THS7316 Datasheet, PDF (17/28 Pages) Texas Instruments – 3-Channel HDTV Video Amplifier With 5th-Order Filters and 6-dB Gain

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APPLICATION INFORMATION (continued)
THS7316
SLOS521 – MARCH 2007
3.3 V
Y’ / G’ Out
75 W
DAC/
Encoder
Y’ / G’
R
HDTV
720p/1080i ^P’_B ^{/ B’}
Y’P’_BP’_R
R
G’B’R’
VGA
SVGA
XGA
P’_R / R’
R
THS7316
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
0.1 mF
P’_B / B’ Out
75 W
P’_R / R’ Out
75 W
+
3 V to 5 V ^{22 mF}
Figure 31. Typical HDTV Y'P'_BP'_R / G'B'R' System with DC-Coupled Line Driving
75 W
75 W
75 W
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 THS7316 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 THS7316. While the THS7316 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 THS7316 design minimizes 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 0 V ±1 V. While there is some question as to whether this voltage is at the output
of the amplifier or at the receiver, it is generally regarded to be measured at the receiver 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 140-mV input level shift, meeting this requirement is
accomplished. Thus, elimination of the large output ac-coupling capacitor can be done while still meeting the EIA
specification. This can save significant PCB area and costs.
Note that the THS7316 can drive the line with dc-coupling regardless 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 THS7316 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 THS7316 output pins should be kept below 20-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 330-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 is to ensure
full inter-operability with the receiving video system. This ensures that regardless of the reference dc voltage
used on the transmit side, the receive side will re-establish the dc reference voltage to its own requirements, and
meets EIA specifications.
Like the dc-output mode of operation, each line should have a 75-Ω source termination resistor in series with the
ac-coupling capacitor. If 2 lines are to be driven, it is best to have each line use its own capacitor and resistor
rather than sharing these components as shown in Figure 32. This helps ensure line-to-line dc isolation and the
potential problems as stipulated previously. Using a single 1000-µF capacitor for 2-lines can be done, but there
is a chance for interference to be created between the two receivers.
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