Data Sheet

AD8003

Rev. C | Page 13 of 16

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.

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.