A8091/AD8092

Rev. B | Page 16 of 20

The other extreme is a full white video signal. The blanking

intervals and sync tips of such a signal have negative-going

excursions in compliance with the composite video

specifications. The combination of horizontal and vertical

blanking intervals limit such a signal to being at the highest

(white) level for a maximum of about 75% of the time.

AD8091

5V

+

4.99k

Ω

10

µF

47

µF

+

4.99k

Ω

10k

Ω

75

Ω

1k

Ω

1k

Ω

75

Ω

75

Ω

COMPOSITE

VIDEO IN

+

220

µF

0.1

µF

3

2

4

6

7

1000

µF

0.1

µF10µF

+

As a result of the duty cycles between the two extremes, 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

varying duty cycle without distortion.

Figure 36. Single-Supply Composite Video Line Driver

The feedback circuit provides unity gain for the dc biasing of

the input and provides a gain of 2 for any signals that are in the

video bandwidth. The output is ac-coupled and terminated to

drive the line.

Some circuits use a sync tip clamp 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 like

sync tip compression unless they are driven by a source with a

very low output impedance. The AD8091/AD8092 have

adequate signal swing when running on a single 5 V supply to

handle an ac-coupled composite video signal.

The capacitor values were selected for providing minimum tilt

or field time distortion of the video signal. These values would

be required for video that is considered to be studio or

broadcast quality. However, if a lower consumer grade of video,

sometimes referred to as consumer video, is all that is desired,

the values and the cost of the capacitors can be reduced by as

much as a factor of 5 with minimum visible degradation in the

picture.

The input to the circuit in Figure 36 is a standard composite

(1 V p-p) video signal that has the blanking level at ground. The

input network level shifts the video signal by means of ac

coupling. The noninverting input of the op amp is biased to half

of the supply voltage.