THS7364 datasheet(34/47 Pages) TI

34 / 47 page

THS7364

SBOS530 – AUGUST 2010

www.ti.com

There are many reasons dc-coupling is desirable,

In the same way as the dc output mode of operation

including reduced costs, PCB area, and no line tilt. A

discussed previously, each line should have a 75-

Ω

common question is whether or not there are any

source

termination

resistor

series

with

the

drawbacks to using dc-coupling. There are some

ac-coupling capacitor. This 75-

Ω resistor should be

potential issues that must be examined, such as the

placed next to the THS7364 output to minimize

dc current bias as discussed above. Another potential

capacitive loading effects. If two lines are to be

risk is whether this configuration meets industry

driven, it is best to have each line use its own

standards.

EIA/CEA-770

stipulates

that

the

capacitor and resistor rather than sharing these

back-porch shall be 0 V ± 1 V as measured at the

components.

This

configuration

helps

ensure

receiver. With a double-terminated load system, this

line-to-line dc isolation and eliminates the potential

requirement implies a 0 V ± 2 V level at the video

problems as described previously. Using a single,

amplifier output. The THS7364 can easily meet this

1000-mF capacitor for two lines is permissible, but

requirement without issue. However, in Japan, the

there is a chance for interference between the two

EIAJ CP-1203 specification stipulates a 0 V ± 0.1 V

receivers.

level with no signal. This requirement can be met with

Lastly, because of the edge rates and frequencies of

the THS7364 in shutdown mode, but while active it

operation, it is recommended (but not required) to

cannot

meet

this

specification

without

output

place a 0.1-mF to 0.01-mF capacitor in parallel with

ac-coupling.

AC-coupling

the

output

essentially

the large 220-mF to 1000-mF capacitor. These large

ensures that the video signal works with any system

value capacitors are most commonly aluminum

and any specification. For many modern systems,

electrolytic. It is well-known that these capacitors

however, dc-coupling can satisfy most needs.

have significantly large equivalent series resistance

(ESR), and the impedance at high frequencies is

OUTPUT MODE OF OPERATION:

rather large as a result of the associated inductances

AC-COUPLED

involved with the leads and construction. The small

0.1-mF

0.01-mF

capacitors

help

pass

these

A very common method of coupling the video signal

high-frequency signals (greater than 1 MHz) with

to the line is with a large capacitor. This capacitor is

much lower impedance than the large capacitors.

typically between 220 mF and 1000 mF, although

470 mF is very typical. The value of this capacitor

Although it is common to use the same capacitor

must be large enough to minimize the line tilt (droop)

values for all the video lines, the frequency bandwidth

and/or

field

tilt

associated

with

ac-coupling

of the chroma signal in a S-Video system is not

described previously in this document. AC-coupling is

required to go as low (or as high of a frequency) as

performed for several reasons, but the most common

the luma channels. Thus, the capacitor values of the

is to ensure full interoperability with the receiving

chroma line(s) can be smaller, such as 0.1 mF.

video system. This approach ensures that regardless

of the reference dc voltage used on the transmitting

Figure 82 shows a typical configuration where the

side,

the

receiving

side

re-establishes

the

input is ac-coupled and the output is also ac-coupled.

reference voltage to its own requirements.

AC-coupled inputs are generally required when

current-sink DACs are used or the input is connected

to an unknown source, such as when the THS7364 is

used as an input device.

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