8
PER INPUT
8
8
8
8
8
8
8
8
OUT 00–07
OUT 00–07
ONE
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
OUT 08–15
AD8108
AD8109
AD8108
AD8109
OR
OR
R
8
R
8
16 OUTPUTS
08–15
00–07
TERM
OUT 00–15
TERM
8
8
8
8
R
R
R
TERM
TERM
TERM
OUT 08–15
8
8
8
8
8
8
8
8
8
8
8
8
8
8
OUT 16–23
8
8
At some point, the number of outputs that are wire-ORed becomes
too great to maintain system performance. This will vary according
to which system specifications are most important. For example,
a 64 × 8 crosspoint can be created with eight AD8108/AD8109s.
This design will have 64 separate inputs and have the correspond-
ing outputs of each device wire-ORed together in groups of eight.
Using additional crosspoint devices in the design can lower the
number of outputs that must be wire-ORed together. Figure 9
shows a block diagram of a system using eight AD8108s and
two AD8109s to create a nonblocking, gain-of-two, 64 × 8 cross-
point that restricts the wire-ORing at the output to only four
outputs. The rank 1 wire-ORed devices are the AD8108,
which has a higher disabled output impedance than the AD8109.
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 accumu-
late 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.
IN 08–15
IN 00–07
IN 16–23
IN 40–47
IN 48–55
IN 56–63
IN 24–31
IN 32–39
×
8
8
8
8
8
8
8
8
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
AD8108
RANK 1
(64:16)
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
1k
1k
16
×
16
AD8108/AD8109
8 NONBLOCKING
4
4
1k
1k
16 BLOCKING
RANK 2
AD8109
AD8109
4
4
4
4
OUT 00–07
NONBLOCKING
ADDITIONAL
8 OUTPUTS
(SUBJECT TO
BLOCKING)