AD8116 Analog Devices, AD8116 Datasheet - Page 14

• Manufacturer Part Number: AD8116
• Description: 200 Mhz, 16 3 16 Buffered Video Crosspoint Switch
• Manufacturer: Analog Devices

AD8116
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 input circuits to a
crosspoint and among the output circuits. Techniques will be
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:
where s = jω is the Laplace transform variable, Asel(s) is the
amplitude of the crosstalk-induced signal in the selected chan-
nel 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. 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 AD8116, 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 AD8116 outputs
where the measurement can be made.
First, we can measure the crosstalk terms associated with driv-
ing a test signal into each of the other 15 inputs one at a time.
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; and then three at a time, etc., 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
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 AD8116s 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
term is “all hostile” crosstalk. This term means that all other sys-
tem channels are driven in parallel, and the crosstalk to the selected
channel is measured. In general, this will yield the worst crosstalk
number, but this is not always the case.
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 one-channel or two-channel crosstalk measurements.
|XT| = 20 log
10
(Asel(s)/Atest(s))
Input and Output Crosstalk
The flexible programming capability of the AD8116 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. The input to IN07 is
just terminated to ground and no signal is applied.
All the other inputs are driven in parallel with the same test
signal (practically provided by a distribution amplifier), but
all other outputs except OUT07 are disabled. Since grounded
IN07 is programmed to drive OUT07, there should be no
signal present. Any signal that is present can be attributed to
the other 15 hostile input signals, because no other outputs
are driven. 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 which is
terminated to ground. Thus OUT07 should not have a signal
present since it is listening to a quiet input. Any signal mea-
sured at the OUT07 can be attributed to the output crosstalk
of the other 15 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 imped-
ance 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 cur-
rent flow to create magnetically induced 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 will be given by:
where R
between the test signal circuit and the selected circuit, and s is
the Laplace transform variable.
From the equation it can be observed that this crosstalk mecha-
nism has a high pass nature; it can be also 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 AD8116 is specified with excellent
differential gain and phase when driving a standard 150 Ω video
load, the crosstalk will be higher than the minimum due to the
high output currents. These currents will induce crosstalk via
the mutual inductance of the output pins and bond wires of the
AD8116.
S
is the source resistance, C
|XT| = 20 log
10
[(R
M
S
is the mutual capacitance
C
M
) × s]