REV. B

AD8041

–13–

APPLICATIONS

RGB Buffer

The AD8041 can provide buffering of RGB signals that include

ground while operating from a single 3 V or 5 V supply.

The signals that drive an RGB monitor are usually supplied by

current output DACs that operate from a 5 V only supply. These

can triple DACs like the ADV7120 and ADV7122 from Analog

Devices or integrate into the graphics controller IC as in most

PCs these days.

During the horizontal blanking interval, the currents output

from the DACs go to zero and the RGB signals are pulled to

ground via the termination resistors. If more than one RGB

monitor is desired, it cannot simply be connected in parallel

because it will provide an additional termination. Therefore,

buffering must be provided before connecting a second monitor.

Since the RGB signals include ground as part of their dynamic

output range, it has previously been required to use a dual-

supply op amp to provide this buffering. In some systems, this is

the only component that requires a negative supply, so it can be

quite inconvenient to incorporate this multiple monitor feature.

Figure 11 shows a schematic of one channel of a single-supply,

gain-of-two buffer for driving a second RGB monitor. No cur-

rent is required when the amplifier output is at ground. The

termination resistor at the monitor helps pull the output down

at low voltage levels.

6

4

7

3

2

AD8041

1k

75

10 F

8

75

R, G OR B

NC

0.1 F

1k

PRIMARY RGB

MONITOR

75

SECOND RGB

MONITOR

3V OR 5V

Figure 11. Single-Supply RGB Buffer

Figure 12 is an oscilloscope photo of the circuit in Figure 11

operating from a 3 V supply and driven by the blue signal of a

color bar pattern. Note that the input and output are at ground

during the horizontal blanking interval. The RGB signals are

specified to output a maximum of 700 mV peak. The output of

the AD8041 is 1.4 V with the termination resistors providing a

divide-by-two. The red and green signals can be buffered in the

same manner with duplication of this circuit.

GND

GND

10

0%

100

90

5 s

500mV

500mV

Figure 12. 3 V, RGB Buffer

Single-Supply Composite Video Line Driver

Figure 13 shows a schematic of a single-supply gain-of-two

composite video line driver. Since the sync tips of a composite

video signal extend below ground, the input must be ac-coupled

and shifted positively to provide signal swing during these nega-

tive excursions in a single-supply configuration.

The input is terminated in 75

Ω and ac-coupled via C

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 AD8041.

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 extremes

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

plied 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.

6

4

7

3

2

AD8041

1k

10k

10 F

5V

75

COMPOSITE

VIDEO IN

NC

0.1 F

75

8

1000 F

0.1 F

4.99k

10 F

4.99k

47 F

1k

220 F

75

COAX

75

Figure 13. Single-Supply Composite Video Line Driver

The AD8041 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.