ADA4861-3_16 Datasheet, PDF (16/17 Pages) Analog Devices – High Speed, Low Cost, Triple Op Amp

SINGLE-SUPPLY OPERATION
The ADA4861-3 can also be operated from a single power
supply. Figure 53 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. Capacitor C5 prevents constant
current from being drawn through the gain set resistor and
enables the ADA4861-3 at dc to provide unity gain to the input
midsupply voltage, thereby establishing the output voltage dc
operating point. Capacitor C6 is the output coupling capacitor.
For more information on single-supply operation of op amps,
see www.analog.com/library/analogDialogue/archives/35-
02/avoiding/.
+5V
C2
C3
1µF
2.2µF
+5V
V_IN
R1
50Ω
R2
50kΩ
R4
50kΩ
R3
1kΩ
C1
22µF
C4
0.01µF
C6
220µF
R5
75Ω
R6
75Ω
V_OUT
C5
22µF
ADA4861-3
–V_S
Figure 53. Single-Supply Video Driver Schematic
ADA4861-3
POWER SUPPLY BYPASSING
Careful attention must be paid to bypassing the power supply
pins of the ADA4861-3. 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 proximity to the ADA4861-3 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 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.
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 ADA4861-3
can operate at up to 730 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, go to: www.analog.com and
www.analog.com/library/analogDialogue/archives/39-
09/layout.html.
Rev. A | Page 15 of 16