Datasheet
OUTPUT MODE OF OPERATION:
THS7365
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.................................................................................................................................................................................................. SBOS467 – MARCH 2009
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 THS7365 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-770 stipulates that the back-porch capacitor and resistor rather than sharing these
shall be 0 V ± 1 V as measured at the receiver. With components. This configuration helps ensure
a double-terminated load system, this requirement line-to-line dc isolation and eliminates the potential
implies a 0 V ± 2 V level at the video amplifier output. problems as described previously. Using a single,
The THS7365 can easily meet this requirement 1000- µ F capacitor for two lines is permissible, but
without issue. However, in Japan, the EIAJ CP-1203 there is a chance for interference between the two
specification stipulates a 0 V ± 0.1 V level with no receivers.
signal. This requirement can be met with the
Lastly, because of the edge rates and frequencies of
THS7365 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- µ F to 0.01- µ F capacitor in parallel with
ac-coupling. AC-coupling the output essentially
the large 220- µ F to 1000- µ F 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
rather large as a result of the associated inductances
AC-COUPLED
involved with the leads and construction. The small
0.1- µ F to 0.01- µ F 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 µ F and 1000 µ F, although 470
µ F is very typical. The value of this capacitor must be
Although it is common to use the same capacitor
large enough to minimize the line tilt (droop) and/or
values for all the video lines, the frequency bandwidth
field tilt associated with ac-coupling as described
of the chroma signal in a S-Video system is not
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 µ F.
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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