AD8177 Datasheet, PDF (37/40 Pages) Analog Devices – 500 MHz, Triple 16 × 5 Video Crosspoint Switch

AD8177
IPn
OPn
INn
ONn
50Ω
Figure 53. Fly-By Input Termination
(Grounds for the two transmission lines shown
must be tied together close to the INn pin.)
If multiple AD8177s are to be driven in parallel, a fly-by input
termination scheme is very useful, but the distance from each
AD8177 input to the driven input transmission line is a stub
that should be minimized in length and parasitics using the
discussed guidelines.
When driving the AD8177 single-endedly, the undriven input is
often terminated with a resistance to balance the input stage.
By terminating the undriven input with a resistor of one-half
the characteristic impedance, the input stage is perfectly balanced
(25 Ω, for example, to balance the two parallel 50 Ω terminations
on the driven input). However, due to the feedback in the input
receiver, there is high speed signal current leaving the undriven
input. To terminate this high speed signal, proper transmission
line techniques should be used.
AD8177
One solution is to adjust the trace width to create a transmission
line of half the characteristic impedance and terminate the far
end with this resistance (25 Ω in a 50 Ω system). This is not
often practical as trace widths become large. In most cases, the
best practical solution is to place the half-characteristic impedance
resistor as close as possible (preferably less than 1.5 cm away)
and to reduce the parasitics of the stub (by removing the ground
plane under the stub, for example). In either case, the designer
must decide if the layout complexity created by a balanced,
terminated solution is preferable to simply grounding the undriven
input at the ball with no trace.
The examples discussed so far are for input termination, but the
theory is similar for output back-termination. Taking the AD8177
as an ideal voltage source, any distance of routing between the
AD8177 and a back-termination resistor is an impedance
mismatch that potentially creates reflections. For this reason,
back-termination resistors should also be placed close to the
AD8177. In practice, because back-termination resistors are
series elements, they can be placed close to the AD8177 outputs.
Finally, the AD8177 pinout allows the user to bring the outputs
out as surface traces to the back-termination resistors. The
designer can avoid creating stubs and reflections by keeping the
AD8177 output signal path on the surface of the board. A stub
is created when a top-to-bottom via connection is made on the
output signal path that is perpendicular to the signal flow.
Rev. 0 | Page 37 of 40