Datasheet

AD8104/AD8105
Rev. 0 | Page 33 of 36
there is only one way to drive a test signal into all 31 other
inputs in parallel.
Each of these cases is legitimately different from the others and
may yield a unique value, depending on the resolution of the
measurement system, but it is hardly practical to measure all
these terms and then 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 outputs (not used for measure-
ment) are taken into consideration, the numbers rather quickly
grow to astronomical proportions. If a larger crosspoint array of
multiple AD8104/AD8105s 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 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 one-channel or two-channel crosstalk
measurements.
Input and Output Crosstalk
Capacitive coupling is voltage-driven (dV/dt), but is generally a
constant ratio. Capacitive crosstalk is proportional to input or
output voltage, but this ratio is not reduced by simply reducing
signal swings. Attenuation factors must be changed by changing
impedances (lowering mutual capacitance), or destructive
canceling must be utilized by summing equal and out of phase
components. For high input impedance devices such as the
AD8104/AD8105, capacitances generally dominate input-
generated crosstalk.
Inductive coupling is proportional to current (dI/dt), and often
scales as a constant ratio with signal voltage, but also shows a
dependence on impedances (load current). Inductive coupling
can also be reduced by constructive canceling of equal and out
of phase fields. In the case of driving low impedance video
loads, output inductances contribute highly to output crosstalk.
The flexible programming capability of the AD8104/AD8105
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 pair (IN07 in the middle for this example) can be
programmed to drive OUT07 (also in the middle). The inputs
to IN07 are just terminated to ground (via 50  or 75 ) and no
signal is 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. Since grounded IN07
is programmed to drive OUT07, no signal should be present.
Any signal that is present can be attributed to the other 31
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.
Effect of Impedances on Crosstalk
The input side crosstalk can be influenced by the output
impedance of the sources that drive the inputs. The lower the
impedance of the drive source, the lower the magnitude of the
crosstalk. The dominant crosstalk mechanism on the input side
is capacitive coupling. The high impedance inputs do not have
significant current flow to create magnetically induced crosstalk.
However, significant current can flow through the input termi-
nation resistors and the loops that drive them. Thus, the PC
board on the input side can contribute to magnetically coupled
crosstalk.
From a circuit standpoint, the input crosstalk mechanism looks
like a capacitor coupling to a resistive load. For low frequencies,
the magnitude of the crosstalk is given by
[
]
sCRXT
M
S
×= )(log20
10
where:
R
S
is the source resistance.
C
M
is the mutual capacitance between the test signal circuit and
the selected circuit.
s is the Laplace transform variable.
From the preceding equation, it can be observed that this
crosstalk mechanism has a high-pass nature; it can also be
minimized by reducing the coupling capacitance of the input
circuits and lowering the output impedance of the drivers. If the
input is driven from a 75  terminated cable, the input crosstalk
can be reduced by buffering this signal with a low output
impedance buffer.