LMH6580 Datasheet, PDF (17/24 Pages) National Semiconductor (TI) – 8x4 500MHz Analog Crosspoint Switch, Gain of 1, Gain of 2

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FIGURE 3. Input Expansion with Separate Termination
Resistors
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FIGURE 5. Input Expansion Frequency Response
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R_OUT. Capacitive loads of
5 pF to 120 pF are the most critical, causing ringing, frequency
response peaking and possible oscillation. Since most ca-
pacitive loading is due to undesired parasitic capacitances the
values of the capacitive loading will not usually be known ex-
actly. It is best to start with a conservative value of R_OUT and
decrease the value until the bandwidth shows slight peaking.
At this point the value of the isloation resistor will be deter-
mined by whether flat frequency response or maximum band-
width is the desired goal. Smaller values of R_OUT will produce
some peaking, but maximum bandwidth. Larger resistor val-
ues will decrease bandwidth and suppress peaking.
As starting values, a capacitive load of 5 pF should have
around 75 Ω of isolation resistance. A value of 120 pF would
require around 12Ω. When driving transmission lines, the out-
put termination resistor is normally sufficient.
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FIGURE 4. Input Expansion Frequency Response
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