THS7364_10 Datasheet, PDF (34/47 Pages) Texas Instruments – 6-Channel Video Amplifier with 3 SD and 3 Full-HD Filters with 6-dB Gain

THS7364
SBOS530 – AUGUST 2010
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/CEA-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 THS7364 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 THS7364 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 mF and 1000 mF, although
470 mF 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.
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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 THS7364 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-mF 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-mF to 0.01-mF capacitor in parallel with
the large 220-mF to 1000-mF 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-mF to 0.01-mF 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 mF.
Figure 82 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 THS7364 is
used as an input device.
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