AD8045ACPZ-R2 datasheet(24/26 Pages) AD

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AD8045

Data Sheet

Rev. B | Page 22 of 24

PRINTED CIRCUIT BOARD LAYOUT

Laying out the printed circuit board (PCB) is usually the last step

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

critical. A brilliant design can be rendered useless because of a

poor or sloppy layout. Since the AD8045 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 all must be addressed

to ensure optimal performance.

SIGNAL ROUTING

The AD8045 LFCSP features the new low distortion pinout with

a dedicated feedback pin and allows a compact layout. The

dedicated feedback pin reduces the distance from the output to

the inverting input, which greatly simplifies the routing of the

feedback network.

When laying out the AD8045 as a unity-gain amplifier, it is

recommended that a short, but wide, trace between the dedicated

feedback pin and the inverting input to the amplifier be used to

minimize stray parasitic inductance.

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.

POWER SUPPLY BYPASSING

Power supply bypassing is a critical aspect of the PCB design

process. For best performance, the AD8045 power supply pins

need to be properly bypassed.

A parallel connection of capacitors from each of the power supply

pins to ground works 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 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 larger value capacitor. It is recommended

for the AD8045 that a 0.1 µF ceramic 0508 case be used. 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 may be used depending on the circuit requirements.

Additional smaller value capacitors help to 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

capacitors 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

to improve PSRR and to maintain distortion performance in

crowded or difficult layouts. It is brought to the attention of the

designer here as another option to improve performance.

Minimizing the trace length and widening the trace from the

capacitors to the amplifier reduce the trace inductance. A series

inductance with the parallel capacitance can form a tank circuit,

which can introduce high frequency ringing at the output. This

additional inductance can also contribute to increased distortion

due to high frequency compression at the output. The use of vias

should be minimized in the direct path to the amplifier power

supply pins since vias can introduce parasitic inductance, which

can lead to instability. When required, use multiple large diameter

vias because this lowers the equivalent parasitic inductance.

GROUNDING

The use of ground and power planes is encouraged as a method

of proving low impedance returns for power supply and signal

currents. Ground and power planes can also help to reduce stray

trace inductance and to provide a low thermal path for the

amplifier. Ground and power planes should not be used under

any of the pins of the AD8045. The mounting pads and the

ground or power planes can form a parasitic capacitance at the

amplifiers input. Stray capacitance on the inverting input and

the feedback resistor form a pole, which degrades the phase

margin, leading to instability. Excessive stray capacitance on

the output also forms a pole, which degrades phase margin.

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