THS7372 Datasheet, PDF (33/49 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
To allow for both dc- and ac-coupling in the same part, the THS7372 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
output of the THS7372. 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 looking
from the THS7372 input perspective is very high, then simply adding a 1-kΩ resistor to GND ensures proper
operation of the THS7372.
INPUT MODE OF OPERATION: AC BIAS
Sync-tip clamps work very well for signals that have horizontal and/or vertical syncs associated with them;
however, some video signals do not have a sync embedded within the signal. If ac-coupling of these signals is
desired, then a dc bias is required to properly set the dc operating point within the THS7372. This function is
easily accomplished with the THS7372 by simply adding an external pull-up resistor to the positive power supply,
as shown in Figure 87.
+3.3 V
Input
C_IN
0.1 mF
R_PU
Input
Pin
+3.3 V
800 kW
Internal
Circuitry
Level
Shift
Figure 87. AC-Bias Input Mode Circuit Configuration
The dc voltage appearing at the input pin is equal to Equation 1:
V =V
800 kW
DC
S
800
kW
+
R
PU
(1)
The THS7372 allowable input range is approximately 0 V to (V_S+ – 1.5 V), allowing for a very wide input voltage
range. As such, the input dc bias point is very flexible, with the output dc bias point being the primary factor. For
example, if the output dc bias point is desired to be 1.6 V on a 3.3-V supply, then the input dc bias point should
be (1.6 V – 300 mV)/2 = 0.65 V. Thus, the pull-up resistor calculates to approximately 3.3 MΩ, resulting in
0.644 V. If the output dc-bias point is desired to be 1.6 V with a 5-V power supply, then the pull-up resistor
calculates to approximately 5.36 MΩ.
Keep in mind that the internal 800-kΩ resistor has approximately a ±20% variance. As such, the calculations
should take this variance into account. For the 0.644-V example above, using an ideal 3.3-MΩ resistor, the input
dc bias voltage is approximately 0.644 V ±0.1 V.
The value of the output bias voltage is very flexible and is left to each individual design. It is important to ensure
that the signal does not clip or saturate the video signal. Thus, it is recommended to ensure the output bias
voltage is between 0.9 V and (V_S+ – 1 V). For 100% color saturated CVBS or signals with Macrovision^®, the
CVBS signal can reach up to 1.23 V_PP at the input, or 2.46 V_PP at the output of the THS7372. In contrast, other
signals are typically 1 V_PP or 0.7 V_PP at the input which translate to an output voltage of 2 V_PP or 1.4 V_PP. The
output bias voltage must account for a worst-case situation, depending on the signals involved.
Copyright © 2011, Texas Instruments Incorporated
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