LMH6572_05 Datasheet, PDF (10/12 Pages) National Semiconductor (TI) – Triple 2:1 High Speed Video Multiplexer

Application Notes (Continued)
The drawback in this case is the increased capacitive load
presented to the output of each LMH6572 due to the off-
state capacitance of the LMH6572.
Other Applications
The LMH6572 may be utilized in systems that involve a
single RGB channel as well whenever there is a need to
switch between different “flavors” of a single RGB input.
Here are some examples:
1. RGB positive polarity, negative polarity switch
2. RGB full resolution, high-pass filter switch
In each of these applications, the same RGB input occupies
one set of inputs to the LMH6572 and the other “flavor”
would be tied to the other input set.
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor. Figure 6 shows the use of a
series output resistor, R_OUT, to stabilize the amplifier output
under capacitive loading. Capacitive loads of 5 to 120 pF are
the most critical, causing ringing, frequency response peak-
ing and possible oscillation. Figure 7gives a recommended
value for selecting a series output resistor for mitigating
capacitive loads. The values suggested in the charts are
selected for .5 dB or less of peaking in the frequency re-
sponse. This gives a good compromise between settling
time and bandwidth. For applications where maximum fre-
quency response is needed and some peaking is tolerable,
the value of R_OUT can be reduced slightly from the recom-
mended values.
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FIGURE 7. Recommended R_OUT vs. Capacitive Load
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FIGURE 6. Decoupling Capacitive Loads
www.national.com
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FIGURE 8. Frequency Response vs. Capacitive Load
LAYOUT CONSIDERATIONS
Whenever questions about layout arise, use the LMH730151
evaluation board as a guide. To reduce parasitic capaci-
tances, ground and power planes should be removed near
the input and output pins. For long signal paths controlled
impedance lines should be used, along with impedance
matching elements at both ends. Bypass capacitors should
be placed as close to the device as possible. Bypass capaci-
tors from each rail to ground are applied in pairs. The larger
electrolytic bypass capacitors can be located farther from the
device; however, the smaller ceramic capacitors should be
placed as close to the device as possible. In Figure 1 and
Figure 2, the capacitor between V⁺ and V^− is optional, but is
recommended for best second harmonic distortion. Another
way to enhance performance is to use pairs of .01 µF and
.1 µF ceramic capacitors for each supply bypass.
POWER DISSIPATION
The LMH6572 is optimized for maximum speed and perfor-
mance in the small form factor of the standard SSOP pack-
age. To achieve its high level of performance, the LMH6572
10