THS7360_1 Datasheet, PDF (27/43 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Fiters and High Gain

THS7360
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The THS7360 allowable input range is approximately
0 V to (V_S+ – 1.5 V), allowing for a very wide input
voltage range. As such, the input dc bias point is very
flexible, with the output dc bias point being the
primary factor. For example, if the output dc bias
point on a SD channel is desired to be 1.6 V on a
3.3-V supply, then the input dc bias point should be
(1.6 V – 120 mV)/5.6 = 0.264 V. Thus, the pull-up
resistor calculates to approximately 9.31 MΩ,
resulting in 0.261 V. If the output dc-bias point is
desired to be 1.6 V with a 5-V power supply, then the
pull-up resistor calculates to approximately 14.3 MΩ.
Keep in mind that the internal 800-kΩ resistor has
approximately a ±20% variance. As such, the
calculations should take this variance into account.
For the 0.261-V example above, using an ideal
9.31-MΩ resistor, the input dc bias voltage is
approximately 0.261 V ± 0.05 V.
The value of the output bias voltage is very flexible
and is left to each individual design. It is important to
ensure that the signal does not clip or saturate the
video signal. Thus, it is recommended to ensure the
output bias voltage is between 0.9 V and (V_S+ – 1 V).
For 100% color saturated CVBS or signals with
Macrovision^®, the CVBS signal can reach up to
1.23 V_PP at the input, or 2.46 V_PP at the output of the
THS7360. In contrast, other signals are typically
1 V_PP or 0.7 V_PP at the input, which translate to an
output voltage of 2 V_PP or 1.4 V_PP, respectively. The
output bias voltage must account for a worst-case
situation, depending on the signals involved.
One other issue that must be taken into account is
the dc-bias point is a function of the power supply. As
such, there is an impact on system PSRR. To help
reduce this impact, the input capacitor combines with
the pull-up resistance to function as a low-pass filter.
Additionally, the time to charge the capacitor to the
final dc bias point is a function of the pull-up resistor
SLOS674 – JUNE 2010
and the input capacitor size. Lastly, the input
capacitor forms a high-pass filter with the parallel
impedance of the pull-up resistor and the 800-kΩ
resistor. In general, it is good to have this high-pass
filter at approximately 3 Hz to minimize any potential
droop on a P’_B or P’_R signal. A 0.1-mF input capacitor
with a 9.31-MΩ pull-up resistor equates to
approximately a 2.2-Hz high-pass corner frequency.
This mode of operation is recommended for use with
chroma (C’), P’_B, P’_R, U’, and V’ signals. This method
can also be used with sync signals if desired. The
benefit of using the STC function over the ac-bias
configuration on embedded sync signals is that the
STC maintains a constant back-porch voltage as
opposed to a back-porch voltage that fluctuates
depending on the video content. Because the
high-pass corner frequency is a very low 2.2 Hz, the
impact on the video signal is negligible relative to the
STC configuration.
OUTPUT MODE OF OPERATION:
DC-COUPLED
The THS7360 incorporates a rail-to-rail output stage
that can be used to drive the line directly without the
need for large ac-coupling capacitors. This design
offers the best line tilt and field tilt (droop)
performance because no ac-coupling occurs. Keep in
mind that if the input is ac-coupled, then the resulting
tilt as a result of the input ac-coupling continues to be
seen on the output, regardless of the output coupling.
The 80-mA output current drive capability of the
THS7360 is designed to drive two video lines
simultaneously—essentially, a 75-Ω load—while
keeping the output dynamic range as wide as
possible. Figure 47 shows the THS7360 driving two
video lines while keeping the output dc-coupled.
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): THS7360
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