THS7373_10 Datasheet, PDF (35/50 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
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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 0.644-V example above, using an ideal
3.3-MΩ resistor, the input dc bias voltage is
approximately 0.644 V ± 0.1 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
THS7373. 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. 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 power-supply
rejection ratio (PSRR). To help reduce this impact,
the input capacitor combines with the pull-up
resistance to function as a low-pass filter.
SBOS506 – DECEMBER 2009
Additionally, the time to charge the capacitor to the
final dc bias point is a function of the pull-up resistor
and the input capacitor. 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-μF input capacitor with a
3.3-MΩ pull-up resistor equates to approximately a
2.5-Hz high-pass corner frequency.
AC biasing is recommended for use with component
video P’_B, P’_R, U’, or V’ signals because these signals
either have no embedded sync or the sync is a
mid-level sync rather than a bottom-level sync. This
method can also be used with sync signals if desired.
The benefit of using the STC function is that it
maintains a constant back-porch voltage as opposed
to a back-porch voltage that fluctuates depending on
the video content. Because the input corner
frequency is a very low 2.5 Hz, the input corner
frequency is also a very low 2.5 Hz, which is
respectable (relative to a STC configuration).
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS7373
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