THS7315_14 Datasheet, PDF (9/28 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier with 5th-Order Filters and 5.2-V/V Gain

THS7315
www.ti.com
SLOS532 – JUNE 2007
APPLICATION INFORMATION (continued)
Note that the Y' term is used for the luma channels throughout this document rather than the more common
luminance (Y) term. The reason for this usage is to account for the definition of luminance as stipulated by the
CIE (International Commission on Illumination). 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'). Therefore, true luminance (Y) is not maintained, and thus a difference in
terminology arises.
This rationale is also used for the chroma (C') term. Chroma is derived from the nonlinear R'G'B' terms and
therefore it is also nonlinear. True chominance (C) is derived from linear RGB, and thus the difference between
chroma (C') and chrominance (C) exists. The color difference signals (P'_B/ P'_R/U'/V') are also referenced this way
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 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 approach is consistent with the Y'P'_BP'_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. Since 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, respectively. 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; this
configuration may not always be the case for all systems.
INPUT MODE OF OPERATION—DC
The THS7315 allows for both ac-coupled and dc-coupled inputs. Many DACs or video encoders can be
dc-connected to the THS7315. One of the drawbacks to dc-coupling, however, occurs when 0 V is applied to the
input. Although the THS7315 allows for a 0-V input signal with no issues, the output swing of a traditional
amplifier cannot yield a 0-V signal, resulting in possible clipping. This condition is true for any single-supply
amplifier because of the output transistor limitations. Both CMOS and bipolar transistors cannot go to 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 both the horizontal and vertical sync amplitudes on the video signal. A problem
occurs if the video signal receiver uses an 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 effect may result in a picture with an overly bright display and too much color saturation.
Other AGC circuits use the chroma burst amplitude for amplitude control, and a reduction in the sync signals
does not alter the proper gain setting. However, it is good engineering design practice to ensure that saturation
and/or 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 signal aberrations.
To eliminate saturation or clipping problems, the THS7315 has a 230 mV output level shift feature. This feature
takes the input voltage and adds an internal level shift to the signal. The THS7315 rail-to-rail output stage can
create this output level while connected to a typical video load. This process ensures that no saturation or
clipping of the sync signal occurs. This level shift is constant, regardless of the input signal. For example, if a
0.5-V input is applied, the output is at (0.5 V × 5.2 V/V) + 0.23 V = 2.92 V.
The fixed internal gain of 5.2 V/V (14.3 dB) dictates what the allowable linear input voltage range can be without
clipping concerns. For example, if the power supply is set to 3 V, the maximum output is about 2.9 V while
driving a significant amount of current. Thus, to avoid clipping, the allowable input will be [ (3.1 V – 0.23 V) / 5.2
V/V) ] = 0.55 V. This relationship holds true up to the maximum recommended 5 V power supply that allows an
approximate input range of [ (4.9 V – 0.23 V) / 5.2 V/V) ] = 0.9 V while avoiding clipping on the output.
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