ADA4859-3

Rev. 0 | Page 14 of 16

VIDEO LINE DRIVER

The ADA4859-3 was designed to excel in video driver applications.

Figure 36 shows a typical schematic for a video driver operating

on bipolar supplies.

1

2

3

4

11

12

CHARGE

PUMP

10

9

56

7

8

16

15

14

13

1µF

+

10µF

0.1µF

1µF

75Ω

75Ω

75Ω

75Ω

75Ω

75Ω

PD

Figure 36. Video Driver Schematic

In applications that require multiple video loads be driven

simultaneously, the ADA4859-3 can deliver. Figure 37 shows

the ADA4856-3 configured with two video loads, and Figure 38

shows the large signal performance for multiple video loads.

75Ω

CABLE

75Ω

CABLE

75Ω

75Ω

75Ω

75Ω

0.1µF

10µF

75Ω

CABLE

75Ω

+

–

Figure 37. Video Driver Schematic for Two Video Loads

6.5

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

1

10

100

1000

FREQUENCY (MHz)

Figure 38. Large Signal Frequency Response for Various Loads

POWER-DOWN

The ADA4859-3 is equipped with a PD (power-down) pin for

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

the quiescent supply current when an amplifier is not active.

The power-down threshold levels are derived from ground

level. The amplifiers are powered down when the voltage

applied to the PD pin is greater than a certain voltage from

ground. In a 5 V supply application, the voltage is greater than 2 V,

and in a 3.3 V supply application, the voltage is greater than 1.5 V.

The amplifier is enabled whenever the PD pin is left floating

(not connected). If the PD pin is not used, it is best to leave it

floating or connected to ground. Note that the power-down feature

does not control the charge pump output voltage and current.

Table 5. Power-Down Voltage Control

PD Pin

5 V

3.3 V

Not Active

<1.5 V

<1 V

Active

>2 V

>1.5 V

LAYOUT CONSIDERATIONS

As is the case with all high speed applications, careful attention

to printed circuit board (PCB) layout details prevents associated

board parasitics from becoming problematic. Proper RF design

technique is mandatory. The PCB should have a ground plane

covering all unused portions of the component side of the board to

provide a low impedance return path. Removing the ground

plane on all layers from the area near the input and output pins

reduces stray capacitance. Locate termination resistors and loads as

close as possible to their respective inputs and outputs. Keep

input and output traces as far apart as possible to minimize

coupling (crosstalk) through the board. Adherence to

microstrip or stripline design techniques for long signal traces

(greater than about 1 inch) is recommended.

POWER SUPPLY BYPASSING

Careful attention must be paid to bypassing the power supply

pins of the ADA4859-3. Use high quality capacitors with low

equivalent series resistance (ESR), such as multilayer ceramic

capacitors (MLCCs), to minimize supply voltage ripple and

power dissipation. A large, usually tantalum, 10 μF to 47 μF

capacitor located in proximity to the ADA4859-3 is required to

provide good decoupling for lower frequency signals. In

addition, locate 0.1 μF MLCC decoupling capacitors as close to

each of the power supply pins as is physically possible, no more

than 1/8-inch away. The ground returns should terminate

immediately into the ground plane. Locating the bypass capacitor

return close to the load return minimizes ground loops and

improves performance.