ADA4856-3

Rev. 0 | Page 16 of 20

SINGLE-SUPPLY OPERATION

POWER DOWN

The ADA4856-3 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

pin. When used in single-supply applications, this is especially

useful with conventional logic levels. The amplifier is enabled

In a 5 V single-supply application, the typical threshold voltage

is +3.75 V, and in a 3.3 V dual-supply application, the typical

threshold voltage is +2 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

positive supply

The ADA4856-3 can operate in single-supply applications.

Figure 46 shows the schematic for a single 5 V supply video

driver. Resistors R2 and R4 establish the midsupply reference.

Capacitor C2 is the bypass capacitor for the midsupply reference.

Capacitor C1 is the input coupling capacitor, and C6 is the output

coupling capacitor. Capacitor C5 prevents constant current from

being drawn through the internal gain set resistor. Resistor R3

sets the ac input impedance of the circuit.

For more information on 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.

C2

1µF

R2

50kΩ

R4

50kΩ

R3

1kΩ

C1

22µF

R1

50Ω

C6

220µF

R5

75Ω

R6

75Ω

C5

22µF

ADA4856-3

+5V

C3

2.2µF

C4

0.01µF

+5V

Table 6. Power-Down Voltage Control

PD Pin

5 V

±2.5 V

3.3 V

Not Active

>3.75 V

>1.25 V

>2.05 V

Active

<2 V

<0 V

<1.3 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) though the board. Adherence to microstrip

or stripline design techniques for long signal traces (greater than

about 1 inch) is recommended.

Figure 46. AC-Coupled, Single-Supply Video Driver Schematic

In addition, the ADA4856-3 can be configured in dc-coupled,

single-supply operation. The common-mode input voltage

can go about 200 mV below ground, which makes it a true single-

supply part. However, in video applications, the black level is set

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

the ground level as well. This part has a rail-to-rail output stage;

it can go as close as 100 mV from either rail. Figure 47 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.

POWER SUPPLY BYPASSING

Careful attention must be paid to bypassing the power supply pins

of the ADA4856-3. 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 ADA4856-3 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.

R1

3.74kΩ

U1

R2

76.8Ω

R3

75Ω

R4

75Ω

ADA4856-3

5V

C1

10µF

C2

0.1µF

5V

Figure 47. DC-Coupled Single Supply Video Driver Schematic