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+

0.1 F

m

3Vto5V

5

8

7

6

VS+

GND

CH.2IN

CH.3IN

CH.1IN

CH.3OUT

CH.2OUT

CH.1OUT

DAC/

Encoder

1

2

3

4

Y’ /G’

75 W

3.3V

75 W

HDTV

720p/1080i

Y’P’ P’

G’B’R’

VGA

SVGA

XGA

B

R

R

R

R

THS7316

P’

Out

Y’

Out

/G’

75 W

75 W

75 W

P’

Out

75 W

22 F

m

OUTPUT MODE OF OPERATION – AC COUPLED

THS7316

SLOS521 – MARCH 2007

APPLICATION INFORMATION (continued)

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.

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.

17

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