AD8115AST datasheet(18/26 Pages) AD

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AD8114/AD8115

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REV. 0

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 AD8114/

AD8115, 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 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 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

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 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 addi-

tion to the circuit board to which they are mounted. It is impor-

tant 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 output. It can also occur from input to

output. Techniques will be discussed for diagnosing which part

of a system is contributing to crosstalk.

Measuring Crosstalk

Crosstalk is measured by applying a signal to one or more chan-

nels and measuring the relative strength of that signal on a de-

sired selected channel. The measurement is usually expressed as

dB down from the magnitude of the test signal. The crosstalk is

expressed by:

|XT| = 20 log10 (Asel(s)/Atest(s))

where s = jw is the Laplace transform variable, Asel(s) is the

amplitude of the crosstalk-induced signal in the selected channel

and Atest(s) is the amplitude of the test signal. It can be seen

that crosstalk is a function of frequency, but not a function of

the magnitude of the test signal (to first order). In addition, the

crosstalk signal will have a phase relative to the test signal asso-

ciated with it.

A network analyzer is most commonly used to measure crosstalk

over a frequency range of interest. It can provide both magni-

tude and phase information about the crosstalk signal.

As a crosspoint system or device grows larger, the number of

theoretical crosstalk combinations and permutations can be-

come extremely large. For example, in the case of the 16

× 16

matrix of the AD8114/AD8115, we can examine the number of

crosstalk terms that can be considered for a single channel, say

IN00 input. IN00 is programmed to connect to one of the

AD8114/AD8115 outputs where the measurement can be made.

First, we can measure the crosstalk terms associated with driv-

ing a test signal into each of the other 15 inputs one at a time,

while applying no signal to IN00. We can then measure the

crosstalk terms associated with driving a parallel test signal into

all 15 other inputs taken two at a time in all possible combina-

tions; and then three at a time, etc., until, finally, there is only

one way to drive a test signal into all 15 other inputs in parallel.

Each of these cases is legitimately different from the others and

might yield a unique value depending on the resolution of the

measurement system, but it is hardly practical to measure all

these terms and then to specify them. In addition, this describes

the crosstalk matrix for just one input channel. A similar cross-

talk matrix can be proposed for every other input. In addition, if

the possible combinations and permutations for connecting

inputs to the other (not used for measurement) outputs are

taken into consideration, the numbers rather quickly grow to

astronomical proportions. If a larger crosspoint array of multiple

AD8114/AD8115s is constructed, the numbers grow larger still.

Obviously, some subset of all these cases must be selected to

be used as a guide for a practical measure of crosstalk. One

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