THS7314 Datasheet, PDF (15/23 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier With 5th-Order Filters and 6-dB Gain

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THS7314
SLOS513 – DECEMBER 2006
APPLICATION INFORMATION (continued)
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 helps ensure a very
robust STC system.
Input
Input
Pin
0.1 mF
+Vs +Vs
Comparator ^{STC LPF}
+
-
800 kW
Internal
Circuitry
+
-
145 mV Level
Shifter
Figure 31. Equivalent AC Sync Tip Clamp Input Circuit
When the AC Sync-Tip-Clamp (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
THS7314 will source 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
fast. If the difference is small, then the source current is also small to account for minor voltage droop.
But, what happens if the input signal goes above the 0-V input level? The problem is 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 will discharge 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 is often seen by looking at the tilt (aka droop) of a constant luma signal being applied and
looking at 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 small, the amount of tilt is low which is good. But, the amount of time for the system to
capture the sync signal could be too long. This 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-coupling and ac-coupling in the same part, the THS7314 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 can cause the current sink transistor to saturate and cause potential problems with
the signal. This resistor is large enough as to not impact a dc-coupled DAC termination. For discharging an
ac-coupled source, Ohm’s Law is used. If the video signal is 1 V, then there is 1 V / 800 kΩ = 1.25-µA of
discharge current. If more hum rejection is desired or there is a loss of sync occurring, then decrease the 0.1-µF
input coupling capacitor. A decrease form 0.1 µF to 0.047 µF increases the hum rejection by a factor of 2.1.
Alternatively an external pull-down resistor to ground may be added which 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 to ring and this ringing may appear
on the output of the THS7314. Because most DACs or encoders use resistors to establish the voltage, which
are typically less than 300-Ω, then meeting the <1kΩ requirement is easily done. But, if the source impedance
looking from the THS7314 input perspective is high, then add a 1-kΩ resistor to GND to ensure proper operation
of the THS7314.
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