THS7374 Datasheet, PDF (20/37 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 6-dB Gain

THS7374
SLOS590 – JULY 2008....................................................................................................................................................................................................... www.ti.com
THS7374 sources up to 3-mA of current to increase
the input voltage level on the THS7374 input side of
the coupling capacitor. As soon as the voltage goes
above the 0-V level, the loop stops sourcing current
and becomes very high impedance.
One of the concerns about the sync tip clamp level is
how the clamp reacts to a sync edge that has
overshoot—common in VCR signals or reflections
found in poor printed circuit board (PCB) layouts.
Ideally, the STC should not react to the overshoot
voltage of the input signal. Otherwise, this issue could
result in clipping on the rest of the video signal
because it may raise the bias voltage too much.
To help minimize this input signal overshoot problem,
the control loop in the THS7374 has an internal
low-pass filter as shown in Figure 38. This filter
reduces the response time of the STC circuit. This
delay is a function of how far the voltage is below
ground, but in general it is about an 800-ns delay.
This filter slows down the response of the control loop
so as not to clamp on the input overshoot voltage, but
rather the flat portion of the sync signal.
Input
0.1 mF ^Input
Pin
+V_S
+V_S
STC LPF
g_m
800 kW
Internal
Circuitry
Level
Shift
Figure 38. Equivalent AC Sync Tip Clamp Input
Circuit
As a result of this delay, the sync may have an
apparent voltage shift. The amount of shift depends
on the amount of droop in the signal as dictated by
the input capacitor and the STC current flow.
Because the sync is primarily for timing purposes,
with synchronization occurring on the edge of the
sync signal, this shift is transparent in most systems.
While this feature may not fully eliminate overshoot
issues on the input signal in case of severe overshoot
and/or ringing, the STC system should help minimize
improper clamping levels. As an additional method to
help minimize this issue, an external capacitor (such
as 10 pF to 47 pF) to ground in parallel with the
external termination resistors can help filter overshoot
problems.
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 appearing 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 feature 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 THS7374
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 3 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, if the input signal goes above the 0-V input
level a problem arises. The problem is that the video
signal is always above this level and must not be
altered in any way. But 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 issue is often seen by looking
at the tilt (droop) of a constant luma signal being
applied and observing the resulting output level. The
associated change in luma level from the beginning
of the video line to the end of the video line is the
amount of line tilt (droop).
If the discharge current is very small, then 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 called 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-coupling and ac-coupling in the
same part, the THS7374 incorporates an 800-kΩ
resistor to ground. Although a true constant-current
sink is preferred over a resistor, there are significant
issues when the voltage is near ground. This
condition can cause the current sink transistor to
saturate and cause potential problems with the signal.
Also, this resistor is large enough to not impact a
dc-coupled DAC termination. For discharging an
ac-coupled source, Ohm’s Law is applied. If the video
signal is 1 V, then there is 1 V/800 kΩ = 1.25-µA of
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