ADA4855-3

Data Sheet

Rev. A | Page 16 of 20

RGB VIDEO DRIVER

Figure 51 shows a typical RGB driver application using dual

supplies. The gain of the amplifier is set at +2, where RF = RG =

1 kΩ. The amplifier inputs are terminated with shunt 75 Ω

resistors, and the outputs have series 75 Ω resistors for proper

video matching. In Figure 51, the PD pin is not shown connected

to any signal source for simplicity. If the power-down function

is not used, it is recommended that the PD pin be tied to the

positive supply or be left floating (not connected).

ADA4855-3

PD

16

15

14

13

5

6

7

8

12

11

10

9

1

2

3

4

0.1µF

0.1µF

0.1µF

0.1µF

10µF

0.1µF

+

0.1µF

10µF

75Ω

1kΩ

1kΩ

1kΩ 1kΩ

75Ω

75Ω

75Ω

1kΩ

1kΩ

75Ω

75Ω

–VS

–VS

+VS

+VS

VOUT (R)

VIN (R)

VIN (G)

VIN (B)

VOUT (G)

VOUT (B)

Figure 51. RGB Video Driver

DRIVING MULTIPLE VIDEO LOADS

Each amplifier in the ADA4855-3 can drive up to three video

loads simultaneously, as shown in Figure 52. When driving

three video loads, the ADA4855-3 maintains its excellent

performance for 0.1 dB flatness and 3 dB bandwidth. Figure 53

shows the large signal frequency response of the ADA4855-3

with three different load configurations: 150 Ω, 75 Ω and 50 Ω.

75Ω

CABLE

75Ω

CABLE

75Ω

75Ω

75Ω

VOUT2

VOUT1

–VS

+VS

VIN

0.1µF

0.1µF

10µF

10µF

75Ω

CABLE

75Ω

75Ω

75Ω

CABLE

75Ω

75Ω

VOUT3

+

–

RF

1kΩ

RG

1kΩ

ADA4855-3

0.1µF

Figure 52. Video Driver Schematic for Triple Video Loads

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

110

100

FREQUENCY (MHz)

200

RL =150Ω

VOUT =2V p-p

G = 2

RL =75Ω

RL =50Ω

Figure 53. Large Signal Frequency Response vs. Loads

PD (POWER-DOWN) PIN

The ADA4855-3 is equipped with a PD (power-down) pin

for all three amplifiers. This allows the user to reduce the

quiescent supply current when an amplifier is inactive. The

power-down threshold levels are derived from the voltage

applied to the +VS pin. When used in single-supply applications,

this is especially useful with conventional logic levels. The

amplifier is enabled when the voltage applied to the PD pin is

greater than +VS − 1.25 V. In a single-supply application, the

voltage threshold is typically +3.75 V, and in a ±2.5 V dual-

supply application, the voltage threshold is typically +1.25 V.

The amplifier is also enabled when the PD pin is left floating (not

connected). However, the amplifier is powered down when the

voltage on the PD pin is lower than 2.5 V from +VS. If the PD

pin is not used, it is best to connect it to the positive supply.

Table 7. Power-Down Voltage Control

PD Pin

5 V

±2.5 V

3 V

Not Active

>3.75 V

>1.25 V

>1.75 V

Active

<2 V

<0 V

<1 V