ADA4855-3

Rev. 0 | Page 17 of 20

SINGLE-SUPPLY OPERATION

POWER SUPPLY BYPASSING

The ADA4855-3 is designed for a single power supply. Figure 54

shows the schematic for a single 5 V supply video driver. The

input signal is ac-coupled into the amplifier via C1. Resistor R2

and Resistor R4 establish the input midsupply reference for the

amplifier. C5 prevents constant current from being drawn

through the gain set resistor. C6 is the output coupling capacitor.

For more information on ac-coupled single-supply operation of

op amps, see Avoiding Op-Amp Instability Problems in Single-

Supply Applications, Analog Dialogue, Volume 35, Number 2,

March-May, 2001, at www.analog.com.

Careful attention must be paid to bypassing the power supply

pins of the ADA4855-3. High quality capacitors with low

equivalent series resistance (ESR), such as multilayer ceramic

capacitors (MLCCs), should be used to minimize supply voltage

ripple and power dissipation. A large, usually tantalum, 2.2 μF

to 47 μF capacitor located in close proximity to the ADA4855-3

is required to provide good decoupling for lower frequency

signals. The actual value is determined by the circuit transient

and frequency requirements. In addition, 0.1 μF MLCC decoupling

capacitors should be located as close to each of the power supply

pins and across both supplies 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.

C2

1µF

R2

50kΩ

R4

50kΩ

R3

1kΩ

C1

22µF

U1

R1

75Ω

R5

1kΩ

R6

1kΩ

C6

220µF

R7

75Ω

R8

75Ω

C5

22µF

ADA4855-3

5V

C3

10µF

C4

0.01µF

5V

LAYOUT

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. The ADA4855-3

can operate at up to 410 MHz; therefore, proper RF design

techniques must be employed. 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 and under the

input and output pins reduces stray capacitance. Signal lines

connecting the feedback and gain resistors should be kept as short

as possible to minimize the inductance and stray capacitance

associated with these traces. Termination resistors and loads

should be located as close as possible to their respective inputs

and outputs. Input and output traces should be kept 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 1 inch) is recommended. For more

information on high speed board layout, see A Practical Guide

to High-Speed Printed-Circuit-Board Layout, Analog Dialogue,

Volume 39, September 2005, at www.analog.com.

Figure 54. AC-Coupled, Single-Supply Video Driver Schematic

Another way to configure the ADA4855-3 in single-supply

operation is dc-coupled. The common-mode input voltage can

go ~200 mV below ground, which makes it a true single-supply

amplifier. However, in video applications, the black level is set at

0 V, which means that the output of the amplifier must go to

ground level as well. The ADA4855-3 has a rail-to-rail output

that can swing to within 100 mV from either rail. Figure 55

shows the schematic for adding 50 mV dc offset to the input

signal so that the output is not clipped while still properly

terminating the input with 75 Ω.

R1

3.74kΩ

U1

R2

76.8Ω

R3

1kΩ

R4

1kΩ

R5

75Ω

R6

75Ω

ADA4855-3

5V

C1

10µF

C2

0.1µF

5V

Figure 55. DC-Coupled, Single-Supply Video Driver Schematic