THS7360 datasheet(29/43 Pages) TI

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THS7360

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SLOS674 – JUNE 2010

filter channels. One way to help ensure this condition

In the same way as the dc output mode of operation

is satisfied is to make sure the 75-

Ω source resistor is

discussed previously, each line should have a 75-

Ω

placed within 0.5-inches to each THS7360 output pin.

source

termination

resistor

series

with

the

If a large ac-coupling capacitor is used, the capacitor

ac-coupling capacitor. This 75-

Ω resistor should be

should be placed after this resistor.

placed next to the THS7360 output to minimize

capacitive loading effects. If two lines are to be

There are many reasons dc-coupling is desirable,

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

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

capacitor and resistor rather than sharing these

common question is whether or not there are any

components.

This

configuration

helps

ensure

drawbacks to using dc-coupling. There are some

line-to-line dc isolation and eliminates the potential

potential issues that must be examined, such as the

problems as described previously. Using a single,

dc current bias as discussed above. Another potential

1000-mF capacitor for two lines is permissible, but

risk is whether this configuration meets industry

there is a chance for interference between the two

standards.

EIA/CEA-770

stipulates

that

the

receivers.

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

receiver. With a double-terminated load system, this

Lastly, because of the edge rates and frequencies of

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

operation, it is recommended (but not required) to

amplifier output. The THS7360 can easily meet this

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

requirement without issue. However, in Japan, the

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

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

value capacitors are most commonly aluminum

level with no signal. This requirement can be met with

electrolytic. It is well-known that these capacitors

the THS7360 in shutdown mode, but while active it

have significantly large equivalent series resistance

cannot

meet

this

specification

without

output

(ESR), and the impedance at high frequencies is

ac-coupling.

AC-coupling

the

output

essentially

rather large as a result of the associated inductances

ensures that the video signal works with any system

involved with the leads and construction. The small

and any specification. For many modern systems,

0.1-mF

0.01-mF

capacitors

help

pass

these

however, dc-coupling can satisfy most needs.

high-frequency signals (greater than 1 MHz) with

much lower impedance than the large capacitors.

OUTPUT MODE OF OPERATION:

Although it is common to use the same capacitor

AC-COUPLED

values for all the video lines, the frequency bandwidth

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

A very common method of coupling the video signal

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

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

the luma channels. Thus, the capacitor values of the

typically between 220 mF and 1000 mF, although

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

470 mF is very typical. The value of this capacitor

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

Figure 48 shows a typical configuration where the

and/or

field

tilt

associated

with

ac-coupling

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

described previously in this document. AC-coupling is

AC-coupled inputs are generally required when

performed for several reasons, but the most common

current-sink DACs are used or the input is connected

is to ensure full interoperability with the receiving

to an unknown source, such as when the THS7360 is

video system. This approach ensures that regardless

used as an input device.

of the reference dc voltage used on the transmitting

side,

the

receiving

side

re-establishes

the

reference voltage to its own requirements.

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