Datasheet

ADV3200/ADV3201
Rev. 0 | Page 34 of 36
Effect of Impedances on Crosstalk
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 PCB 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
[
sCRXT
M
S
×= )(log20
10
]
(5)
where:
R
S
is the source resistance.
C
M
is the mutual capacitance between the test signal circuit and
the selected circuit.
s is the Laplace transform variable.
From the preceding 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  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 ADV3200/ADV3201 are specified
with excellent differential gain and phase when driving a
standard 150  video load, the crosstalk will be higher than the
minimum obtainable due to the high output currents. These
currents induce crosstalk via the mutual inductance of the
output pins and bond wires of the ADV3200/ADV3201.
From a circuit standpoint, the 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
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×=
L
XY
R
s
MXT
10
log20 (6)
where:
M
XY
is the mutual inductance of Output X to Output Y.
R
L
is the load resistance on the measured output.
s is the Laplace transform variable.
This crosstalk mechanism can be minimized by keeping
the mutual inductance low and increasing R
L
. The mutual
inductance can be kept low by increasing the spacing of the
conductors and minimizing their parallel length.
PCB Layout
Extreme care must be exercised to minimize additional
crosstalk generated by system circuit boards. The areas that
must be carefully detailed are grounding, shielding, signal
routing, and supply bypassing.
The input and output signals have minimum crosstalk if they
are located between ground planes on layers above and below
and are separated by ground in between. Locate vias 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 termin-
ation resistors and the output series back-termination resistors.
To the extent possible, separate these signals as soon as they
emerge from the IC package.
PCB TERMINATION LAYOUT
As frequencies of operation increase, proper routing of trans-
mission line signals becomes more important. The bandwidth
of the ADV3200/ADV3201 is large enough so that using high
impedance routing does not provide a flat in-band frequency
response for practical signal trace lengths. It is necessary for
the user to choose a characteristic impedance suitable for the
application and to properly terminate the input and output
signals of the ADV3200/ADV3201. Traditionally, video
applications use 75  single-ended environments.
For flexibility, the ADV3200/ADV3201 does not contain on-
chip termination resistors. This flexibility in application comes
with some board layout challenges. The distance between the
termination of the input transmission line and the ADV3200/
ADV3201 die is a high impedance stub and causes reflections
of the input signal. With some simplification, it can be shown
that these reflections cause peaking of the input at regular
intervals in frequency, dependent on the propagation speed (v
P
)
of the signal in the chosen board material and the distance (d)
between the termination resistor and the ADV3200/ADV3201.
If the distance is great enough, these peaks can occur in band.
In fact, practical experience shows that these peaks are not
high-Q, and should be pushed out to three or four times the
desired bandwidth in order to not have an effect on the signal.
For a board designer using FR4 (v
P
= 144 × 10
6
m/s), this means
that the ADV3200/ADV3201 input should be placed no farther
than 2 cm after the termination resistors and, preferably, should
be placed even closer. Therefore, 2 cm PCB routing equates to
d = 2 × 10
−2
m in the calculations.
( )
d
vn
f
P
PEAK
4
12 ×
+
= (7)
where n = {0, 1, 2, 3, …}.
In some cases, it is difficult to place the termination close to
the ADV3200/ADV3201 due to space constraints and large
resistor footprints. A better solution in this case is to maintain a
controlled transmission line past the ADV3200/ADV3201
inputs and to terminate the end of the line. This method is
known as fly-by termination. The input impedance of the
ADV3200/ADV3201 is large enough, and the stub length inside
the package is small enough, that this works well in practice.