Datasheet
OUTPUT MODE OF OPERATION:
THS7365
www.ti.com
.................................................................................................................................................................................................. SBOS467 – MARCH 2009
Macrovision
®
, the CVBS signal can reach up to For 720p, 1080i, and 1080p signals, or for the the
1.23 V
PP
at the input, or 2.46 V
PP
at the output of the 480i, 576i, 480p, and 576p signals with sync present
THS7365. In contrast, other signals are typically on the P ’
B
and P ’
R
channels, the lowest voltage of the
1 V
PP
or 0.7 V
PP
at the input which translate to an sync is – 300 mV below the midpoint reference
output voltage of 2 V
PP
or 1.4 V
PP
. The output bias voltage of 0 V. The P ’
B
and P ’
R
signals allow a signal
voltage must account for a worst-case situation, to be as low as – 350 mV below the midpoint
depending on the signals involved. reference voltage of 0 V. This allowance corresponds
to 100% yellow for P ’
B
signal or 100% cyan for P ’
R
One other issue that must be taken into account is
signal . Because the P ’
B
and P ’
R
signal voltage can
the dc-bias point is a function of the power supply. As
be lower than the sync voltage, there exists a
such, there is an impact on system PSRR. To help
potential for clipping of the signal for a short period of
reduce this impact, the input capacitor combines with
time if the signals drop below the sync voltage.
the pull-up resistance to function as a low-pass filter.
Additionally, the time to charge the capacitor to the The THS7365 does include a 150-mV input level
final dc bias point is a function of the pull-up resistor shift, or 300 mV at the output, that should mitigate
and the input capacitor size. Lastly, the input any clipping issues. For example, if a STC is used,
capacitor forms a high-pass filter with the parallel then the bottom of the sync is 300 mV at the output.
impedance of the pull-up resistor and the 800-k Ω If the signal does go the lowest level, or 50 mV lower
resistor. In general, it is good to have this high-pass than the sync at the input, then the instantaneous
filter at approximately 3 Hz to minimize any potential output is ( – 50 mV + 150 mV) × 2 = 200 mV at the
droop on a P ’
B
, P ’
R
, or non-sync B ’ or R ’ signal. A output.
0.1- µ F input capacitor with a 3.3-M Ω pull-up resistor
Another potential risk is that if this signal (100%
equates to approximately a 2.5-Hz high-pass corner
yellow for P ’
B
or 100% cyan for P ’
R
) exists for several
frequency.
pixels, then the STC circuit engages to raise the
This mode of operation is recommended for use with voltage back to 0 V at the input. This function can
chroma (C ’ ), P ’
B
, P ’
R
, U ’ , V ’ , and non-sync R'G'B' cause a 50-mV level shift at the input midway through
signals. This method can also be used with sync the active video signal. This effect is undesirable and
signals if desired. The benefit of using the STC can cause errors in the decoding of the signal.
function over the ac-bias configuration on embedded
It is therefore recommended to use ac bias mode for
sync signals is that the STC maintains a constant
component P ’
B
and P ’
R
signals when ac-coupling is
back-porch voltage as opposed to a back-porch
desired.
voltage that fluctuates depending on the video
content. Because the high-pass corner frequency is a
very low 2.5 Hz, the impact on the video signal is
negligible relative to the STC configuration.
DC-COUPLED
One question may arise over the P ’
B
and P ’
R The THS7365 incorporates a rail-to-rail output stage
channels. For 480i, 576i, 480p, and 576p signals, a
that can be used to drive the line directly without the
sync may or may not be present. If no sync exists
need for large ac-coupling capacitors. This design
within the signal, then it is obvious that ac-bias is the
offers the best line tilt and field tilt (droop)
preferred method of ac-coupling the signal.
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
THS7365 is designed to drive two video lines
simultaneously — essentially, a 75- Ω load — while
keeping the output dynamic range as wide as
possible. Figure 120 shows the THS7365 driving two
video lines while keeping the output dc-coupled.
Copyright © 2009, Texas Instruments Incorporated Submit Documentation Feedback 37
Product Folder Link(s): THS7365