THS7372 Datasheet, PDF (36/49 Pages) Texas Instruments – 4-Channel Video Amplifier with One CVBS and Three Full-HD Filters with 6-dB Gain

THS7372
SBOS578 – AUGUST 2011
www.ti.com
at the video amplifier output. The THS7372 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 THS7372 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 μF and 1000 μF, although 470 μF 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.
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
THS7372 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-μF
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-μF to 0.01-μF capacitor in parallel with the large 220-μF to 1000-μF 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-μF to 0.01-μF 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 μF.
Figure 89 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 THS7372 is used as an input device.
CVBS
Y'/G'
P’_B/B'
P’_R/R'
+V
0.1 mF⁽¹⁾
37.4 W
+V
+2.7 V to
0.1 mF⁽¹⁾
+5 V
37.4 W
+V
0.1 mF⁽¹⁾
37.4 W
+V
0.1 mF⁽¹⁾
THS7372
1 CVBS IN
2 NC
3 ^VS+
4 NC
5 FHD1 IN
6 FHD2 IN
7 FHD3 IN
CVBS OUT 14
DIS CVBS 13
GND 12
DIS FHD 11
FHD1 OUT 10
FHD2 OUT 9
FHD3 OUT 8
Disable
CVBS
Disable
FHD
37.4 W
R_PU
R_PU
75 W
CVBS Out
330 mF⁽²⁾
75 W
Y'/G’ Out
330 mF⁽²⁾
75 W
330 mF^{(2) P'B/B’ Out}
75 W
P'_R/R’ Out
330 mF⁽²⁾
75 W
75 W
75 W
75 W
+2.7 V to +5 V
(1) AC-coupled input is shown in this example. DC-coupling is also allowed as long as the DAC output voltage is within the allowable linear
input and output voltage range of the THS7372. To apply dc-coupling, remove the 0.1-μF input capacitors and the R_PU pull-up resistors.
(2) This example shows an ac-coupled output. DC-coupling is also allowed by simply removing these capacitors.
Figure 89. Typical AC Input System Driving AC-Coupled Video Lines
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