AD8115AST datasheet(22/32 Pages) AD

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AD8114/AD8115

Rev. B | Page 22 of 32

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 will effectively form an 8 × 8 differential

crosspoint 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 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 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 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

will undoubtedly be 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/AD8115s.

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

propagates across a stray capacitance (e.g., 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 will 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.

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

multichannel system are generally shared by the various

channels. 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 common impedance.

All these sources of crosstalk are vector quantities, so the

magnitudes cannot simply be 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 AD8114/AD8115 circuit, it is required that it be

mounted to some sort of circuit board 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 intrinsic

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

crosspoint and among the output. 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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