Datasheet

THS7364
SBOS530 –AUGUST 2010 www.ti.com
There are many reasons dc-coupling is desirable, In the same way as the dc output mode of operation
including reduced costs, PCB area, and no line tilt. A discussed previously, each line should have a 75-Ω
common question is whether or not there are any source termination resistor in series with the
drawbacks to using dc-coupling. There are some ac-coupling capacitor. This 75-Ω resistor should be
potential issues that must be examined, such as the placed next to the THS7364 output to minimize
dc current bias as discussed above. Another potential capacitive loading effects. If two lines are to be
risk is whether this configuration meets industry driven, it is best to have each line use its own
standards. EIA/CEA-770 stipulates that the capacitor and resistor rather than sharing these
back-porch shall be 0 V ± 1 V as measured at the components. This configuration helps ensure
receiver. With a double-terminated load system, this line-to-line dc isolation and eliminates the potential
requirement implies a 0 V ± 2 V level at the video problems as described previously. Using a single,
amplifier output. The THS7364 can easily meet this 1000-mF capacitor for two lines is permissible, but
requirement without issue. However, in Japan, the there is a chance for interference between the two
EIAJ CP-1203 specification stipulates a 0 V ± 0.1 V receivers.
level with no signal. This requirement can be met with
Lastly, because of the edge rates and frequencies of
the THS7364 in shutdown mode, but while active it
operation, it is recommended (but not required) to
cannot meet this specification without output
place a 0.1-mF to 0.01-mF capacitor in parallel with
ac-coupling. AC-coupling the output essentially
the large 220-mF to 1000-mF capacitor. These large
ensures that the video signal works with any system
value capacitors are most commonly aluminum
and any specification. For many modern systems,
electrolytic. It is well-known that these capacitors
however, dc-coupling can satisfy most needs.
have significantly large equivalent series resistance
(ESR), and the impedance at high frequencies is
OUTPUT MODE OF OPERATION:
rather large as a result of the associated inductances
AC-COUPLED
involved with the leads and construction. The small
0.1-mF to 0.01-mF capacitors help pass these
A very common method of coupling the video signal
high-frequency signals (greater than 1 MHz) with
to the line is with a large capacitor. This capacitor is
much lower impedance than the large capacitors.
typically between 220 mF and 1000 mF, although
470 mF is very typical. The value of this capacitor
Although it is common to use the same capacitor
must be large enough to minimize the line tilt (droop)
values for all the video lines, the frequency bandwidth
and/or field tilt associated with ac-coupling as
of the chroma signal in a S-Video system is not
described previously in this document. AC-coupling is
required to go as low (or as high of a frequency) as
performed for several reasons, but the most common
the luma channels. Thus, the capacitor values of the
is to ensure full interoperability with the receiving
chroma line(s) can be smaller, such as 0.1 mF.
video system. This approach ensures that regardless
of the reference dc voltage used on the transmitting Figure 82 shows a typical configuration where the
side, the receiving side re-establishes the dc input is ac-coupled and the output is also ac-coupled.
reference voltage to its own requirements. 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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