OUTPUT MODE OF OPERATION: AC

75 W

Y'/G'Out

CVBS/Sync

CVBS

+3.3V

+3.3V

R

DAC/Encoder

SDTV

CVBS

S-Video

Y'P' P'

R'G'B'

B

R

Y'/G'

R

P' /B'

B

R

P' /R'

R

R

1

2

3

4

5

6

7

14

13

12

11

10

9

8

CH1OUT

CH2OUT

CH3OUT

CH4OUT

V

S+

BYPASS

NC

CH1IN

CH2IN

CH3IN

CH4IN

GND

DISABLE

NC

THS7374

ToGPIOController

orGND

75 W

75 W

P’ /B'Out

B

75 W

75 W

P’ /R'Out

R

75 W

75 W

75 W

+3Vto+5V

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

R

PU

+3.3V

R

PU

330 F

m

(2)

75 W

CVBSOut1

75 W

330 F

m

(2)

75 W

Y'Out1

75 W

330 F

m

(2)

75 W

P' Out1

B

75 W

330 F

m

(2)

75 W

P' Out1

R

75 W

330 F

m

(2)

330 F

m

(2)

330 F

m

(2)

330 F

m

(2)

THS7374

This

configuration

helps

ensure

line-to-line

dc

COUPLED

isolation and avoids the potential problems discussed

above. Using a single 1000-

µF capacitor for two lines

A very common method of coupling the video signal

can be done, but there is a chance for ground loops

to the line is the use of a large capacitor. This

and interference to be created between the two

capacitor is typically between 220

µF and 1000 µF,

receivers.

although 470

µF is very common. The value of this

capacitor must be large enough to minimize the line

Lastly, because of the edge rates and frequencies of

tilt (droop) and/or field tilt associated with ac coupling

operation, it is recommended (but not required) to

as

described

previously

in

this

document.

AC

place a 0.1-

µF to 0.01-µF capacitor in parallel with

coupling is done for several reasons, but most often

the large 220-

µF to 1000-µF capacitor. These

to ensure full interoperability with the receiving video

large-value

capacitors

are

generally

aluminum

system. AC coupling also ensures adherence to video

electrolytic. It is well-known that these capacitors

standard specifications. It ensures that regardless of

have significantly large equivalent series resistance

the reference dc voltage used on the transmit side,

(ESR), and the impedance at high frequencies is

the receive side re-establishes the dc reference

rather large because of the associated inductances

voltage to its own requirements.

involved with the leads and construction. The small

0.1-

µF to 0.01-µF capacitors help pass these

As with the dc output mode of operation discussed

high-frequency (> 1-MHz) signals with much lower

previously, each line should have a 75-

Ω source

impedance than the large capacitors.

termination resistor in series with the ac coupling

capacitor. If two lines are to be driven, it is best to

Although it is common to use the same capacitor

have each line use its own capacitor and resistor

values for all the video lines, the frequency bandwidth

rather than sharing these components, as shown in

of the chroma signal in a s-video system are not

Figure 41.

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.

(1) An ac-coupled input is shown in this example. DC coupling is also allowed as long as the DAC output voltage is within the allowable

linear input and output voltage range of the THS7374. To dc-couple, remove the 0.1

µF input capacitors and R

(2) An ac-coupled output is shown in this example. DC coupling is also allowed by simply removing these capacitors.

Figure 41. Typical SDTV AC-Input System Driving Two AC-Coupled Video Lines

Copyright © 2008, Texas Instruments Incorporated

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