THS7314 Datasheet, PDF (17/23 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)
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.
3.3 V
DAC/
CVBS
Encoder
(THS8200)
R
SDTV
CVBS
Y’
S-Video Y’
S-Video C’
R
480i/576i
Y’P’_BP’_R
G’B’R’
C’
R
THS7314
1 ^{CH.1 IN}
2 ^{CH.2 IN}
3 ^{CH.3 IN}
4 ^VS+
CH.1 OUT ₈
CH.2 OUT ₇
CH.3 OUT ₆
GND ₅
3.3 V
CVBS
Out
75 W
75 W
Y’
Out
S-Video
C’
75 W
Out
75 W
75 W
75 W
Figure 33. Typical SDTV CVBS/Y'/C' System with DC-Coupled Line Driving
One concern of dc-coupling is if the line is terminated to ground. If the ac-bias input configuration is used, the
output of the THS7314 will have a dc-bias on the output. With 2 lines terminated to ground, this creates a
dc-current path to exist which results in a slightly decreased high output voltage swing and resulting in an
increase in power dissipation of the THS7314. While the THS7314 was designed to operate with a junction
temperature of up to 125 C, care must be taken to ensure that the junction temperature does not exceed this
level or else long term reliability could suffer. Although this configuration only adds less then 10 mW of power
dissipation per channel, the overall low power dissipation of the THS7314 design will minimize potential thermal
issues even when using the SOIC package at high ambient temperatures.
Another concern of DC coupling is the blanking level voltage of the video signal. The EIA specification dictates
that the blanking level shall be 0V ± 1V. While there is some question as to whether this voltage is at the output
of the amplifier or at the reciever, it is generally regarded to be measured at the reciever side of a system as the
rest of the specification voltage requirements are given with doubly terminated connections present. With the
rail-to-rail output swing capability, combined with the 145-mV input level shift, meeting this requirement is easily
accomplished. Thus, elimination of the large output AC coupling capacitor can be done while still meeting the
EIA specification. This can save PCB area and costs.
Note that the THS7314 can drive the line with dc-coupling irregardless of the input mode of operation. The only
requirement is to make sure the video line has proper termination in series with the output – typically 75-Ω. This
helps isolate capacitive loading effects from the THS7314 output. Failure to isolate capacitive loads may result in
instabilities with the output buffer potentially causing ringing or oscillations to appear. The stray capacitance
appearing directly at the THS7314 output pins should be kept below 25-pF.
OUTPUT MODE OF OPERATION – AC COUPLED
The most common method of coupling the video signal to the line is with the use of a large capacitor. This
capacitor is typically between 220-µF and 1000-µF, although 470-µF is common. This value of this capacitor
must be this large to minimize the line tilt (droop) and/or field tilt associated with ac-coupling as described
previously in this document. AC-coupling is done for several reasons, but the most common reason to do this is
to ensure full inter-operability with the receiving video system. This minimizes possible ground loops. It also
ensures that irregardless of the reference dc voltage used on the transmit side, the receive side will re-establish
the dc reference voltage to its own requirements.
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