AD8113
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.
On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8113 is specified with excellent
differential gain and phase when driving a standard 150 Ω video
load, the crosstalk will be higher than the minimum obtainable
due to the high output currents. These currents will induce
crosstalk via the mutual inductance of the output pins and bond
wires of the AD8113.
Input and Output Crosstalk
The flexible programming capability of the AD8113 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 (also in the middle). The
input to IN07 is just terminated to ground (via 50 Ω or 75 Ω)
and no signal is applied.
From a circuit standpoint, this output crosstalk mechanism
looks like a transformer with a mutual inductance between the
windings that drives a load resistor. For low frequencies, the
magnitude of the crosstalk is given by
XT = 20 log10 Mxy × s R
(
)
L
where Mxy is the mutual inductance of output X to output Y
and RL is the load resistance on the measured output. This
crosstalk mechanism can be minimized by keeping the mutual
inductance low and increasing RL. The mutual inductance can
be kept low by increasing the spacing of the conductors and
minimizing their parallel length.
All the other inputs are driven in parallel with the same test
signal (practically provided by a distribution amplifier), with all
other outputs except OUT07 disabled. Since grounded IN07 is
programmed to drive OUT07, no signal should be present. Any
signal that is present can be attributed to the other 15 hostile input
signals, because no other outputs are driven (they are all disabled).
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.
PCB Layout
Extreme care must be exercised to minimize additional crosstalk
generated by the system circuit board(s). The areas that must be
carefully detailed are grounding, shielding, signal routing, and
supply bypassing.
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 (far away
from IN00), 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 16 hostile outputs. Again, this
method can be modified to measure other channels and other
crosspoint matrix combinations.
The packaging of the AD8113 is designed to help keep the
crosstalk to a minimum. Each input is separated from each other
input by an analog ground pin. All of these AGNDs should be
directly connected to the ground plane of the circuit board.
These ground pins provide shielding, low impedance return
paths, and physical separation for the inputs. All of these help to
reduce crosstalk.
Each output is separated from its two neighboring outputs by an
analog supply pin of one polarity or the other. Each of these analog
supply pins provides power to the output stages of only the two
nearest outputs. These supply pins provide shielding, physical
separation, and a low impedance supply for the outputs. Individual
bypassing of each of these supply pins with a 0.01 µF chip capaci-
tor directly to the ground plane minimizes high frequency output
crosstalk via the mechanism of sharing common impedances.
Effect of Impedances on Crosstalk
The input side crosstalk can be influenced by the output imped-
ance 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. How-
ever, significant current can flow through the input termination
resistors and the loops that drive them. Thus, the PC board on
the input side can contribute to magnetically coupled crosstalk.
Each output also has an on-chip compensation capacitor that
is individually tied to the nearby analog ground pins AGND00
through AGND07. This technique reduces crosstalk by prevent-
ing the currents that flow in these paths from sharing a common
impedance on the IC and in the package pins. These AGNDxx
signals should all be connected directly to the ground plane.
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
The input and output signals will have minimum crosstalk if they
are located between ground planes on layers above and below,
and separated by ground in between. Vias should be located as
close to the IC as possible to carry the inputs and outputs to the
inner layer. The input and output signals surface at the input
termination resistors and the output series back-termination
resistors. To the extent possible, these signals should also be
separated as soon as they emerge from the IC package.
XT = 20 log10 R C × s
(
)
[
S
M
]
where RS is the source resistance, CM 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 mecha-
nism has a high-pass nature; it can also be 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 Ω
terminated cable, the input crosstalk can be reduced by buffering
this signal with a low output impedance buffer.
REV. A
–21–
ADI [ ADI ]
