THS7320 Datasheet, PDF (22/35 Pages) Texas Instruments – 3-Channel ED Filter Video Amplifier with 4-V/V Gain

THS7320
SBOS565B – JULY 2011 – REVISED SEPTEMBER 2012
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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'_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. 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 OPERATION
The THS7320 allows for dc-coupled inputs. Most DACs or video encoders can be dc-connected to the THS7320
with essentially any DAC termination resistance desired for the system. One of the potential drawbacks to dc-
coupling is when 0 V is applied to the input from the DAC. Although the input of the THS7320 allows for a 0-V
input signal without issue, the output swing of a traditional amplifier cannot yield a 0-V signal that results in
possible clipping of the signal. 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 may use the chroma burst amplitude for amplitude control. For this situation, 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 that may not be
desirable.
To eliminate saturation or clipping problems, the THS7320 has a 150-mV output level shift feature. This feature
takes the input voltage and adds an internal +37.5-mV shift to the input signal. Because of the 12-dB (4-V/V)
gain, the resulting output with a 0-V applied input signal is approximately 150 mV. The THS7320 rail-to-rail output
stage can create this output level while connected to a typical video load. This configuration ensures the sync
signal clipping or saturation does not occur. This shift is constant, regardless of the input signal. The equation for
this level shift is V_OUT = (V_IN × 4 V/V) + 0.15 V. For example, if a 0.5-V input is applied, the output is (0.5 V × 4
V/V) + 0.15 V = 2.15 V.
Because the internal gain is fixed at +12 dB (4 V/V), it dictates the allowable linear input voltage range. 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 ([2.8 V – 0.15 V]/4) = 0.66 V. This range is valid
for up to the maximum recommended 5-V power supply that allows approximately a ([4.8 V – 0.15 V]/4) = 1.16-V
input range while avoiding clipping on the output.
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