AD8091/AD8092

Rev. C | Page 14 of 20

2

1

6

5

7

2

3

6

AD8092

AD8091

C2

50pF

C1

50pF

R5

2kΩ

R4

2kΩ

R6

1kΩ

R3

2kΩ

R2

2kΩ

R1

3kΩ

3

AD8092

Figure 33. 2 MHz Biquad Band-Pass Filter

The frequency response of the circuit is shown in Figure 34.

0

–10

–20

–30

–40

10k

100k

1M

10M

100M

FREQUENCY (Hz)

Figure 34. Frequency Response of 2 MHz Band-Pass Biquad Filter

SYNC STRIPPER

Synchronizing pulses are sometimes carried on video signals so

as not to require a separate channel to carry the synchronizing

information. However, for some functions, such as A/D

conversion, it is not desirable to have the sync pulses on the

video signal. These pulses reduce the dynamic range of the

video signal and do not provide any useful information for such

a function.

A sync stripper removes the synchronizing pulses from a video

signal while passing all the useful video information. Figure 35

shows a practical single-supply circuit that uses only a single

AD8091. It is capable of directly driving a reverse terminated

video line.

The video signal plus sync is applied to the noninverting input

with the proper termination. The amplifier gain is set equal to 2

via the two 1 kΩ resistors in the feedback circuit. A bias voltage

must be applied to R1 for the input signal to have the sync

pulses stripped at the proper level.

The blanking level of the input video pulse is the desired place

to remove the sync information. The amplifier multiplies this

level by 2. This level must be at ground at the output in order

for the sync stripping action to take place. Because the gain of

the amplifier from the input of R1 to the output is −1, a voltage

come out at ground.

AD8091

+

R2

1kΩ

R1

1kΩ

+0.8V

100Ω

TO A/D

3V OR 5V

3

2

4

6

7

10µF

0.1µF

GROUND

+0.4V

VIDEO WITH SYNC

GROUND

VIDEO WITHOUT SYNC

Figure 35. Sync Stripper

SINGLE-SUPPLY COMPOSITE VIDEO LINE DRIVER

Many composite video signals have their blanking level at

ground and have video information that is both positive and

negative. Such signals require dual-supply amplifiers to pass

them. However, by ac level-shifting, a single-supply amplifier

can be used to pass these signals. The following complications

may arise from such techniques.

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

cycle require larger dynamic swing capacity than their

(bounded) peak-to-peak amplitude after they are ac-coupled.

As a worst case, the dynamic signal swing approaches twice the

peak-to-peak value. One of two conditions that define the

maximum dynamic swing requirements is a signal that is

mostly low but goes high with a duty cycle that is a small

fraction of a percent. The opposite condition defines the second

condition.

The worst case of composite video is not quite this demanding.

One bounding condition is a signal that is mostly black for an

entire frame but has a white (full amplitude) minimum width

spike at least once in a frame.

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.

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.