AD8143
Rev. 0 | Page 21 of 24
Typically, the input signals are received over 100 Ω differential
transmission lines. A 100 Ω differential transmission line is
readily realized on the printed circuit board using two well-
matched, closely-spaced 50 Ω single-ended traces that are
coupled through the ground plane. The traces that carry the
single-ended output signals are most often 75 Ω for video
signals. Output signal connections should include series
termination resistors that are matched to the impedance
of the line they are driving.
Broadband power supply decoupling networks should be placed
as close as possible to the supply pins. Small surface-mount
ceramic capacitors are recommended for these networks, and
tantalum capacitors are recommended for bulk supply
decoupling.
Minimizing Parasitic Reactances in the Feedback Network
Parasitic trace capacitance and inductance are both reduced
when the traces that connect the feedback network together are
reduced in length. Removing the copper from all planes below
the traces reduces trace capacitance, but increases trace inductance
because the loop area formed by the trace and ground plane is
increased. A reasonable compromise that works well is to void
all copper directly under the feedback loop traces and component
pads with margins on each side approximately equal to one
trace width. Combining this technique with minimizing trace
lengths is effective in keeping parasitic trace reactances in the
feedback loop to a minimum. Additionally, all components used
in the feedback network should be in 0402 surface-mount
packages.
Figure 45 illustrates the magnified view of a proven
feedback network layout that provides excellent performance. Note
that the internal layers are not shown.
It is strongly recommended that the layout shown in
Figure 45,
or something very similar, be used for the three AD8143
feedback networks.
A conservative estimate for feedback-loop trace capacitance in
each loop of the layout shown in
Figure 45 is 2 pF. This value is
viewed as the minimum load capacitance and is reflected in the
frequency response and transient response plots.
Maximizing Heat Removal
The AD8143 pinout includes ground connections on its corner
pins to facilitate heat removal. These pins should be connected
to the exposed paddle on the underside of the AD8143 and to a
ground plane on the component side of the board. Additionally,
a 5 × 5 array of thermal vias connecting the exposed paddle to
internal ground planes should be placed inside the PCB pad
that is soldered to the exposed paddle. Using these techniques
is highly recommended in all applications, and is required in
±12 V applications where power dissipation is the greatest.
Figure 45 illustrates how to optimize the circuit board layout
for heat removal.
Designs must often conform to design-for-manufacturing
(DFM) rules that stipulate how to lay out PCBs in such a way
as to facilitate the manufacturing process. Some of these rules
require thermal relief on pads that connect to planes, and the
rules may preclude the use of the technique illustrated in
Figure 45.
In these cases, the ground pins should be connected to the exposed
paddle and component-side ground plane using techniques that
conform to the DFM requirements.
GND
CFR
CFG
RFG
RGG
RGB
RGR
RFB
RFR
CFB
= CIRCUIT SIDE
= COMPONENT SIDE
05538-043
Figure 45. Recommended Layout for Feedback Loops and Grounding