AD8109AST datasheet(23/33 Pages) AD

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AD8108/AD8109

Rev. B | Page 22 of 32

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 proximity in a system, as will

undoubtedly be the case in a system that uses the AD8108/

AD8109, 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

AD8108/AD8109s.

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 AD8108/AD8109 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 outputs. 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

channels and measuring the relative strength of that signal on a

desired selected channel. The measurement is usually expressed

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

is expressed by:

()

(

)

Atest

Asel

log

where

s = jω 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

associated with it.

A network analyzer is most commonly used to measure

crosstalk over a frequency range of interest. It can provide both

magnitude and phase information about the crosstalk signal.

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

theoretical crosstalk combinations and permutations can

become extremely large. For example, in the case of the 8 × 8

matrix of the AD8108/AD8109, 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

AD8108/AD8109 outputs where the measurement can be made.

We can first measure the crosstalk terms associated with driving

a test signal into each of the other seven inputs one at a time.

We can then measure the crosstalk terms associated with

driving a parallel test signal into all seven other inputs taken

two at a time in all possible combinations, and then three at a

time, etc., until there is only one way to drive a test signal into

all seven other inputs.

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

crosstalk 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

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