THS7372 Datasheet, PDF (30/49 Pages) Texas Instruments – 4-Channel Video Amplifier with One CVBS and Three Full-HD Filters with 6-dB Gain

THS7372
SBOS578 – AUGUST 2011
www.ti.com
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.
INPUT MODE OF OPERATION: DC
The inputs to the THS7372 allow for both ac- and dc-coupled inputs. Many DACs or video encoders can be
dc-connected to the THS7372. One of the drawbacks to dc-coupling arises when 0 V is applied to the input.
Although the input of the THS7372 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 THS7372 has a 150-mV input level shift feature. This feature
takes the input voltage and adds an internal +150-mV shift to the signal. Because the THS7372 also has a gain
of 6 dB (2 V/V), the resulting output with a 0-V applied input signal is approximately 300 mV. The THS7372
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 1-V input is applied, the output is 2.3 V.
Because the internal gain is fixed at +6 dB, the gain 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 approximately
2.9 V while driving a significant amount of current. Thus, to avoid clipping, the allowable input is ([2.9 V/2] – 0.15
V) = 1.3 V. This range is valid for up to the maximum recommended 5-V power supply that allows approximately
a ([4.9 V/2] – 0.15 V) = 2.3 V input range while avoiding clipping on the output.
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Copyright © 2011, Texas Instruments Incorporated