Datasheet

THS7373
www.ti.com SBOS506A –DECEMBER 2009–REVISED AUGUST 2012
Keep in mind that the internal 800-kΩ resistor has Additionally, the time to charge the capacitor to the
approximately a ±20% variance. As such, the final dc bias point is a function of the pull-up resistor
calculations should take this variance into account. and the input capacitor. Lastly, the input capacitor
For the 0.644-V example above, using an ideal 3.3- forms a high-pass filter with the parallel impedance of
MΩ resistor, the input dc bias voltage is the pull-up resistor and the 800-kΩ resistor. In
approximately 0.644 V ± 0.1 V. general, it is good to have this high-pass filter at
approximately 3 Hz to minimize any potential droop
The value of the output bias voltage is very flexible
on a P’
B
or P’
R
signal. A 0.1-μF input capacitor with a
and is left to each individual design. It is important to
3.3-MΩ pull-up resistor equates to approximately a
ensure that the signal does not clip or saturate the
2.5-Hz high-pass corner frequency.
video signal. Thus, it is recommended to ensure the
output bias voltage is between 0.9 V and (V
S+
– 1 V). AC biasing is recommended for use with component
For 100% color saturated CVBS or signals with video P’
B
, P’
R
, U’, or V’ signals because these signals
Macrovision
®
, the CVBS signal can reach up to either have no embedded sync or the sync is a mid-
1.23 V
PP
at the input, or 2.46 V
PP
at the output of the level sync rather than a bottom-level sync. This
THS7373. In contrast, other signals are typically method can also be used with sync signals if desired.
1 V
PP
or 0.7 V
PP
at the input which translate to an The benefit of using the STC function is that it
output voltage of 2 V
PP
or 1.4 V
PP
. The output bias maintains a constant back-porch voltage as opposed
voltage must account for a worst-case situation, to a back-porch voltage that fluctuates depending on
depending on the signals involved. the video content. Because the input corner
frequency is a very low 2.5 Hz, the input corner
One other issue that must be taken into account is
frequency is also a very low 2.5 Hz, which is
the dc-bias point is a function of the power supply. As
respectable (relative to a STC configuration).
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.
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