THS7315 Datasheet, PDF (12/26 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)
To allow for both dc-coupling and ac-coupling in the same part, the THS7315 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. (For example, voltage near ground can cause the current sink transistor to saturate and
produce potential signal problems.) This resistor is large enough to not impact a dc-coupled DAC termination.
For discharging an ac-coupled source, Ohm's Law is utilized. If the video signal is 0.5 V, then there will be 0.5 V
/ 800 kΩ = 0.625 μA of discharge current. If more hum rejection is desired or there is a loss of sync occurring,
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, decreasing the
overall resistance and ultimately increasing 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. This ringing may appear on
the THS7315 output. Because most DACs or encoders use resistors that are typically ≤ 500 Ω to establish the
voltage, meeting the < 1 kΩ requirement is easily done. However, if the source impedance looking from the
THS7315 input is very high, then simply adding a 1-kΩ resistor to GND will ensure proper operation of the
THS7315.
INPUT MODE OF OPERATION —AC BIAS
Sync-tip clamps work very well for signals that have horizontal and/or vertical syncs associated with them. Some
video signals, on the other hand, 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 THS7315. This function is
easily accomplished with the THS7315 by simply adding an external pull-up resistor to the positive power
supply, as shown in Figure 7.
+3.3 V
+3.3 V
Internal
Circuitry
Input
C_IN
9.31 MW
Input
Pin
+
800 kW
-
Internal Level
Shifter
Figure 7. AC-Bias Input Mode Circuit Configuration
The dc voltage appearing at the input pin is approximately equal to:
ǒ Ǔ V_DC + V_S
800k
800k ) R_PU
(1)
The allowable input range of the THS7315 is very wide: approximately (+V_S – 1.5 V). The input range is limited
by the allowable output voltage range and the internal gain. As such, the input dc bias point is very flexible, with
the output dc bias point being the primary factor. For example, if the desired output dc bias point is 1.6 V on a
3.3-V supply, then the input dc bias point should be (1.6 V – 230 mV) / 5.2 = 0.263 V. Consequently, the pull-up
resistor calculates to be about 9.31 MΩ, resulting in 0.261 V. If the desired input dc-bias point is 2.4 V with a 5-V
power supply, then the pull-up resistor calculates to be about 8.66 MΩ.
Keep in mind that the internal 800-kΩ resistor has approximately a ±20% variance. As such, the calculations
should account for this variance. For the 0.261 V example above, using an ideal 9.31-MΩ resistor, the input dc
bias voltage is about 0.261 V ±0.05 V, which translates to an output bias voltage of about 1.64 V ±0.26 V.
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