LMH6738 Datasheet, PDF (12/20 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Op Amp

LMH6738
SNOSAC1E – APRIL 2004 – REVISED MARCH 2013
www.ti.com
High speed amplifiers are sensitive to capacitance between the inverting input and ground or power supplies.
This shows up as gain peaking at high frequency. The capacitor raises device gain at high frequencies by
making R_G appear smaller. Capacitive output loading will exaggerate this effect. In general, avoid introducing
unnecessary parasitic capacitance at both the inverting input and the output.
One possible remedy for this effect is to slightly increase the value of the feedback (and gain set) resistor. This
will tend to offset the high frequency gain peaking while leaving other parameters relatively unchanged. If the
device has a capacitive load as well as inverting input capacitance using a series output resistor as described in
DRIVING CAPACITIVE LOADS will help.
LAYOUT CONSIDERATIONS
Whenever questions about layout arise, use the evaluation board as a guide. The LMH730275 is the evaluation
board for the LMH6738.
To reduce parasitic capacitances ground and power planes should be removed near the input and output pins.
Components 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 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 possible. The LMH6738 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, especially if the
supply traces are thin and /or long. In Figure 22 and Figure 23 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 0.1 μF ceramic capacitors
for each supply bypass.
VIDEO PERFORMANCE
The LMH6738 has been designed to provide excellent performance 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 performance 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 25 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.
2
1.8
225 LFPM FORCED AIR
1.6
1.4
1.2
1
STILL AIR
0.8
0.6
0.4
0.2
0
-40 -20 0
20 40 60 80 100
TEMPERATURE (°C)
Figure 28. Maximum Power Dissipation
POWER DISSIPATION
The LMH6738 is optimized for maximum speed and performance in the small form factor of the standard SSOP-
16 package. To achieve its high level of performance, the LMH6738 consumes an appreciable amount of
quiescent current which cannot be neglected when considering the total package power dissipation limit. The
quiescent current contributes to about 40° C rise in junction temperature when no additional heat sink is used (V_S
= ±5V, all 3 channels on). Therefore, it is easy to see the need for proper precautions to be taken in order to
make sure the junction temperature’s absolute maximum rating of 150°C is not violated.
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