AD8113
There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
blue) directly from the image sensors. RGB is also the usual
format used by computers internally for graphics. RGB can also
be converted to Y, R–Y, B–Y format, sometimes called YUV
format. These three-circuit video standards are referred to as
component analog video.
raises the issue that a system’s crosstalk 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 when attempting to minimize the effect of crosstalk.
In addition, crosstalk can occur among the inputs to a cross-
point and among the outputs. It can also occur from input to
output. Techniques will be discussed for diagnosing which part
of a system is contributing to crosstalk.
The component video standards require three crosspoint chan-
nels per video channel to handle the switching function. In a
fashion similar to the two-circuit video formats, the inputs and
outputs are assigned in groups of three and the appropriate logic
programming is performed to route the video signals.
Measuring Crosstalk
Crosstalk is measured by applying a signal to one or more chan-
nels 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
CROSSTALK
Many systems, such as studio audio or broadcast video, that
handle numerous analog signal channels, have strict requirements
for keeping the various signals from influencing any of the others in
the system. Crosstalk is the term used to describe the coupling
of the signals of other nearby channels to a given channel.
XT = 20 log10 Asel s Atest s
( )
( )
)
(
where s = jw 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 (to first order). In addition,
the crosstalk signal will have a phase relative to the test signal
associated with it.
When there are many signals in close proximity in a system, as
will undoubtedly be the case in a system that uses the AD8113,
the crosstalk issues can be quite complex. A good understanding
of the nature of crosstalk and some definition of terms is required
in order to specify a system that uses one or more AD8113s.
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.
Types of Crosstalk
Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field,
and sharing of common impedances. This section will explain
these effects.
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 AD8113, look at the number of crosstalk terms that can
be considered for a single channel, say the IN00 input. IN00
is programmed to connect to one of the AD8113 outputs where
the measurement can be made.
Every conductor can be both a radiator of electric fields and a
receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter propa-
gates across a stray capacitance (e.g., free space) and couples with
the receiver and induces a voltage. This voltage is an unwanted
crosstalk signal in any channel that receives it.
First, the crosstalk terms associated with driving a test signal into
each of the other 15 inputs can be measured one at a time, while
applying no signal to IN00. Then the crosstalk terms associated
with driving a parallel test signal into all 15 other inputs can be
measured two at a time in all possible combinations, 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 in parallel.
Currents flowing in conductors create magnetic fields that circulate
around the currents. These magnetic fields then generate voltages
in any other conductors whose paths they link. The undesired
induced voltages in these other channels are crosstalk signals. The
channels that crosstalk can be said to have a mutual inductance
that couples signals from one channel to another.
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 specify them. In addition, this describes
the crosstalk matrix for just one input channel. A similar cross-
talk 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 measurement) are
taken into consideration, the numbers rather quickly grow to
astronomical proportions. If a larger crosspoint array of multiple
AD8113s is constructed, the numbers grow larger still.
The power supplies, grounds, and other signal return paths of a
multichannel system are generally shared by the various chan-
nels. 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 com-
mon impedance.
All these sources of crosstalk are vector quantities, so the mag-
nitudes 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.
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 will
yield the worst crosstalk number, but this is not always the case,
due to the vector nature of the crosstalk signal.
Areas of Crosstalk
A practical AD8113 circuit must be mounted to some sort of
circuit board in order to connect it to power supplies and
measurement equipment. Great care has been taken to create a
characterization board (also available as an evaluation board) that
adds minimum crosstalk to the intrinsic device. This, however,
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REV. A
ADI [ ADI ]
