AD8108AST datasheet(17/28 Pages) AD

17 / 28 page

AD8108/AD8109

–17–

REV. 0

8 INPUTS

IN 00–07

AD8108

AD8109

ONE

TERMINATION

PER INPUT

16 OUTPUTS

OUT 00–15

AD8108

AD8109

Figure 42. 8

× 16 Crosspoint Array Using Two AD8108s

(Unity Gain) or Two AD8109s (Gain-of-Two)

Figure 43 illustrates a 16

× 16 crosspoint array, while a 24 × 24

crosspoint is illustrated in Figure 44. 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

OUT 00–07

OUT 08–15

00–07

RTERM

08–15

RTERM

IN 08–15

8 8

Figure 43. 16

× 16 Crosspoint Array Using Four

AD8108s or AD8109s

IN 00–07

IN 08–15

IN 16–23

RTERM

OUT 16–23

OUT 08–15

OUT 00–07

8 8

Figure 44. 24

× 24 Crosspoint Array Using Nine AD8108s

or AD8109s

At some point, the number of outputs that are wire-ORed be-

comes 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 corresponding outputs of each device wire-ORed to-

gether in groups of eight.

Using additional crosspoint devices in the design can lower the

number of outputs that have to be wire-ORed together. Figure

45 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.

IN 00–07

IN 08–15

IN 16–23

IN 24–31

IN 32–39

IN 40–47

IN 48–55

IN 56–63

RANK 2

16:8 NONBLOCKING

16:16 BLOCKING

OUT 00–07

NONBLOCKING

ADDITIONAL

8 OUTPUTS

(SUBJECT TO

BLOCKING)

RANK 1

(64:16)

AD8109

AD8108

Figure 45. Nonblocking 64

× 8 Array with Gain-of-Two

(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 tech-

nique. 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.

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