AD8114/AD8115 Data Sheet
Rev. C | Page 22 of 25
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 the
device be mounted to some sort of circuit board to connect it
to the power supplies and the measurement equipment. This
requirement, however, raises the issue that the crosstalk of a
system 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 are 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
( ) ( )
( )
sAtest
sAselXT
10
log20=
where:
s = jω is the Laplace transform variable.
Asel(s) is the amplitude of the crosstalk-induced signal in the
selected channel.
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 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 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, and so on, 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/
AD8115 devices is constructed, the numbers grow larger still.
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 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 one
nearest neighbor or by the two nearest neighbors on either side.
These crosstalk measurements are generally 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.
