AD8074/AD8075

–10–

RGB Buffer for Second Monitor

The RGB signals for PC monitors are driven through coax

cables whose characteristic impedance is 75

Ω. The graphics

chip will generally have current-source output drivers that should

be double terminated with a 75

Ω shunt termination at each end.

On the transmit end, the shunt terminations are provided to

ground close to the graphics IC, while the monitor terminates

its end via internal termination resistors. While this scheme works

well and is virtually foolproof for a single monitor, it leaves no

means for passively connecting a second monitor to the same source.

A second monitor that is connected simply in parallel will pro-

vide an extra set of terminations that will upset the signal levels.

To keep costs low, most computer monitors do not have the ability

to open-circuit the terminations in order that an additional monitor

can be connected to the same signal, as is done in some studio-

type TV monitors.

A way around this problem is to connect the first monitor to the

RGB channels in the standard fashion, and then to provide a

triple gain-of-two buffer to drive the second monitor. The AD8075

is designed to provide this function and also provide excellent

high-frequency performance for high-resolution graphics signals.

Figure 3 shows a schematic of this circuit.

The outputs of the AD8075 are low impedance voltage sources

and are therefore series-terminated with 75

Ω resistors. The

internal resistors in Monitor #2 provide the terminations at its

end. The overall effect of this type of termination scheme is to

divide the signal amplitude by two. This is compensated by the

gain of two provided by the AD8075.

0.1 F

–5V

0.1 F

–5V

0.1 F

–5V

+

25 F

0.1 F

+5V

0.1 F

+5V

0.1 F

+5V

+

25 F

AD8075

75

75

75

75

75

75

75

75

75

MONITOR #1

MONITOR #2

INTERNAL

TERMINATIONS

INTERNAL

TERMINATIONS

75

75

75

R

G

B

PC GRAPHICS IC

CURRENT SOURCE

OUTPUT DRIVERS

Figure 3. Buffer

Rev. B