THS7376 Datasheet, PDF (43/61 Pages) Texas Instruments – 4-Channel Video Amplifier with One SD and Three HD 8th-Order Filters with 6-dB Gain

THS7376
www.ti.com
SBOS692 – JUNE 2013
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 THS7376
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, thus supplying up to 6
mA of current. Therefore, 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, what happens if the input signal goes above the 0-V input level? The problem is that the video signal is
always above this level and must not be altered in any way. Thus, 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 effect is often seen by comparing the tilt (droop) of a constant luma signal being applied and the
resulting output level. The associated change in luma level from the beginning and end of the video line is the
amount of line tilt (droop).
If the discharge current is very small, 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 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- and ac-coupling in the same device, the THS7376 incorporates an 800-kΩ resistor to
ground. Although a true constant-current sink is preferred over a resistor, there can be issues when the voltage
is near ground. This configuration can cause the current sink transistor to saturate and cause potential problems
with the signal. The 800-kΩ resistor is large enough 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 simply decrease the
0.1-μF input coupling capacitor. A decrease from 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 that 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 ringing, which may appear on the
device output. Because most DACs or encoders use resistors (typically less than 300 Ω) to establish the voltage,
meeting the less than 1-kΩ requirement is easily done. However, if the source impedance is very high, then
simply adding a 1-kΩ resistor to GND ensures proper device operation.
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