AD8074/AD8075
Each of the six outputs has a 75 Ω series resistor that is used to
reverse-terminate the output transmission line. The correspond-
ing outputs are then wired in parallel and delivered to the output
cable. The termination resistors in this position help to isolate
the off capacitance of the disabled device’s outputs from loading
the enabled device’s outputs. The gain-of-two of the AD8075
compensates for the signal halving that occurs as a result of the
output terminations.
A major area of focus should be the power distribution system.
There should be a full ground plane that provides the reference
and return paths for both the inputs and outputs. The ground
also provides isolation between the input signals to minimize the
crosstalk. This ground plane should cover as wide an area as
possible and be minimally interrupted in order to keep its
impedance to a minimum.
The power planes should also be as broad as possible to provide
minimal inductance, which is required for high-slew-rate sig-
nals. These power planes layers should be spaced closely to the
ground plane to increase the interplane capacitance between the
supplies and ground.
A select signal is provided directly to the OE of the second
AD8075 and an inverted version is used to drive the other device’s
OE. This will ensure that only one device is active at a time. Since
there is a total of 150 Ω in series between any two outputs, it is
not essential to be overly concerned about the exact timing of
the making and breaking of the enable signals.
Each supply pin should be bypassed with a low inductance
0.1 µF ceramic capacitance with minimal excess circuit length
to minimize the series impedance. A 25 µF tantalum electro-
lytic capacitor will supply a charge reservoir for lower frequency,
high-amplitude transitions.
Additional inputs can easily be added to the circuit shown to
make wider multiplexers. The outputs of all of the devices will
be wired in parallel, and the logic must allow that only one output
be enabled at a time.
The input and output signals should be run as directly as pos-
sible in order to minimize the effects of parasitics. If they must
run over a longer distance of more than a few centimeters, con-
trolled impedance PCB traces should be used to minimize the
effect of reflections due to mismatches in impedance and the
proper termination should be provided.
If it is desired to make a triple 3:1 multiplexer, a triple 2:1 mul-
tiplexer, like the AD8185 can be used along with the AD8075.
The same general guidelines for input and output treatment
should be followed and the logic must perform the proper function.
If it is desired to design such a multiplexer at unity gain, the
AD8074 should be used. For a triple 3:1 multiplexer, an
AD8183 (triple 2:1 mux) can be combined with an AD8074 to
provide this function.
To avoid excess crosstalk, the above recommendations should
be followed carefully. The power system and signal routing are
the most important aspects of preventing excess crosstalk.
Beyond these techniques, shielding can be provided by ground
traces between adjacent signals, especially those that travel
parallel over long distances.
Layout and Grounding
The AD8074 and AD8075 are extreme bandwidth, high-slew-rate
devices that are designed to drive up to the highest resolution
monitors and provide excellent resolution. To realize their full
performance potential, it is essential to adhere to the best prac-
tices of high-speed PCB layout.
–12–
REV. A
ROCHESTER [ Rochester Electronics ]
