LMH6733 Datasheet, PDF (13/16 Pages) Texas Instruments – Single Supply, 1.0 GHz, Triple Operational Amplifier

FIGURE 6. AC Coupled Single Supply Video Amplifier
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AC-COUPLED VIDEO
The LMH6733 can be used as an AC-coupled single supply
video amplifier for driving 75Ω coax with a gain of 2. The input
signal is nominally 0.7V or 1.0V for component YP_RP_B and
RGB, depending on the presence of a sync. R₁, R₂, and R₃
simply set the input to the center of the input linear range while
C_IN AC couples the video onto the op amp’s input.
As can be seen in Figure 6, amplifier U1 is used in a positive
gain configuration set for a closed loop gain of 2. The feed-
back resistor R_F is 340Ω. The gain resistor is created from the
parallel combination of R_G and R₄, giving a Thevenin equiv-
alent of 340Ω connected to 2.5V.
The 75Ω back termination resistor R_O divides the signal such
that V_OUT equals a buffered version of V_IN. The back termi-
nation will eliminate any reflection of the signal that comes
from the load. The input termination resistor, R_T, is optional –
it is used only if matching of the incoming line is necessary.
In some applications, it is recommended that a small valued
ceramic capacitor be used in parallel with C_O which is itself
electrolytic because of its rather large value. The ceramic cap
will tend to shunt the inductive behavior of this electrolytic cap,
C_O, at higher frequencies for an improved overall, low-
impedance output.
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was
not intentionally added. It comes about from electrical inter-
action between conductors. Parasitic capacitance can be
reduced but never entirely eliminated. Most parasitic capaci-
tances that cause problems are related to board layout or lack
of termination on transmission lines. Please see the section
on Layout Considerations for hints on reducing problems due
to parasitic capacitances on board traces. Transmission lines
should be terminated in their characteristic impedance at both
ends.
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 gen-
eral, 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 capaci-
tive load as well as inverting input capacitance using a series
output resistor as described in the section on “Driving Capac-
itive Loads” will help.
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 LMH6733.
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
LMH6733 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 LMH6733 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
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