THS7376_14 Datasheet, PDF (47/62 Pages) Texas Instruments – 4-Channel Video Amplifier with One SD and Three HD 8th-Order Filters with 6-dB Gain

THS7376
www.ti.com
SBOS692 – JUNE 2013
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 field tilt associated with ac-coupling as described previously. AC-coupling is
performed for several reasons, but the most common reason 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 in the DC-Coupled Output Mode of Operation , 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
device output to minimize capacitive loading effects. If two lines are to be driven, having each line use its own
capacitor and resistor rather than sharing these components is best. 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, TI recommends (but does not require) placing 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. 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 using the same capacitor values for all video lines is a common practice, 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 can be smaller, such as 0.1 μF.
Figure 135 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 device is used as an input device.
CVBS
R
Y’, G’
R
P’^B, B’
R
P’^R, R’
R
0.1 mF⁽¹⁾
0.1 mF⁽¹⁾
+3.3 V
0.1 mF⁽¹⁾ 3.3 MW
+3.3 V
0.1 mF^{(1) 3.3 MW}
1 CVBS IN
CVBS OUT 14
2 HD CH1 IN ^{HD CH1 OUT} 13
3 HD CH2 IN HD CH2 OUT 12
4 ^{HD CH3 IN HD CH3 OUT} 11
5 ^GND
VS+ ₁₀
6 ^DISABLE HD BYPASS 9
7 ^NC
NC ₈
75 W
330 mF⁽²⁾
75 W
330 mF⁽²⁾
75 W
330 mF⁽²⁾
75 W
330 mF⁽²⁾
CVBS
Out
Y’, G’
Out
P’_B, B’
Out
P’_R, R’
Out
To GPIO
+3 V to 5 V
Controller Or GND
(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 device. 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 135. Typical AC Input System Driving AC-Coupled Video Lines
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