AD8117_07 AD [Analog Devices], AD8117_07 Datasheet - Page 34

• Manufacturer Part Number: AD8117_07
• Manufacturer: AD [Analog Devices]

AD8117/AD8118
On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8117/AD8118 are 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 AD8117/AD8118.
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:
M
R
This crosstalk mechanism can be minimized by keeping the
mutual inductance low and increasing R
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 the system circuit board(s). The areas
that must be carefully detailed are grounding, shielding, signal
routing, and supply bypassing.
The packaging of the AD8117/AD8118 is designed to help keep
the crosstalk to a minimum. On the BGA substrate, each pair is
carefully routed to predominately couple to each other, with
shielding traces separating adjacent signal pairs. The ball grid
array is arranged such that similar board routing can be achieved.
Only the outer two rows are used for signals, such that vias can
be used to take the input rows to a lower signal plane if desired.
The input and output signals 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.
PCB Termination Layout
As frequencies of operation increase, the importance of proper
transmission line signal routing becomes more important. The
bandwidth of the AD8117/AD8118 is large enough 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 properly terminate the input
and output signals of the AD8117/AD8118. Traditionally, video
applications have used 75 Ω single-ended environments. RF
L
XY
is the load resistance on the measured output.
is the mutual inductance of Output X to Output Y.
XT
=
20
log
10
⎛
⎜
⎜
⎝
M
XY
×
R
s
L
⎞
⎟
⎟
⎠
L
. The mutual
Rev. A | Page 34 of 36
applications are generally 50 Ω single-ended (and board
manufacturers have the most experience with this application).
CAT-5 cabling is usually driven as differential pairs of 100 Ω
differential impedance.
For flexibility, the AD8117/AD8118 do not contain on-chip
termination resistors. This flexibility in application comes with
some board layout challenges. The distance between the termi-
nation of the input transmission line and the AD8117/AD8118
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
signal in the chosen board material and the distance (d)
between the termination resistor and the AD8117/AD8118. 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
AD8117/AD8118 input should be placed no farther than 1.5 cm
after the termination resistors, and preferably should be placed
even closer. The BGA substrate routing inside the AD8117/
AD8118 is approximately 1 cm in length and adds to the stub
length, so 1.5 cm PCB routing equates to d = 2.5 × 10
calculations.
where n = {0, 1, 2, 3, …}.
In some cases, it is difficult to place the termination close to the
AD8117/AD8118 due to space constraints, differential routing,
and large resistor footprints. A preferable solution in this case is
to maintain a controlled transmission line past the AD8117/
AD8118 inputs and terminate the end of the line. This is known
as fly-by termination. The input impedance of the AD8117/
AD8118 is large enough and stub length inside the package is
small enough that this works well in practice. Implementation
of fly-by input termination often includes bringing the signal in
on one routing layer, then passing through a filled via under the
AD8117/AD8118 input ball, then back out to termination on
another signal layer. In this case, care must be taken to tie the
reference ground planes together near the signal via if the signal
layers are referenced to different ground planes.
Figure 72. Fly-By Input Termination, Grounds for the Two Transmission Lines
f
PEAK
Shown Must be Tied Together Close to the INn Pin.
=
75Ω
(
2
n
+
4
1
d
)
×
V
INn
IPn
P
P
= 144 × 106 m/s), this means the
AD8117/
AD8118
OPn
ONn
P
) of the
−2
m in the