THS7365 Datasheet, PDF (34/53 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7365
SBOS467 – MARCH 2009.................................................................................................................................................................................................. 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 THS7365 allow for both ac- and
dc-coupled inputs. Many DACs or video encoders can
be dc-connected to the THS7365. One of the
drawbacks to dc-coupling is when 0 V is applied to
the input. Although the input of the THS7365 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
THS7365 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 THS7365
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 THS7365 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.
The input impedance of the THS7365 in this mode of
operation is dictated by the internal, 800-kΩ
pull-down resistor, as shown in Figure 117. Note that
the internal voltage shift does not appear at the input
pin; it only shows at the output pin.
+V_S
Input
Pin
800 kW
Internal
Circuitry
Level
Shift
Figure 117. Equivalent DC Input Mode Circuit
34
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