THS7368_14 Datasheet, PDF (27/41 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Filters and 6-dB Gain

THS7368
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channels. One way to help ensure this condition is
satisfied is to make sure the 75-Ω source resistor is
placed next to each THS7368 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 costs, PCB area, and no line 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 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 level at the video amplifier output.
The THS7368 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
signal. This requirement can be met with the
THS7368 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
typically 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.
SBOS497 – DECEMBER 2009
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 75-Ω resistor should be
placed next to the THS7368 output to minimize
capacitive loading 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.
Lastly, because of the edge rates and frequencies of
operation, it is recommended (but not required) to
place a 0.1-μF to 0.01-μF capacitor in parallel with
the large 220-μF to 1000-μF capacitor. These large
value capacitors are most commonly aluminum
electrolytic. It is well-known that these capacitors
have significantly large equivalent series resistance
(ESR), and the impedance at high frequencies is
rather large as a result of the associated inductances
involved with the leads and construction. The small
0.1-μF to 0.01-μF capacitors help pass these
high-frequency signals (greater than 1 MHz) with
much lower impedance than the large capacitors.
Although it is common to use the same capacitor
values for all the video lines, the frequency bandwidth
of the chroma signal in a S-Video system is not
required to go as low (or as high of a frequency) as
the luma channels. Thus, the capacitor values of the
chroma line(s) can be smaller, such as 0.1 μF.
Figure 42 shows a typical configuration where the
input is ac-coupled and the output is also ac-coupled.
AC-coupled inputs are generally required when
current-sink DACs are used or the input is connected
to an unknown source, such as when the THS7368 is
used as an input device.
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