AD8115AST datasheet(23/32 Pages) AD

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AD8114/AD8115

Rev. B | Page 23 of 32

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 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

driving 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

combinations, then three at a time, etc., until 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

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

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

common method is to measure all hostile crosstalk. This term

means that the crosstalk to the selected channel is measured

while all other system channels are driven in parallel. In general,

this will yield the worst crosstalk number, but this is not always

the case due to the vector nature of the crosstalk signal.

Other useful crosstalk measurements are those created by one

nearest neighbor or by the two nearest neighbors on either side.

These crosstalk measurements will generally be higher than

those of more distant channels, so they can serve as a worst-case

measure for any other 1-channel or 2-channel crosstalk

measurements.

Input and Output Crosstalk

The flexible programming capability of the AD8114/AD8115

can be used to diagnose whether crosstalk is occurring more on

the input side or the output side. Some examples are illustrative.

A given input channel (IN07 in the middle for this example)

can be programmed to drive OUT07 (also in the middle). The

input to IN07 is just terminated to ground (via 50 Ω or 75 Ω)

and no signal is applied.

All the other inputs are driven in parallel with the same test

signal (practically that is provided by a distribution amplifier),

with all other outputs except OUT07 disabled. Since grounded

IN07 is programmed to drive OUT07, no signal should be

present. Any signal that is present can be attributed to the other

15 hostile input signals because no other outputs are driven.

(They are all disabled.) Thus, this method measures the all-

hostile input contribution to crosstalk into IN07. Of course, the

method can be used for other input channels and combinations

of hostile inputs.

For output crosstalk measurement, a single input channel is

driven (IN00, for example) and all outputs other than a given

output (IN07 in the middle) are programmed to connect to

IN00. OUT07 is programmed to connect to IN15 (far away

from IN00), which is terminated to ground. Thus OUT07

should not have a signal present since it is listening to a quiet

input. Any signal measured at the OUT07 can be attributed to

the output crosstalk of the other 16 hostile outputs. Again, this

method can be modified to measure other channels and other

crosspoint matrix combinations.

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