AD8108ASTZ datasheet(21/27 Pages) AD

21 / 27 page

Data Sheet

AD8108/AD8109

Rev. C | Page 21 of 27

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 AD8108 devices

and two AD8109 devices 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 AD8108

devices, which have higher disabled output impedance than the

AD8109.

�

RANK 2

× 8 NONBLOCKING

× 16 BLOCKING

RANK 1

(64:16)

AD8109

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

Ω

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 AD8109 devices, 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

accumulate, and the bandwidth limitations of the devices

compound. In addition, the extra devices consume more

current and take up more board space. Once again, the overall

system design specifications 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 high level of integration of the AD8108/AD8109 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 effectively forms a 4 × 4 differential crosspoint

switch.

Programming such a device requires 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 are 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.

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