Datasheet

THS7360
www.ti.com SLOS674 –JUNE 2010
filter channels. One way to help ensure this condition In the same way as the dc output mode of operation
is satisfied is to make sure the 75-Ω source resistor is discussed previously, each line should have a 75-Ω
placed within 0.5-inches to each THS7360 output pin. source termination resistor in series with the
If a large ac-coupling capacitor is used, the capacitor ac-coupling capacitor. This 75-Ω resistor should be
should be placed after this resistor. placed next to the THS7360 output to minimize
capacitive loading effects. If two lines are to be
There are many reasons dc-coupling is desirable,
driven, it is best to have each line use its own
including reduced costs, PCB area, and no line tilt. A
capacitor and resistor rather than sharing these
common question is whether or not there are any
components. This configuration helps ensure
drawbacks to using dc-coupling. There are some
line-to-line dc isolation and eliminates the potential
potential issues that must be examined, such as the
problems as described previously. Using a single,
dc current bias as discussed above. Another potential
1000-mF capacitor for two lines is permissible, but
risk is whether this configuration meets industry
there is a chance for interference between the two
standards. EIA/CEA-770 stipulates that the
receivers.
back-porch shall be 0 V ± 1 V as measured at the
receiver. With a double-terminated load system, this Lastly, because of the edge rates and frequencies of
requirement implies a 0-V ± 2-V level at the video operation, it is recommended (but not required) to
amplifier output. The THS7360 can easily meet this place a 0.1-mF to 0.01-mF capacitor in parallel with
requirement without issue. However, in Japan, the the large 220-mF to 1000-mF capacitor. These large
EIAJ CP-1203 specification stipulates a 0-V ± 0.1-V value capacitors are most commonly aluminum
level with no signal. This requirement can be met with electrolytic. It is well-known that these capacitors
the THS7360 in shutdown mode, but while active it have significantly large equivalent series resistance
cannot meet this specification without output (ESR), and the impedance at high frequencies is
ac-coupling. AC-coupling the output essentially rather large as a result of the associated inductances
ensures that the video signal works with any system involved with the leads and construction. The small
and any specification. For many modern systems, 0.1-mF to 0.01-mF capacitors help pass these
however, dc-coupling can satisfy most needs. high-frequency signals (greater than 1 MHz) with
much lower impedance than the large capacitors.
OUTPUT MODE OF OPERATION:
Although it is common to use the same capacitor
AC-COUPLED
values for all the video lines, the frequency bandwidth
of the chroma signal in a S-Video system is not
A very common method of coupling the video signal
required to go as low (or as high of a frequency) as
to the line is with a large capacitor. This capacitor is
the luma channels. Thus, the capacitor values of the
typically between 220 mF and 1000 mF, although
chroma line(s) can be smaller, such as 0.1 mF.
470 mF is very typical. The value of this capacitor
must be large enough to minimize the line tilt (droop)
Figure 48 shows a typical configuration where the
and/or field tilt associated with ac-coupling as
input is ac-coupled and the output is also ac-coupled.
described previously in this document. AC-coupling is
AC-coupled inputs are generally required when
performed for several reasons, but the most common
current-sink DACs are used or the input is connected
is to ensure full interoperability with the receiving
to an unknown source, such as when the THS7360 is
video system. This approach ensures that regardless
used as an input device.
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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