AD8108/AD8109 Data Sheet
Rev. C | Page 22 of 27
CROSSTALK
Many systems, such as 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.
When there are many signals in proximity in a system, as is
undoubtedly be the case in a system that uses the AD8108/
AD8109, the crosstalk issues can be quite complex. A good
understanding of the nature of crosstalk and some definition
of terms is required to specify a system that uses one or more
AD8108/AD8109 devices.
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 explains
these effects.
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 (for example, free space)
and couples with the receiver and induces a voltage. This
voltage is an unwanted crosstalk signal in any channel that
receives it.
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.
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 in one of
these paths, a voltage that is developed across the impedance
becomes an input crosstalk signal for other channels that share
the common impedance.
All these sources of crosstalk are vector quantities, so the
magnitudes 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.
Areas of Crosstalk
A practical AD8108/AD8109 circuit must be mounted to some
sort of circuit board to connect it to the power supplies and the
measurement equipment. Take great care to create a board that
adds minimum crosstalk to the intrinsic device. Note that the
crosstalk of a system 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 of
crosstalk when attempting to minimize its effect.
In addition, crosstalk can occur among the inputs to a crosspoint
and among the outputs. It can also occur from input to output.
Techniques are 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:
( ) ( )
( )
sAtestsAselXT
10
log20=
where s = jω is the Laplace transform variable, As el(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 has 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 8 × 8
matrix of the AD8108/AD8109, 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
AD8108/AD8109 outputs where the measurement can be made.
We can first measure the crosstalk terms associated with driving
a test signal into each of the other seven inputs one at a time.
We can then measure the crosstalk terms associated with
driving a parallel test signal into all seven other inputs taken
two at a time in all possible combinations, and then three at a
time, and so on, until there is only one way to drive a test signal
into all seven 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
