LMH6739 Datasheet, PDF (8/10 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Video Buffer

Application Section (Continued)
UNITY GAIN COMPENSATION
With a current feedback PGB like the LMH6739, the feed-
back 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
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FIGURE 8. Decoupling Capacitive Loads
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
output 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
compromise between settling time and bandwidth. For appli-
cations 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 imped-
ance 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
applied in pairs. The larger electrolytic bypass capacitors
can be located farther from the device, the smaller ceramic
capacitors should be placed as close to the device as pos-
sible. 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 degrade second order harmonic performance, espe-
cially 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 harmonic 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
variety 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
capacitances from the amplifier output stage. Figure 4
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