REV. C

–12–

AD8042

+5V

0.1µF

10 F

8

3

2

1000 F

0.1 F

75

COAX

75

75

1k

75

1000 F

0.1 F

4

6

5

1

7

1k

1k

220 F

220 F

10 F

4.99k

4.99k

75

10k

COMPOSITE

VIDEO

IN

1k

75

Figure 7. 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 will approach 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 every-

where. The blanking intervals and sync tips of such a signal will

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 ex-

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

multiplied by a gain of two 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 in order to lower

the amount of dynamic signal swing required. However, these

circuits can have artifacts like 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 like 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 to be observed is that the sync tips become compressed

before the differential gain and differential phase are adversely

affected. Thus, 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 differ-

ential gain and differential phase were not significantly adversely

affected until the difference between the peak video output and

the supply reached 0.6 V. Thus, the highest video level should

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

Taking the above into account, 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 fre-

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

coupling points—especially at the output—are quite small. In

order 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 observ-

able change in the picture quality.

Single-Ended-to-Differential Driver

Using a cross-coupled single-ended-to-differential converter, the

AD8042 makes a good general purpose differential line driver.

This can be used for applications such as driving category 5 twisted

pair wire which is becoming common for data communications

in buildings. Figure 8 shows a configuration for a circuit that

performs this function that can be used for video transmission

over a differential pair or various data communication purposes.

50m

AMP1

8

3

2

6

5

7

4

–5V

1

AD8042

100

0.1 F

10 F

60.4

60.4

1k

49.9

1k

1k

1k

1k

1k

121

0.1 F

10 F

AMP2

+5V

Figure 8. Single-Ended-to-Differential Twisted

Pair Line Driver