AD8003ACPZ-R2(RevA) Просмотр технического описания (PDF) - Analog Devices

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AD8003ACPZ-R2
(Rev.:RevA)

Triple, 1.5 GHz Op Amp

Analog Devices 

PRINTED CIRCUIT BOARD LAYOUT

Printed circuit board (PCB) layout is usually one of the last

steps in the design process and often proves to be one of the

most critical. A high performance design can be rendered

mediocre due to poor or sloppy layout. Because the AD8003

can operate into the RF frequency spectrum, high frequency

board layout considerations must be taken into account. The

PCB layout, signal routing, power supply bypassing, and

grounding must all be addressed to ensure optimal

performance.

LOW DISTORTION PINOUT

The AD8003 LFCSP features ADI’s low distortion pinout. The

pinout lowers the second harmonic distortion and simplifies the

circuit layout. The close proximity of the noninverting input

and the negative supply pin creates a source of second harmonic

distortion. Physical separation of the noninverting input pin

and the negative power supply pin reduces this distortion.

By providing an additional output pin, the feedback resistor

can be connected directly between the feedback pin and the

inverting input. This greatly simplifies the routing of the

feedback resistor and allows a more compact circuit layout,

which reduces its size and helps to minimize parasitics and

increase stability.

SIGNAL ROUTING

To minimize parasitic inductances, ground planes should be

used under high frequency signal traces. However, the ground

plane should be removed from under the input and output pins

to minimize the formation of parasitic capacitors, which

degrades phase margin. Signals that are susceptible to noise

pickup should be run on the internal layers of the PCB, which

can provide maximum shielding.

EXPOSED PADDLE

The AD8003 features an exposed paddle, which lowers the

thermal resistance by approximately 40% compared to a

standard SOIC plastic package. The paddle can be soldered

directly to the ground plane of the board. Thermal vias or heat

pipes can also be incorporated into the design of the mounting

pad for the exposed paddle. These additional vias improve the

thermal transfer from the package to the PCB. Using a heavier

weight copper also reduces the overall thermal resistance path

to ground.

AD8003

POWER SUPPLY BYPASSING

Power supply bypassing is a critical aspect of the PCB design

process. For best performance, the AD8003 power supply pins

need to be properly bypassed.

Each amplifier has its own supply pins brought out for the

utmost flexibility. Supply pins can be commoned together or

routed to a dedicated power plane. Commoned supply connections

can also reduce the need for bypass capacitors on each supply

line. The exact number and values of the bypass capacitors are

dictated by the design specifications of the actual circuit.

A parallel combination of different value capacitors from each

of the power supply pins to ground tends to work the best.

Paralleling different values and sizes of capacitors helps to

ensure that the power supply pins see a low ac impedance across

a wide band of frequencies. This is important for minimizing

the coupling of noise into the amplifier. Starting directly at the

power supply pins, the smallest value and physical-sized

component should be placed on the same side of the board as

the amplifier, and as close as possible to the amplifier, and

connected to the ground plane. This process should be repeated

for the next largest capacitor value. It is recommended that a

0.1 μF ceramic 0508 case be used for the AD8003. The 0508

offers low series inductance and excellent high frequency

performance. The 0.1 μF case provides low impedance at high

frequencies. A 10 μF electrolytic capacitor should be placed in

parallel with the 0.1 μF. The 10 μF capacitor provides low ac

impedance at low frequencies. Smaller values of electrolytic

capacitors can be used depending on the circuit requirements.

Additional smaller value capacitors help provide a low impedance

path for unwanted noise out to higher frequencies but are not

always necessary.

Placement of the capacitor returns (grounds), where the

capacitors enter into the ground plane, is also important.

Returning the capacitor grounds close to the amplifier load is

critical for distortion performance. Keeping the capacitors

distance short, but equal from the load, is optimal for

performance.

In some cases, bypassing between the two supplies can help

improve PSRR and maintain distortion performance in

crowded or difficult layouts. Designers should note this as

another option for improving performance.

Rev. A | Page 13 of 16

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