AD8112-EVALZ Analog Devices Inc, AD8112-EVALZ Datasheet - Page 25

• Manufacturer Part Number: AD8112-EVALZ
• Manufacturer: Analog Devices Inc

Specifications of AD8112-EVALZ

• Lead Free Status / Rohs Status: Compliant

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 (not in close
proximity to IN00), which is terminated to ground. Therefore,
OUT07 should not have a signal present because it is listening
to a quiet input. Any signal measured at the OUT07 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.
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.
Therefore, 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 is given by
where:
R
C
the selected circuit.
s is the Laplace transform variable.
S
M
is the source resistance.
is the mutual capacitance between the test signal circuit and
| XT | = 20 log
10
[( R
S
C
M
) × s ]
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From the equation, it can be observed that this crosstalk
mechanism 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 imped-
ance buffer.
On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8112 is specified with excellent
differential gain and phase when driving a standard 150 Ω video
load, the crosstalk is higher than the minimum obtainable
crosstalk due to the high output currents. These currents induce
crosstalk via the mutual inductance of the output pins and bond
wires of the AD8112.
From a circuit standpoint, this output crosstalk mechanism
looks like a transformer with a mutual inductance between
the windings that drive a load resistor. For low frequencies,
the magnitude of the crosstalk is given by
where:
Mxy is the mutual inductance of output x to output y.
R
This crosstalk mechanism can be minimized by keeping the
mutual inductance low and increasing R
tance can be kept low by increasing the spacing of the conductors
and minimizing their parallel length.
L
is the load resistance on the measured output.
|
XT
|
=
20
log
10
(
Mxy
×
s
/
R
L
)
L
. The mutual induc-
AD8112