THS7372 Datasheet, PDF (31/49 Pages) Texas Instruments – 4-Channel Video Amplifier with One CVBS and Three Full-HD Filters with 6-dB Gain

THS7372
www.ti.com
SBOS578 – AUGUST 2011
The input impedance of the THS7372 in this mode of operation is dictated by the internal, 800-kΩ pull-down
resistor, as shown in Figure 85. Note that the internal voltage shift does not appear at the input pin; it only shows
at the output pin.
+V_S
Input
Pin
Internal
Circuitry
800 kW
Level
Shift
Figure 85. Equivalent DC Input Mode Circuit
INPUT MODE OF OPERATION: AC SYNC TIP CLAMP
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. To
account for this scenario, the THS7372 incorporates a sync-tip clamp circuit. This function requires a capacitor
(nominally 0.1 μF) to be in series with the input. Although the term sync-tip-clamp is used throughout this
document, it should be noted that the THS7372 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 THS7372 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 THS7372 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 THS7372 internal control loop sources up to 6 mA of
current to increase the input voltage level on the THS7372 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, noise, DAC overshoot, 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.
Copyright © 2011, Texas Instruments Incorporated
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