AD8115ASTZ datasheet(21/25 Pages) AD

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Data Sheet

AD8114/AD8115

Rev. C | Page 21 of 25

Using additional crosspoint devices in the design can lower the

number of outputs that must be wire-OR’ed together. Figure 49

shows a block diagram of a system using eight AD8114 devices

and two AD8115 devices to create a nonblocking, gain-of-2,

128 × 16 crosspoint that restricts the wire-OR’ing at the

output to only four outputs.

Additionally, by using the lower eight outputs from each of the

two Rank 2 AD8115 devices, a blocking 128 × 32 crosspoint

array can be realized. There are, however, some drawbacks to

this technique. The offset voltages of the various cascaded

devices accumulates, 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 AD8114/AD8115 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 AD8114/AD8115 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 AD8114/AD8115 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 voltage. 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 AD8114/AD8115, eight differential

video channels can be assigned to the 16 inputs and 16 outputs.

This effectively forms an 8 × 8 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 AD8114/AD8115

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.

CROSSTALK

Many systems, such as broadcast video, that handle numerous

analog signal channels have strict requirements for keeping the

various signals from influencing any of the others in the system.

Crosstalk is the term used to describe the coupling of the

signals of other nearby channels to a given channel.

When there are many signals in close proximity in a system, as

is undoubtedly the case in a system that uses the AD8114/

AD8115, the crosstalk issues can be quite complex. A good

understanding of the nature of crosstalk and some definition

of terms is required to specify a system that uses one or more

AD8114/AD8115 devices.

Types of Crosstalk

Crosstalk can be propagated by means of any of three methods.

These fall into the categories of electric field, magnetic field,

and sharing of common impedances. This section explains

these effects.

Every conductor can be both a radiator of electric fields and a

receiver of electric fields. The electric field crosstalk mechanism

occurs when the electric field created by the transmitter propagates

across a stray capacitance (for example, free space) and couples

with the receiver and induces a voltage. This voltage is an

unwanted crosstalk signal in any channel that receives it.

Currents flowing in conductors create magnetic fields that circulate

around the currents. These magnetic fields then generate voltages

in any other conductors whose paths they link. The undesired

induced voltages in these other channels are crosstalk signals.

The channels that crosstalk can be said to have a mutual

inductance that couples signals from one channel to another.

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