THS7319_14 Datasheet, PDF (19/34 Pages) Texas Instruments – 3-Channel, Very Low Power Video Amplifiers with EDTV Filters and 6-dB Gain

THS7319
www.ti.com.............................................................................................................................................................. SBOS468A – JUNE 2009 – REVISED JULY 2009
DC-COUPLED OUTPUT
The THS7319 incorporates a rail-to-rail output stage
that can drive the line directly without the need for
large ac-coupling capacitors. This design offers the
best line tilt and field tilt (droop) performance because
no ac-coupling occurs. Keep in mind that if the input
is ac-coupled, then the resulting tilt as a result of the
input ac-coupling continues to be seen on the output,
regardless of the output coupling. The 70-mA output
current drive capability of the THS7319 is designed to
drive the video line while keeping the output dynamic
range as wide as possible.
One concern of dc-coupling, however, arises if the
line is terminated to ground. If an ac-bias input
configuration is used or if a dc reference from the
DAC is applied, such as S-Video C'/component P'_B/or
component P'_R signals, the output of the THS7319
will have a dc bias on the output, such as 1 V. This
configuration allows a dc current path to flow, such as
1 V/150 Ω = 6.67 mA. The result of this configuration
is a slightly decreased high output voltage swing and
an increase in power dissipation of the THS7319.
While the THS7319 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. Using a 5-V supply, this configuration can
result in an additional power dissipation of
(5 V – 1 V) × 6.67 mA = 26.7 mW per channel. With a
3.3-V supply, this dissipation reduces to 15.3 mW per
channel. The overall low quiescent current of the
THS7319 design minimizes potential thermal issues
even when used at high ambient temperatures, but
power and thermal analysis should always be
examined in any system to ensure that no issues
arise. Be sure to use RMS power and not
instantaneous power when evaluating the thermal
performance.
Note that the THS7319 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 requirement helps isolate
capacitive loading effects from the THS7319 output.
Failure to properly isolate capacitive loads may result
in ringing or oscillation. The stray capacitance
appearing directly at the THS7319 output pins should
be kept below 18-pF. One method to ensure this
condition is to make sure the 75-Ω source resistor is
placed next to each THS7319 output pin.
There are many reasons dc-coupling is desirable,
including reduced costs, printed circuit board (PCB)
area, and no line tilt or field tilt. A common question is
whether or not there are any drawbacks to using
dc-coupling. There are a few potential issues that
must be examined, such as the dc current bias as
discussed above. Another potential risk is whether
this configuration meets industry standards. EIA-770
stipulates that the back-porch shall be 0 V ± 1 V as
measured at the receiver. With a double-terminated
load system, this requirement implies a 0-V ± 2-V
level at the video amplifier output. The THS7319 can
easily meet this requirement without issue. However,
in Japan, the EIAJ CP-1203 specification stipulates a
0-V ± 0.1-V level with no signal. This requirement can
be met with the THS7319 in disable mode, but while
active it cannot meet this specification without output
ac-coupling.
AC-COUPLED OUTPUT
A very common method of coupling the video signal
to the line is with a large capacitor. This capacitor is
typically between 220 µF and 1000 µF, although
470 µF is very typical. The value of this capacitor
must be large enough to minimize the line tilt (droop)
and/or field tilt associated with ac-coupling as
described previously in this document. AC-coupling is
performed for several reasons, but the most common
is to ensure full interoperability with the receiving
video system. This approach ensures that regardless
of the reference dc voltage used on the transmitting
side, the receiving side re-establishes the dc
reference voltage to its own requirements.
In the same way as the dc output mode of operation
discussed previously, each line should have a 75-Ω
source termination resistor in series with the
ac-coupling capacitor. This 75-Ω series resistor
should be placed next to the THS7319 output to
minimize capacitive loading effects.
Because of the edge rates and frequencies of
operation, it is recommended (but not required) to
place a 0.1-µF to 0.01-µF capacitor in parallel with
the large 220-µF to 1000-µF capacitor. These large
value capacitors are most commonly aluminum
electrolytic. It is well-known that these capacitors
have significantly large equivalent series resistance
(ESR), and the impedance at high frequencies is
rather large as a result of the associated inductances
involved with the leads and construction. The small
0.1-µF to 0.01-µF capacitors help pass these
high-frequency signals (greater than 1 MHz) with
much lower impedance than the large capacitors.
Although it is common to use the same capacitor
values for all the video lines, the frequency bandwidth
of the chroma signal in a S-Video system is not
required to go as low (or as high of a frequency) as
the luma channels. Thus, the capacitor values of the
chroma line(s) can be smaller, such as 0.1 µF.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS7319
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