Datasheet

AD8106/AD8107
Rev. 0 | Page 18 of 28
In addition, crosstalk can occur among the inputs to a
crosspoint as well as among the outputs. It can also occur
from input to output. Refer to the
CROSSTALK
Many systems, such as broadcast video, handle numerous
analog signal channels that have strict requirements for keeping
the various signals from influencing others in the system.
Crosstalk is the term used to describe the undesired coupling
between signals of other nearby channels to a given channel.
Input and Output Crosstalk
section for techniques to diagnose 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
When many signals are in close proximity in a system, as is
undoubtedly the case in a system that uses the AD8106/
AD8107, the crosstalk issues can be quite complex. A good
understanding of the nature of crosstalk and its associated
terms is required to specify a system that uses one or more
AD8106s/AD8107s.
(
)()
(
)
sAtestsAselXT /log20
10
=
(1)
Types of Crosstalk
where:
Crosstalk can be propagated by means of one of three methods.
These fall into the categories of electric field, magnetic field, and
sharing of common impedances. This section explains these effects.
s = jw is the Laplace transform variable.
Asel(s) is the amplitude of the crosstalk-induced signal in the
selected channel.
Atest(s) is the amplitude of the test signal.
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
propagates across a stray capacitance (free space, for example),
couples with the receiver, and induces a voltage. This voltage is
an unwanted crosstalk signal in any channel that receives it.
It can be seen that crosstalk is a function of frequency, but not a
function of the test signal’s magnitude (to first order). The crosstalk
signal also has a phase relative to its associated test signal.
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.
Currents flowing into 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 channels are
crosstalk signals. The channels that crosstalk have a mutual
inductance that couples signals from one channel to another.
As a crosspoint system or device grows, the number of theoretical
crosstalk combinations and permutations can become extremely
large. For example, in the case of the 16 x 5 matrix of the
AD8106/AD8107, examine the number of crosstalk terms that
can be considered for a single channel, such as the IN00 input.
IN00 is programmed to connect to one of the AD8106/AD8107
outputs where the measurement can be made.
The power supplies, grounds, and other signal return paths of a
multichannel system are generally shared by the various
channels. When a current from one channel flows into one of
these paths, a voltage develops across the impedance and
becomes an input crosstalk signal for other channels that share
the common impedance.
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.
All these sources of crosstalk are vector quantities, so the magni-
tudes cannot simply be added together to obtain the total crosstalk.
In fact, there are conditions when driving additional circuits in
parallel in a given configuration can actually reduce the crosstalk.
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/AD8107s is constructed, the numbers grow
larger still.
Areas of Crosstalk
A practical AD8106/AD8107 circuit is required to be mounted
to some sort of circuit board to connect 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,
raises the issue that a system’s crosstalk is a combination of the
device’s intrinsic crosstalk and the 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.