MT-059 datasheet(13/17 Pages) AD

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AD8044

REV. B

–12–

ACL (V/V)

1000

100

VS = +5V

< 30% OVERSHOOT

VOUT

VIN

100mV STEP

= 10

= 0

Figure 35. Capacitive Load Drive vs. Closed-Loop Gain

APPLICATIONS

RGB Buffer

The AD8044 can provide buffering of RGB signals that include

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

When driving two monitors from the same RGB video source it

is necessary to provide an additional driver for one of the moni-

tors to prevent the double termination situation that the second

monitor presents. This has usually required a dual-supply op

amp because the level of the input signal from the video driver

goes all the way to ground during horizontal blanking. In single-

supply systems it can be a major inconvenience and expense to

add an additional negative supply.

A single AD8044 can provide the necessary drive capability and

yet does not require a negative supply in this application. Fig-

ure 36 is a schematic that uses three amplifiers out of a single

AD8044 to provide buffering for a second monitor.

The source of the RGB signals is shown to be from a set of three

current output DACs that are within a single-supply graphics

IC. This is typically the situation in most PCs and workstations

that may use either a standalone triple DAC or DACs that are

integrated into a larger graphics chip.

During horizontal blanking, the current output from the DACs

is turned off and the RGB outputs are pulled to ground by the

termination resistors. If voltage sources were used for the RGB

signals, then the termination resistors near the graphics IC

would be in series and the rest of the circuit would remain the

same. This is because a voltage source is an ac short circuit, so a

series resistor is required to make the drive end of the line see

W to ac ground. On the other hand, a current source has a

very high output impedance, so a shunt resistor is required to

make the drive end of the line see 75

W to ground. In either

case, the monitor terminates its end of the line with 75

The circuit in Figure 36 shows minimum signal degradation

when using a single-supply for the AD8044. The circuit per-

forms equally well on either a +3 V or +5 V supply.

10 F

+3V OR +5V

0.1 F

RGB

MONITOR #2

V+

V–

RGB

MONITOR #1

+5V

GRAPHICS

AD8044

Figure 36. Single Supply RGB Video Driver

Figure 37 is an oscilloscope photo of the circuit in Figure 36

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 AD8044 is 1.4 V with the termination resistors providing a

divide-by-two.

VIN

GND

VOUT

100

5 s

500mV

Figure 37. +3 V, RGB Buffer

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