THS7360 Datasheet, PDF (26/41 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Filters and High Gain

THS7360
SLOS674 – JUNE 2010
It should be noted that this STC system is dynamic
and does not rely upon timing in any way. It only
depends on the voltage that appears at the input pin
at any given point in time. The STC filtering helps
minimize level shift problems associated with
switching noises or very short spikes on the signal
line. This architecture helps ensure a very robust
STC system.
When the ac STC operation is used, there must also
be some finite amount of discharge bias current. As
previously described, if the input signal goes below
the 0-V clamp level, the internal loop of the THS7360
sources current to increase the voltage appearing at
the input pin. As the difference between the signal
level and the 0-V reference level increases, the
amount of source current increases
proportionally—supplying up to 6 mA of current.
Thus, the time to re-establish the proper STC voltage
can be very fast. If the difference is very small, then
the source current is also very small to account for
minor voltage droop.
However, what happens if the input signal goes
above the 0-V input level? The problem is that the
video signal is always above this level and must not
be altered in any way. As a result, 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 THS7360 incorporates an 800-kΩ resistor to
ground. Although a true constant current sink is
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
0.5 V, then there is 0.5 V/800 kΩ = 0.625-mA of
discharge current. If more hum rejection is desired or
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there is a loss of sync occurring, then simply
decrease the 0.1-mF input coupling capacitor. A
decrease from 0.1 mF to 0.047 mF 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 THS7360. 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
THS7360 input perspective is very high, then simply
adding a 1-kΩ resistor to GND ensures proper
operation of the THS7360.
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
THS7360. This function is easily accomplished with
the THS7360 by simply adding an external pull-up
resistor to the positive power supply, as shown in
Figure 46.
+3.3 V
Input
C_IN
0.1 mF
R_PU
Input
Pin
+3.3 V
800 kW
Internal
Circuitry
Level
Shift
Figure 46. 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)
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