THS7360 Datasheet, PDF (24/41 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Filters and High Gain

THS7360
SLOS674 – JUNE 2010
Note that the Y’ term is used for the luma channels
throughout this document rather than the more
common luminance (Y) term. This usage accounts for
the definition of luminance as stipulated by the
International Commission on Illumination (CIE). Video
departs from true luminance because a nonlinear
term, gamma, is added to the true RGB signals to
form R’G’B’ signals. These R’G’B’ signals are then
used to mathematically create luma (Y’). Thus,
luminance (Y) is not maintained, providing a
difference in terminology.
This rationale is also used for the chroma (C’) term.
Chroma is derived from the nonlinear R’G’B’ terms
and, thus, it is nonlinear. Chominance (C) is derived
from linear RGB, giving the difference between
chroma (C’) and chrominance (C). The color
difference signals (P’_B/P’_R/U’/V’) are also referenced
in this manner to denote the nonlinear (gamma
corrected) signals.
R’G’B’ (commonly mislabeled RGB) is also called
G’B’R’ (again commonly mislabeled as GBR) in
professional video systems. The Society of Motion
Picture and Television Engineers (SMPTE)
component standard stipulates that the luma
information is placed on the first channel, the blue
color difference is placed on the second channel, and
the red color difference signal is placed on the third
channel. This practice is consistent with the Y'/P'_B/P'_R
nomenclature. Because the luma channel (Y') carries
the sync information and the green channel (G') also
carries the sync information, it makes logical sense
that G' be placed first in the system. Because the
blue color difference channel (P'_B) is next and the red
color difference channel (P'_R) is last, then it also
makes logical sense to place the B' signal on the
second channel and the R' signal on the third
channel, respectfully. Thus, hardware compatibility is
better achieved when using G'B'R' rather than R'G'B'.
Note that for many G'B'R' systems, sync is embedded
on all three channels, but this configuration may not
always be the case in all systems.
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INPUT MODE OF OPERATION: DC
The inputs to the THS7360 allow for both ac- and
dc-coupled inputs. Many DACs or video encoders can
be dc-connected to the THS7360. One of the
drawbacks to dc-coupling is when 0 V is applied to
the input. Although the input of the THS7360 allows
for a 0-V input signal without issue, the output swing
of a traditional amplifier cannot yield a 0-V signal,
resulting in possible clipping. This limitation is true for
any single-supply amplifier because of the
characteristics of the output transistors. Neither
CMOS nor bipolar transistors can achieve 0 V while
sinking current. This transistor characteristic is also
the same reason why the highest output voltage is
always less than the power-supply voltage when
sourcing current.
This output clipping can reduce the sync amplitudes
(both horizontal and vertical sync) on the video
signal. A problem occurs if the video signal receiver
uses an automatic gain control (AGC) loop to account
for losses in the transmission line. Some video AGC
circuits derive gain from the horizontal sync
amplitude. If clipping occurs on the sync amplitude,
then the AGC circuit can increase the gain too
much—resulting in too much luma and/or chroma
amplitude gain correction. This correction may result
in a picture with an overly bright display with too
much color saturation.
Other AGC circuits use the chroma burst amplitude
for amplitude control; reduction in the sync signals
does not alter the proper gain setting. However, it is
good engineering design practice to ensure that
saturation/clipping does not take place. Transistors
always take a finite amount of time to come out of
saturation. This saturation could possibly result in
timing delays or other aberrations on the signals.
To eliminate saturation or clipping problems, the
THS7360 has an input level shift feature. The
resulting output with a 0-V applied input signal is
approximately 120 mV. The THS7360 rail-to-rail
output stage can create this output level while
connected to a typical video load. This configuration
ensures that no saturation or clipping of the sync
signals occur. This shift is constant, regardless of the
input signal. For example, if a 0.1-V input is applied
to the SD channel, the output is 0.1V X 5.6 V/V +
0.12V = 0.68 V.
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