THS7374_16 Datasheet, PDF (25/42 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 6-dB Gain

THS7374
www.ti.com
OUTPUT MODE OF OPERATION: AC
COUPLED
A very common method of coupling the video signal
to the line is the use of a large capacitor. This
capacitor is typically between 220 mF and 1000 mF,
although 470 mF is very common. 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 done for several reasons, but most often
to ensure full interoperability with the receiving video
system. AC coupling also ensures adherence to video
standard specifications. It ensures that regardless of
the reference dc voltage used on the transmit side,
the receive side re-establishes the dc reference
voltage to its own requirements.
As with 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. 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, as shown in
Figure 62.
SLOS590B – JULY 2008 – REVISED JANUARY 2011
This configuration helps ensure line-to-line dc
isolation and avoids the potential problems discussed
above. Using a single 1000-mF capacitor for two lines
can be done, but there is a chance for ground loops
and interference to be created 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-mF to 0.01-mF capacitor in parallel with
the large 220-mF to 1000-mF capacitor. These
large-value capacitors are generally 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 because of the associated inductances
involved with the leads and construction. The small
0.1-mF to 0.01-mF capacitors help pass these
high-frequency (> 1-MHz) signals 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 are 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 mF.
+3.3 V
CVBS
DAC/Encoder
SDTV
CVBS
S-Video
Y'P'BP'R
R'G'B'
Y'/G'
P'_B/B'
P'_R/R'
0.1 mF⁽¹⁾
R
0.1 mF⁽¹⁾
R
+3.3 V
0.1 mF⁽¹⁾
R_PU
R
+3.3 V
0.1 mF⁽¹⁾
R_PU
R
THS7374
1 CH1 IN CH1 OUT 14
2 CH2 IN CH2 OUT 13
3 CH3 IN CH3 OUT 12
4 CH4 IN CH4 OUT 11
5 GND
V_S+ 10
6 DISABLE BYPASS 9
7 NC
NC 8
To GPIO Controller
or GND
+3 V to +5 V
75 W ^{330 mF(2)}
CVBS Out 1
75 W ^{330 mF(2)}
75 W ^{330 mF(2)}
Y' Out 1
75 W ^{330 mF(2)}
75 W ^{330 mF(2)}
75 W ^{330 mF(2) P'B Out 1}
75 W ^{330 mF(2)}
75 W ^{330 mF(2) P'R Out 1}
CVBS/Sync
75 W
Y'/G' Out
75 W
P’_B/B' Out
75 W
P’_R/R' Out
75 W
75 W
75 W
75 W
75 W
(1) An 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 THS7374. To dc-couple, remove the 0.1mF input capacitors and R_PU.
(2) An ac-coupled output is shown in this example. DC coupling is also allowed by simply removing these capacitors.
Figure 62. Typical SDTV AC-Input System Driving Two AC-Coupled Video Lines
Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Link(s): THS7374
Submit Documentation Feedback
25