THS7315_14 Datasheet, PDF (10/28 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier with 5th-Order Filters and 5.2-V/V Gain

THS7315
SLOS532 – JUNE 2007
www.ti.com
APPLICATION INFORMATION (continued)
The THS7315 input impedance in this operating mode is dictated by the internal 800-kΩ pull-down resistor, as
shown in Figure 5. Note that the internal voltage shift does not appear at the input pin, but only at the output pin.
Internal
+V_S
Circuitry
Input
+
800 kW
-
Internal Level
Shifter
Figure 5. 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. These
DACs typically only sink current, rather than the more traditional current-sourcing DAC where the resistor is
referenced to ground. The resulting video signal voltages can be too high for a dc-coupled video buffer to
function properly. To account for this scenario, the THS7315 incorporates a sync-tip clamp (STC) circuit. This
function requires a capacitor (nominally 0.1 μF) to be placed in series with the input. Note that while the term
sync-tip clamp or STC is used throughout this document, it should be noted that the THS7315 is better termed
as a dc-restoration circuit based on how this function is performed. The STC circuit is an active clamp circuit and
not a passive diode clamp function.
The input to the THS7315 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 THS7315 allows a dc-coupled input to also function. Therefore,
the STC is considered transparent because it does not operate unless the input signal goes below ground. The
signal then goes through the same internal level shifter, resulting in an output voltage low level of 230 mV. If the
input signal tries to go below 0 V, the internal control loop of the THS7315 will source up to 2 mA of current to
increase the THS7315 input voltage level on the input side of the coupling capacitor. As soon as the voltage
goes above 0 V, the loop will stop sourcing current and become very high impedance.
One of the concerns about the STC level is how the clamp reacts to a sync edge that has overshoot—a
common effect in VCR signals or reflections found in poor PCB layouts. Ideally, the STC should not react to the
overshoot voltage of the input signal. Otherwise, this effect 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 THS7315 has an internal low-pass
filter as shown in Figure 6. This filter reduces the response time of the STC circuit. This delay is a function of
how far the voltage is below ground, but generally, it is about a 100-ns delay. 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.
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