AD8111AST datasheet(18/28 Pages) AD

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AD8110/AD8111

–18–

REV. 0

Multichannel Video

The excellent video specifications of the AD8110/AD8111

make them ideal candidates for creating composite video cross-

point switches. These can be made quite dense by taking advan-

tage of the AD8110/AD8111’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 AD8110/AD8111 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 AD8110/AD8111, eight differen-

tial video channels can be assigned to the 16 inputs and four to

the outputs. This will effectively form an 8

× 4 differential cross-

point 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 AD8110/AD8111

and the requirements of the system.

There are other analog video formats requiring more than one

analog circuit per video channel. One two-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 chan-

nel, creating a crosspoint system requires assigning one video

channel to two crosspoint channels as in the case of a differen-

tial 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 also be converted to Y, R-Y, B-Y format, sometimes called

YUV format. These three-circuit, video standards are referred

to as component analog video.

The component video standards require three crosspoint chan-

nels per video channel to handle the switching function. In a

fashion similar to the two-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

will undoubtedly be the case in a system that uses the AD8110/

AD8111, the crosstalk issues can be quite complex. A good

understanding of the nature of crosstalk and some definition of

terms is required in order to specify a system that uses one or

more AD8110/AD8111s.

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 will explain 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 propa-

gates across a stray capacitance (e.g., free space) and couples

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

wanted crosstalk signal in any channel that receives it.

Currents flowing in conductors create magnetic fields that circu-

late around the currents. These magnetic fields will then gener-

ate 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.

The power supplies, grounds and other signal return paths of a

multichannel system are generally shared by the various chan-

nels. When a current from one channel flows in one of these

paths, a voltage that is developed across the impedance becomes

an input crosstalk signal for other channels that share the com-

mon impedance.

All these sources of crosstalk are vector quantities, so the magni-

tudes cannot be simply added together to obtain the total

crosstalk. In fact, there are conditions where driving additional

circuits in parallel in a given configuration can actually reduce

the crosstalk.

Areas of Crosstalk

For a practical AD8110/AD8111 circuit, it is required that it be

mounted to some sort of circuit board in order to connect it to

power supplies and measurement equipment. Great care has

been taken to create a characterization board (also available as

an evaluation board) that adds minimum crosstalk to the intrin-

sic device. This, however, raises the issue that a system’s

crosstalk is a combination of the intrinsic crosstalk of the

devices in addition to the circuit board to which they are

mounted. It is important to try to separate these two areas of

crosstalk when attempting to minimize its effect.

In addition, crosstalk can occur among the inputs to a cross-

point and among the outputs. It can also occur from input to

output. Techniques will be discussed for diagnosing which part

of a system is contributing to crosstalk.

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