THS7314_14 Datasheet, PDF (16/31 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier With 5th-Order Filters and 6-dB Gain

THS7314
SLOS513A – DECEMBER 2006 – REVISED MARCH 2011
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INPUT MODE OF OPERATION – AC BIAS
Sync tip clamps work great for signals that have horizontal and/or vertical syncs associated with them. But, 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 THS7314. This function is accomplished with the
THS7314 by adding an external pull-up resistor to the positive power supply as shown in Figure 32.
3.3 V
3.01 MW
3.3 V
Internal
Circuitry
Input
Cin
Input
Pin
+
800 kW
-
145 mV Level
Shifter
Figure 32. AC-Bias Input Mode Circuit Configuration
The dc voltage appearing at the input pin is approxiamately equal to:
ǒ Ǔ V_DC + V_S
800k
800k ) R_PU
(1)
The THS7314 allowable input range is approximately 0-V to (+Vs – 1.5V) which allows for a 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 mid-rail on a 3.3-V supply, then the input dc-bias point
should be (1.65V – 290mV) /2 = 0.68V. Thus, the pull-up resistor calculates to about 3.01-MΩ resulting in
0.693V. If the output dc-bias point is desired to be 0.68-V with a 5-V power supply, then the pull-up resistor
calculates to about 5.1-MΩ.
Keep in mind that the internal 800-kΩ resistor has approximately a ±20% variance. As such, the calculations
should take this into account. For the 0.693-V example above using an ideal 3.01-MΩ resistor, the input dc-bias
voltage is about 0.693V ± 0.11V.
One other issue that must be taken into account is the dc-bias point is a function of the power supply. As such,
there is an impact on PSRR on the system. To help reduce the impact, the input capacitor combined with the
pull-up resistance functions as a low-pass filter. Additionally, the time to charge the capacitor to the final dc-bias
point is also a function of the pull-up resistor and the input capacitor. Lastly, the input capacitor forms a
high-pass filter with the parallel impedance of the pull-up resistor and the 800-kΩ resistor. It is good to have this
high pass filter at about 3-Hz to minimize any potential droop on a P'_B, P'_R, or non-sync B' or R' signal. A 0.1μF
input capacitor with a 3.01-MΩ pull-up resistor equates to about a 2.5-Hz high-pass corner frequency.
This mode of operation is recommended for use with chroma (C'), P’_B, P'_R, U', V', and non-sync B' and/or R'
signals.
OUTPUT MODE OF OPERATION – DC COUPLED
The THS7314 incorporates a rail-to-rail output stage that can be used to drive the line directly without the need
for large ac-coupling capacitors as shown in Figure 33. This offers the best line tilt and field tilt (or droop)
performance since there is no ac-coupling occurring. Keep in mind that if the input is ac-coupled, then the
resulting tilt due to the input ac-coupling will still be seen on the output irregardless of the output coupling. The
80-mA output current drive capability of the THS7314 was designed to drive two video lines simultaneously –
essentially a 75-Ω load – while keeping the output dynamic range as wide as possible.
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