Data Sheet AD8106/AD8107
Rev. A | Page 19 of 22
First, measure the crosstalk terms associated with driving a test
signal into each of the other 15 inputs one at a time. 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; and 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.
Each of these cases is legitimately different from the others and
could yield a unique value depending on the resolution of the
measurement system. However, it is impractical to measure all
of these terms and then to specify them. In addition, this
describes the crosstalk matrix for only one input channel. A
similar crosstalk matrix can be proposed for every other input.
If the possible combinations and permutations for connecting
inputs to the other outputs (not used for measurement) are
taken into consideration, the numbers grow rather quickly to
astronomical proportions. If a larger crosspoint array of
multiple AD8106/AD8107 devices 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, which 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
of the nearest neighbors or by two of the 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
The flexible programming capability of the AD8106/AD8107
can be used to diagnose whether crosstalk is occurring more on
the input side or the output side. For example, a given input
channel, such as IN07 in the middle, can be programmed to
drive OUT01. The input to IN07 is 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 provided by a distribution amplifier) with all
other outputs disabled, except OUT01. Because grounded IN07
is programmed to drive OUT01, no signal should be present. If
any signal is present, it can be attributed to the other 15 hostile
input signals because no other outputs are driven; that is, they
are all disabled. Thus, this method measures the all-hostile input
contribution to crosstalk into IN07. This method can also be used
for other input channels and combinations of hostile inputs.
For output crosstalk measurement, a single input channel (IN00,
for example) is driven and all outputs other than a given output
are programmed to connect to IN00. OUT01 is programmed to
connect to IN15, which is far away from IN00, and is terminated
to ground. As a result, OUT01 should not have a signal present
because it is listening for a quiet input. Any signal measured at
the OUT01 can be attributed to the output crosstalk of the other
seven 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
termination resistors and the loops that drive them. Thus,
the printed circuit 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
(
)( )
sC
RXT
MS
×
=
10
log20
(2)
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.
Equation 2 shows 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 Ω
terminated cable, the input crosstalk can be reduced by
buffering this signal with a low output impedance buffer.
On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8106/AD8107 are specified with
excellent differential gain and phase when driving a standard
150 Ω video load, the crosstalk is higher than the minimum
obtainable because of the high output currents. These currents
induce crosstalk via the mutual inductance of the output pins
and bond wires of the AD8106/AD8107.
