THS7373_10 Datasheet, PDF (37/50 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
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One concern of dc-coupling arises, however, if the
line is terminated to ground. If the ac-bias input
configuration is used, the THS7373 output has a dc
bias. With two lines terminated to ground, this
configuration creates a dc current path that results in
a slightly decreased high output voltage swing and an
increase in power dissipation of the THS7373. While
the THS7373 was designed to operate with a junction
temperature of up to +125°C, care must be taken to
ensure that the junction temperature does not exceed
this level or else long-term reliability could suffer.
If the ac bias places 1.6 V on the output with two
dc-coupled lines connected, then the output current
flow without a signal is (1.6 V/75 Ω) = 21.3 mA per
channel. With a 3.3-V supply, the power dissipation
adds approximately [(3.3 V – 1.6 V) × 21.3 mA] =
36.2 mW per channel. With a 5-V power supply, this
increases to 72.4 mW per channel. The overall low
power dissipation of the THS7373 design minimizes
potential thermal issues even when using the TSSOP
package at high ambient temperatures. However,
power and thermal analysis should always be
examined in any system to ensure no issues arise.
Be sure to use RMS power rather than instantaneous
power when conducting thermal analysis.
Note that the THS7373 can drive the line with
dc-coupling regardless of the input mode of
operation. The only requirement is to make sure the
video line has proper termination in series with the
output (typically 75 Ω). This requirement helps isolate
capacitive loading effects from the THS7373 output.
Failure to properly isolate capacitive loads may result
in ringing or oscillations. The stray capacitance
appearing directly at the THS7373 output pins should
be kept below 20 pF for the 9.5-MHz filter channels
and below 15 pF for the 36-MHz filter channels. One
way to help ensure this condition is satisfied is to
make sure the 75-Ω source resistor is placed next to
each THS7373 output pin. If a large ac-coupling
capacitor is used, the capacitor should be placed
after this resistor.
There are many reasons dc-coupling is desirable,
including reduced system cost, PCB area, no line tilt,
and no field tilt. A common question is whether or not
there are any drawbacks to using dc-coupling. There
are some potential issues that must be examined,
such as the dc current bias as discussed above.
Another potential risk is whether this configuration
SBOS506 – DECEMBER 2009
meets industry standards. EIA-770 stipulates that the
back-porch shall be 0 V ± 1 V as measured at the
receiver. With a double-terminated load system, this
requirement implies a 0 V ± 2 V back-porch level at
the video amplifier output. The THS7373 can easily
meet this requirement without issue. However, in
Japan, the EIAJ CP-1203 specification stipulates a 0
V ± 0.1 V level with no video signal. This requirement
can be met with the THS7373 in shutdown mode, but
while active it cannot meet this specification without
output ac-coupling. AC-coupling the output essentially
ensures that the video signal works with any system
and any specification. For many modern systems,
however, dc-coupling can satisfy most needs.
OUTPUT MODE OF OPERATION:
AC-COUPLED
A very common method of coupling the video signal
to the line is with a large capacitor. This capacitor is
generally between 220 μF and 1000 μF, although
470 μF is very typical. The value of this capacitor
must be large enough to minimize the line tilt (droop)
and/or field tilt associated with ac-coupling as
described previously in this document. AC-coupling is
performed for several reasons, but the most common
is to ensure full interoperability with the receiving
video system. This approach ensures that regardless
of the reference dc voltage used on the transmitting
side, the receiving side re-establishes the dc
reference voltage to its own requirements.
In the same way as the dc output mode of operation
discussed previously, each line should have a 75-Ω
source termination resistor in series with the
ac-coupling capacitor. This resistor should be placed
next to the THS7373 output to minimize stray
capacitive effects. If two lines are to be driven, it is
best to have each line use its own capacitor and
resistor rather than sharing these components. This
configuration helps ensure line-to-line dc isolation and
eliminates the potential problems as described
previously. Using a single, 1000-μF capacitor for two
lines is permissible, but there is a chance for
interference between the two receivers along with the
capacitor potentially placing a capacitive load on the
THS7373 output.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS7373
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