LMH6739_07 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Video Buffer

UNITY GAIN COMPENSATION
With a current feedback Selectable Gain Buffer like the
LMH6739, the feedback resistor is a compromise between
the value needed for stability at unity gain and the optimized
value used at a gain of two. The result of this compromise is
substantial peaking at unity gain. If this peaking is undesirable
a simple RC filter at the input of the buffer will smooth the
frequency response shown as Figure 4. Figure 5 shows the
results of a simple filter placed on the non-inverting input. See
Figure 6 and Figure 7 for another method for reducing unity
gain peaking.
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FIGURE 6. Alternate Unity Gain Compensation
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FIGURE 7. Frequency Response for Circuit in Figure 6
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R_OUT. Figure 8 shows the use
of a series output resistor, R_OUT, to stabilize the amplifier out-
put under capacitive loading. Capacitive loads of 5 to 120 pF
are the most critical, causing ringing, frequency response
peaking and possible oscillation. The charts “Suggested
R_OUT vs. Cap Load” give 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 response. This gives a good com-
promise between settling time and bandwidth. For applica-
tions where maximum frequency response is needed and
some peaking is tolerable, the value of R_OUT can be reduced
slightly from the recommended values.
LAYOUT CONSIDERATIONS
Whenever questions about layout arise, use the evaluation
board as a guide. The LMH730275 is the evaluation board
supplied with samples of the LMH6739.
To reduce parasitic capacitances ground and power planes
should be removed near the input and output pins. Compo-
nents in the feedback loop should be placed as close to the
device as possible. 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 capacitors from each rail to ground are ap-
plied in pairs. The larger electrolytic bypass capacitors can be
located farther from the device, the smaller ceramic capaci-
tors should be placed as close to the device as possible. The
LMH6739 has multiple power and ground pins for enhanced
supply bypassing. Every pin should ideally have a separate
bypass capacitor. Sharing bypass capacitors may slightly de-
grade second order harmonic performance, especially if the
supply traces are thin and /or long. In Figure 1 and Figure 2
C_SS is optional, but is recommended for best second harmon-
ic distortion. Another option to using C_SS is to use pairs of .01
μF and .1 μF ceramic capacitors for each supply bypass.
VIDEO PERFORMANCE
The LMH6739 has been designed to provide excellent per-
formance with production quality video signals in a wide va-
riety of formats such as HDTV and High Resolution VGA.
NTSC and PAL performance is nearly flawless. Best perfor-
mance will be obtained with back terminated loads. The back
termination reduces reflections from the transmission line and
effectively masks transmission line and other parasitic ca-
pacitances from the amplifier output stage. Figure 4 shows a
typical configuration for driving a 75Ω Cable. The amplifier is
configured for a gain of two to make up for the 6 dB of loss in
R_OUT.
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FIGURE 8. Decoupling Capacitive Loads
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