+2.6Vto+5V

Enable

75 W

Out1

Channel1

R

75 W

LPF

3Pole

20MHz

6dB

x2

Level

Shift

Out2

Channel2

R

75 W

75 W

LPF

3Pole

20MHz

6dB

x2

Level

Shift

Channel3

R

Out3

75 W

75 W

LPF

3Pole

20MHz

6dB

x2

Level

Shift

330

F

m

330

F

m

330

F

m

THS7320

SBOS565B – JULY 2011 – REVISED SEPTEMBER 2012

www.ti.com

There are many reasons dc-coupling is desirable, including reduced costs, PCB area reduction, 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 THS7320 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

THS7320 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 THS7320 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

generally 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 an S-Video system is not required to perform at as low (or as high) a frequency as the luma

channels. Thus, the capacitor values of the chroma line(s) can be smaller, such as 0.1

μF.

Figure 58 shows a typical configuration where the input is dc-coupled and the output is also ac-coupled. 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 THS7320 is used as an input device.

Figure 58. Typical DC Input System Driving AC-Coupled Video Lines

24

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