THS7373 Datasheet, PDF (34/49 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
SBOS506 – DECEMBER 2009
However, what happens if the input signal goes
above the 0-V input level? The problem is the video
signal is always above this level and must not be
altered in any way. Thus, if the sync level of the input
signal is above this 0-V level, then the internal
discharge (sink) current reduces the ac-coupled bias
signal to the proper 0-V level.
This discharge current must not be large enough to
alter the video signal appreciably or picture quality
issues may arise. This effect is often seen by looking
at the tilt (droop) of a constant luma signal being
applied and the resulting output level. The associated
change in luma level from the beginning and end of
the video line is the amount of line tilt (droop).
If the discharge current is very small, the amount of
tilt is very low, which is a generally a good thing.
However, the amount of time for the system to
capture the sync signal could be too long. This effect
is also termed hum rejection. Hum arises from the ac
line voltage frequency of 50 Hz or 60 Hz. The value
of the discharge current and the ac-coupling capacitor
combine to dictate the hum rejection and the amount
of line tilt.
To allow for both dc- and ac-coupling in the same
part, the THS7373 incorporates an 800-kΩ resistor to
ground. Although a true constant current sink is
generally preferred over a resistor, there can be
issues when the voltage is near ground. This
configuration can cause the current sink transistor to
saturate and cause potential problems with the signal.
The 800-kΩ resistor is large enough to not impact a
dc-coupled DAC termination. For discharging an
ac-coupled source, Ohm’s Law is used. If the video
signal is 1 V, then there is 1 V/800 kΩ = 1.25 μA of
discharge current. If more hum rejection is desired or
if a loss of sync occurs, then simply decrease the
0.1-μF input coupling capacitor. A decrease from
0.1 μF to 0.047 μF increases the hum rejection by a
factor of 2.1. Alternatively, an external pull-down
resistor to ground may be added that decreases the
overall resistance and ultimately increases the
discharge current.
To ensure proper stability of the ac STC control loop,
the source impedance must be less than 1 kΩ with
the input capacitor in place. Otherwise, there is a
possibility of the control loop ringing, which may
appear on the output of the THS7373. Because most
DACs or encoders use resistors to establish the
voltage, which are typically less than 300 Ω, meeting
the less than 1 kΩ requirement is easily done.
However, if the source impedance looking from the
THS7373 input perspective is very high, then simply
adding a 1-kΩ resistor to GND ensures proper
operation of the THS7373.
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The ac STC function is not recommended for
ac-coupled component video P’_B/P’_R/U’/V’ signals.
These signals either have no embedded sync or they
have a mid-level sync. Using STC on these signals
can cause clipping, saturation, or an apparent voltage
shift in some video signals, such as 100% yellow for
a few pixels in a video frame. For these signals and
ac-input coupling, using the ac-bias mode is
recommended.
INPUT MODE OF OPERATION: AC BIAS
Sync-tip clamps work very well for signals that have
horizontal and/or vertical syncs associated with them;
however, some video signals do not have a sync
embedded within the signal. If ac-coupling of these
signals is desired, then a dc bias is required to
properly set the dc operating point within the
THS7373. This function is easily accomplished with
the THS7373 by simply adding an external pull-up
resistor to the positive power supply, as shown in
Figure 101.
V_S+
V_S+
Input
C_IN
0.1 mF
R_PU
Input
Pin
800 kW
Internal
Circuitry
Level
Shift
Figure 101. AC-Bias Input Mode Circuit
Configuration
The dc voltage appearing at the input pin is equal to
Equation 1:
V_DC = V_S
800 kW
800 kW + R_PU
(1)
The THS7373 allowable input range is approximately
0 V to (V_S+ – 1.5 V), allowing for a very wide input
voltage range. As such, the input dc bias point is very
flexible, with the output dc bias point being the
primary factor. For example, if the output dc bias
point is desired to be 1.6 V on a 3.3-V supply, then
the input dc bias point should be (1.6 V – 300 mV)/2
= 0.65 V. Thus, the pull-up resistor calculates to
approximately 3.3 MΩ, resulting in 0.644 V. If the
output dc-bias point is desired to be 1.6 V with a 5-V
power supply, then the pull-up resistor calculates to
approximately 5.36 MΩ.
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