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OUTPUT MODE OF OPERATION—AC-COUPLED
75 W
+
75 W
+
75 W
+
75 W
+
Y’
OUT2
OUT2
OUT2
75 W
+
Y’
OUT1
OUT1
OUT1
75 W
75 W
75 W
75 W
75 W
75 W
75 W
+
5
8
7
6
1
2
3
4
Y’
P’
B
P’
R
+
R
R
+1.8V
R
+1.8V
+3.3V
+3.3V
9.31MW
9.31MW
0.1 Fm
0.1 Fm
0.1 Fm
0.1 Fm
22 Fm
V
S+
GND
CH.2IN
CH.3IN
CH.1IN
CH.3OUT
CH.2OUT
CH.1OUT
DAC/
Encoder
+3.3V
+1.8V
+1.8V
330 Fm
(1)
330 Fm
(1)
330 Fm
(1)
330 Fm
(1)
330 Fm
(1)
330 Fm
(1)
SDTV
480i/576i
Y’P’ P’
G’B’R’
B R
THS7315
P’
B
P’
B
P’
R
P’
R
THS7315
SLOS532 – JUNE 2007
APPLICATION INFORMATION (continued)
The most common method of coupling the video signal to the line is through the use of a large capacitor. This
capacitor is typically between 220 μ F and 1000 μ F, although 330 μ F is very common. The value of this capacitor
must be this large to minimize the line tilt (droop) and/or field tilt associated with ac coupling as discussed
previously in this document. AC coupling is done for several reasons; generally, it is done to ensure full
interoperability with the receiving video system. This coupling eliminates possible ground loops. It also ensures
that regardless of the reference dc voltage used on the transmit side, the receive side will re-establish the dc
reference voltage to its own requirements.
In the same way that the dc output mode of operation is configured (as discussed earlier), each line should have
a 75- Ω source termination resistor in series with the ac-coupling capacitor. If driving two lines, it is best to have
each line use its own capacitor and resistor rather than sharing these components, as Figure 9 shows. This
configuration helps ensure line-to-line dc isolation and avoids the potential problems discussed earlier. Using a
single 1000- μ F capacitor for two lines can be done, but there is a chance for ground loops and additional
interference to be created between the two receivers.
(1) As a result of the high frequency content of the video signal, it is recommended, but not required, to add a 0.1- μ F or
0.01- μ F capacitor in parallel with these large capators.
(2) Current sinking DAC / Encoder shown. See the application notes.
Figure 9. Typical 480i/576i Y'P'
B
P'
R
AC-Input System Driving Two AC-Coupled Video Lines
14
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