AD8108AST Analog Devices Inc, AD8108AST Datasheet - Page 23

• Manufacturer Part Number: AD8108AST
• Description: IC VIDEO CROSSPOINT SWIT 80LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD8108AST

• Rohs Status: RoHS non-compliant
• Function: Video Crosspoint Switch
• Circuit: 1 x 8:8
• Voltage Supply Source: Dual Supply
• Voltage - Supply, Single/dual (±): ±4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 80-LQFP
• Array Configuration: 8x8
• Number Of Arrays: 1
• Screening Level: Industrial
• Pin Count: 80
• Package Type: LQFP
• Power Supply Requirement: Dual
• Lead Free Status / RoHS Status: Not Compliant

quickly grow to astronomical proportions. If a larger crosspoint
array of multiple AD8108/AD8109s is constructed, the numbers
grow larger still.
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 term
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.
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 1-channel or 2-channel crosstalk
measurements.
Input and Output Crosstalk
The flexible programming capability of the AD8108/AD8109
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 (IN03 in the middle for this example)
can be programmed to drive OUT03. The input to IN03 is just
terminated to ground (via 50 Ω or 75 Ω) and no signal is
applied.
All the other inputs are driven in parallel with the same test
signal (practically this is provided by a distribution amplifier),
with all other outputs except OUT03 disabled. Since grounded
IN03 is programmed to drive OUT03, there should be no signal
present. Any signal that is present can be attributed to the other
seven 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 IN03. Of course, the
method can be used for other input channels and combinations
of hostile inputs.
For output crosstalk measurement, a single input channel is
driven (IN00, for example) and all outputs other than a given
output (IN03 in the middle) are programmed to connect to
IN00. OUT03 is programmed to connect to IN07 (far away
from IN00), which is terminated to ground. Thus OUT03
should not have a signal present since it is listening to a quiet
input. Any signal measured at the OUT03 can be attributed to
the output crosstalk of the other seven hostile outputs. Again,
this method can be modified to measure other channels and
other crosspoint matrix combinations.
Rev. B | Page 23 of 32
Effect of Impedances on Crosstalk
The input side crosstalk can be influenced by the output
impedance 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. However, 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.
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
where
between the test signal circuit and the selected circuit, and is
the Laplace transform variable.
From the equation, it can be observed that this crosstalk
mechanism has a high-pass nature; it can 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.
On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8108/AD8109 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 AD8108/AD8109.
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
where
and
crosstalk mechanism can be minimized by keeping the mutual
inductance low and increasing R
be kept low by increasing the spacing of the conductors and
minimizing their parallel length.
is the load resistance on the measured output. This
is the source resistance,
=
=
is the mutual inductance of Output x to Output y,
20
20
log
log
10
10
(
[
(
×
)
×
L
)
. The mutual inductance can
]
is the mutual capacitance
AD8108/AD8109