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參數(shù)資料
| 型號： | AD8116 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Buffered Video Crosspoint Switch(200MHz,視頻正交開關(guān)) |
| 中文描述： | 緩沖視頻交叉點開關(guān)（200MHz的，視頻正交開關(guān)） |
| 文件頁數(shù)： | 14/28頁 |
| 文件大?。?/td> | 642K |
| 代理商： | AD8116 |
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AD8116
–14–
REV. 0
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:
|
XT
| = 20 log
10
(
Asel
(
s
)/
Atest
(
s
))
where
s
= j
ω
is the Laplace transform variable,
Asel
(
s
) is the
amplitude of the crosstalk-induced signal in the selected channel
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 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
×
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
driving 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
system 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.
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 measured
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
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.
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:
|
XT
| = 20 log
10
[(
R
S
C
M
)
×
s
]
where
R
S
is the source resistance,
C
M
is the mutual capacitance
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
mechanism 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
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
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.
From a circuit standpoint, this output crosstalk mechanism
looks like a transformer with a mutual inductance between the
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相關(guān)代理商/技術(shù)參數(shù) |
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| AD8116-EB | 制造商:Analog Devices 功能描述:AD8116 EVALUATION BOARD - Bulk |
| AD8116JST | 制造商:Analog Devices 功能描述:Analog Video Crosspoint 200MHz 16 x 16 128-Pin LQFP 制造商:Rochester Electronics LLC 功能描述:TQFP 16X16 VIDEO CROSSPOINT SWITCH - Bulk 制造商:Analog Devices 功能描述:SWITCH CROSSPOINT 16X16 SMD 8116 |
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