AD8108 Analog Devices, AD8108 Datasheet - Page 20

• Manufacturer Part Number: AD8108
• Description: 325 Mhz, 8x8 Buffered Video Crosspoint Switch (Gain=1)
• Manufacturer: Analog Devices

AD8108/AD8109
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 AD8108/AD8109 is designed to help keep
the crosstalk to a minimum. Each input is separated from each
other input by an analog ground pin. All of these AGNDs
should be directly connected to the ground plane of the circuit
board. These ground pins provide shielding, low impedance return
paths and physical separation for the inputs. All of these help to
reduce crosstalk.
Each output is separated from its two neighboring outputs by an
analog ground pin in addition to an analog supply pin of one
polarity or the other. Each of these analog supply pins provides
power to the output stages of only the two nearest outputs. These
supply pins and analog grounds provide shielding, physical sepa-
ration and a low impedance supply for the outputs. Individual
bypassing of each of these supply pins, with a 0.01 µF chip capaci-
tor directly to the ground plane, minimizes high frequency output
crosstalk via the mechanism of sharing common impedances.
Each output also has an on-chip compensation capacitor that
is individually tied the nearby analog ground pins AGND00
through AGND07. This technique reduces crosstalk by prevent-
ing the currents that flow in these paths from sharing a common
impedance on the IC and in the package pins. These AGNDxx
signals should all be directly connected to the ground plane.
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The input and output signals will 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 only place the input and output signals surface is at the
input termination resistors and the output series back termination
resistors. These signals should also be separated, to the extent
possible, as soon as they emerge from the IC package.
Evaluation Board
A four-layer evaluation board for the AD8108/AD8109 is avail-
able. The exact same board and external components are used
for each device. The only difference is the device itself, which
offers a selection of a gain of unity or gain of two through the
analog channels. This board has been carefully laid out and tested
to demonstrate the specified high-speed performance of the device.
Figure 10 shows the schematic of the evaluation board. Figure
11 shows the component side silk-screen. The layouts of the
board’s four layers are given in Figures 12, 13, 14, and 15.
The evaluation board package includes the following:
• Fully populated board with BNC-type connectors.
• Windows
• Custom cable to connect evaluation board to PC.
• Disk containing Gerber files of board layout.
via the printer port.
®
based software for controlling the board from a PC