Datasheet

AD8104/AD8105
Rev. 0 | Page 32 of 36
Short-Circuit Output Conditions
Although there is short-circuit current protection on the
AD8104/AD8105 outputs, the output current can reach values
of 80 mA into a grounded output. Any sustained operation
with too many shorted outputs can exceed the maximum die
temperature and can result in device failure (see the
Absolute
Maximum Ratings
section).
Crosstalk
Many systems, such as broadcast video and KVM switches, that
handle numerous analog signal channels, have strict require-
ments 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 close proximity in a system, as
is undoubtedly the case in a system that uses the AD8104/AD8105,
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
crosspoint 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),
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; therefore, 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. Because the AD8104/AD8105 are fully differential
designs, many sources of crosstalk either destructively cancel, or
are common mode to the signal and can be rejected by a
differential receiver.
Areas of Crosstalk
A practical AD8104/AD8105 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 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 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 are discussed in the following sections 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
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
)(
)(
log20
10
sA
sA
XT
TEST
SEL
where:
s = jω, the Laplace transform variable.
A
SEL
(s) is the amplitude of the crosstalk induced signal in the
selected channel.
A
TEST
(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.
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 32 × 16
matrix of the AD8104/AD8105, look at the number of crosstalk
terms that can be considered for a single channel, for example,
the input IN00. IN00 is programmed to connect to one of the
AD8104/AD8105 outputs where the measurement can be made.
First, the crosstalk terms associated with driving a test signal
into each of the other 31 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 31 other
inputs can be measured two at a time in all possible
combinations, then three at a time, and so on, until, finally,