Datasheet

AD8112
Rev. 0 | Page 24 of 28
All these sources of crosstalk are vector quantities; therefore 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 reduce the
crosstalk.
Areas of Crosstalk
A practical AD8112 circuit must 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 characteriza-
tion board (also available as an evaluation board) that adds
minimum crosstalk to the intrinsic device. This, however, raises
the issue that the crosstalk of a system is a combination of the
intrinsic crosstalk of both the devices and the circuit board to
which they are mounted. It is important to try to separate these
two areas when attempting to minimize the effect of crosstalk.
In addition, crosstalk can occur among the inputs as well as the
outputs of a cross-point. It can also occur from input to output.
The following sections describe techniques for measuring and
identifying the source of 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
decibels down from the magnitude of the test signal. The
crosstalk is expressed by
() ()
() sAtestsAselXT /log 20||
10
=
where:
s = jw is the Laplace transform variable.
Asel(s) = the amplitude of the crosstalk induced signal in the
selected channel.
Atest(s) = 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 has 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 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 become
extremely large. For example, in the case of the 16 × 8
matrix of
the AD8112, consider the number of possible sources of crosstalk
terms for a single channel, for example the IN00 input. IN00 is
programmed to connect to one of the AD8112 outputs where
crosstalk can be measured.
To measure this crosstalk, use one of the following two methods.
In the first method, the crosstalk terms associated with driving
a test signal into each of the other 15 inputs is measured one at
a time, while applying no signal to IN00. In the second method,
the crosstalk terms associated with driving a parallel test signal
into all 15 other inputs is measured two at a time in all possible
combinations, then three at a time, and so on, until, finally, there
is only one way to drive a test signal into all 15 other inputs in
parallel.
Each combination is legitimately different from the others and
might yield a unique value, depending on the resolution of the
measurement system. It is not practical to measure and then
specify all these terms. Furthermore, 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 outputs (not used for measurement) are taken into
consideration, the numbers of possibilities quickly grows to
astronomical proportions. If a larger crosspoint array of multiple
AD8112s is constructed, the numbers grow larger still.
Obviously, a 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 means that the
crosstalk to the selected channel is measured while all other
system channels are driven in parallel. In general, this yields 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 the
nearest neighbor or by the two nearest neighbors on either side.
These crosstalk measurements are generally higher than those
of more distant channels, and therefore 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 AD8112 can be used
to diagnose whether crosstalk is greater on the input side or the
output side. For example, to identify the source of crosstalk, the
IN07 input channel can be programmed to drive OUT07, with
the input to IN07 terminated to ground (via 50 Ω or 75 Ω) and
no signal applied.
All the other inputs are driven in parallel with the same test
signal (practically provided by a distribution amplifier), with all
other outputs except OUT07 disabled. Because 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). Therefore, this method measures the all-hostile input
contribution to crosstalk into IN07. This method can be used
for other input channels and combinations of hostile inputs.