AD8042

Rev. D | Page 13 of 16

Single-Supply Composite Video Line Driver

The two op amps of an AD8042 can be configured as a single-

supply dual line driver for composite video. The wide signal

swing of the AD8042 enables this function to be performed

without using any type of clamping or dc restore circuit, which

can cause signal distortion.

Figure 38 shows a schematic for a circuit that is driven by a

single composite video source that is ac-coupled, level-shifted

and applied to both + inputs of the two amplifiers. Each op amp

provides a separate 75 Ω composite video output. To obtain

single-supply operation, ac coupling is used throughout. The

large capacitor values are required to ensure that there is

minimal tilting of the video signals due to their low frequency

(30 Hz) signal content. The circuit shown was measured to have

a differential gain of 0.06% and a differential phase of 0.06°.

voltage divider that provides the dc bias point to the input.

Setting the optimal bias point requires some understanding of

the nature of composite video signals and the video

performance of the AD8042.

+5V

0.1µF

8

3

2

4

6

5

1

7

10µF

4.99kΩ

4.99kΩ

75Ω

100kΩ

1kΩ

1kΩ

1kΩ

1kΩ

75Ω

75Ω

75Ω

75Ω

COAX

75Ω

220µF

220µF

1000µF

0.1µF

10µF

1000µF

0.1µF

COMPOSITE

VIDEO IN

Figure 38. Single-Supply Composite Video Line Driver Using AD8042

Signals of bounded peak-to-peak amplitude that vary in duty

cycle require larger dynamic swing capability than their peak-

to-peak amplitude after ac coupling. As a worst case, the

dynamic signal swing required approaches twice the peak-to-

peak value. The two bounding cases are for a duty cycle that is

mostly low, but occasionally goes high at a fraction of a percent

duty cycle and vice versa.

Composite video is not quite this demanding. One bounding

extreme is for a signal that is mostly black for an entire frame

but has a white (full intensity), minimum width spike at least

once per frame.

The other extreme is for a video signal that is full white

everywhere. The blanking intervals and sync tips of such a

signal have negative going excursions in compliance with

composite video specifications. The combination of horizontal

and vertical blanking intervals limit such a signal to being at its

highest level (white) for only about 75% of the time.

As a result of the duty cycle variations between the two extremes

presented above, a 1 V p-p composite video signal that is

multiplied by a gain of 2 requires about 3.2 V p-p of dynamic

voltage swing at the output for an op amp to pass a composite

video signal of arbitrary duty cycle without distortion.

Some circuits use a sync tip clamp along with ac coupling to

hold the sync tips at a relatively constant level to lower the

amount of dynamic signal swing required. However, these

circuits can have artifacts, such as sync tip compression, unless

they are driven by sources with very low output impedance.

The AD8042 not only has ample signal swing capability to

handle the dynamic range required without using a sync tip

clamp but also has good video specifications such as differential

gain and differential phase when buffering these signals in an

ac-coupled configuration.

To test this, the differential gain and differential phase were

measured for the AD8042 while the supplies were varied. As

the lower supply is raised to approach the video signal, the first

effect observed is that the sync tips become compressed before

the differential gain and differential phase are adversely

affected. Therefore, there must be adequate swing in the

negative direction to pass the sync tips without compression.

As the upper supply is lowered to approach the video, the

differential gain and differential phase was not significantly

affected until the difference between the peak video output

and the supply reached 0.6 V. Therefore, the highest video level

should be kept at least 0.6 V below the positive supply rail.

Therefore, it was found that the optimal point to bias the

noninverting input is at 2.2 V dc. Operating at this point, the

worst-case differential gain is measured at 0.06% and the worst-

case differential phase is 0.06°.

The ac-coupling capacitors used in the circuit at first glance

appear quite large. A composite video signal has a lower frequency

band edge of 30 Hz. The resistances at the various ac coupling

points, especially at the output, are quite small. To minimize

phase shifts and baseline tilt, the large value capacitors are

required. For video system performance that is not to be of the

highest quality, the value of these capacitors can be reduced by a

factor of up to five with only a slightly observable change in the

picture quality.