THS7373_10 Datasheet, PDF (33/50 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
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INPUT MODE OF OPERATION: AC SYNC TIP
CLAMP (STC)
Some video DACs or encoders are not referenced to
ground but rather to the positive power supply. The
resulting video signals are generally at too great a
voltage for a dc-coupled video buffer to function
properly. In other systems, the inputs may be
connecting to an unknown source with unknown dc
reference levels. To account for this scenario, the
THS7373 incorporates a sync-tip clamp circuit. This
function requires a capacitor (nominally 0.1 μF) to be
in series with the input pin. Although the term
sync-tip-clamp is used throughout this document, it
should be noted that the THS7373 would probably be
better termed as a dc restoration circuit based on
how this function is performed. This circuit is an
active clamp circuit and not a passive diode clamp
function.
The input to the THS7373 has an internal control loop
that sets the lowest input applied voltage to clamp at
ground (0 V). By setting the reference at 0 V, the
THS7373 allows a dc-coupled input to also function.
Therefore, the sync-tip-clamp (STC) is considered
transparent because it does not operate unless the
input signal goes below ground. The signal then goes
through the same 150-mV level shifter, resulting in an
output voltage low level of 300 mV. If the input signal
tries to go below 0 V, the internal control loop of the
STC sources up to 6 mA of current to increase the
input voltage level on the THS7373 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 response
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 THS7373 has an internal
low-pass filter, as shown in Figure 100. 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 approximately an 800-ns
delay for the 9.5-MHz filter and approximately a
250-ns delay for the 36-MHz filters. The effect of this
filter is to slow 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.
SBOS506 – DECEMBER 2009
As a result of this delay, 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 sync is
used primarily for timing purposes with syncing
occurring on the edge of the sync signal, this shift is
transparent in most systems.
Input
0.1 mF ^Input
Pin
+V_S
+V_S
STC LPF
g_m
800 kW
Internal
Circuitry
Level
Shift
Figure 100. Equivalent AC Sync-Tip-Clamp Input
Circuit
While this feature may not fully eliminate overshoot
issues on the input signal, in cases of extreme
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 (for example, 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 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 THS7373
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.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS7373
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