ADA4817-1/ADA4817-2

Rev. 0 | Page 16 of 24

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

Laying out the 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 poor layout. Because

the ADA4817-1/ADA4817-2 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 ADA4817-1/ADA4817-2 feature the new low distortion

pinout with a dedicated feedback pin that 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 ADA4817-1/ADA4817-2 as a unity-gain

amplifier, it is recommended that a short, but wide, trace be

placed between the dedicated feedback pins, 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 ADA4817-1/ADA4817-2

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 that a 0.1 μF ceramic 0508 case be used for the

ADA4817-1/ADA4817-2. The 0508 offers low series inductance

and excellent high frequency performance. The 0.1 μF provides

low impedance at high frequencies. A 10 μF electrolytic capa-

citor 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) 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 another option to improve

performance.

Minimizing the trace length and widening the trace from the

capacitors to the amplifier reduces 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 because vias can introduce

parasitic inductance, which can lead to instability. When

required to use vias, choose 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 providing 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. The mounting pads and the ground or power

planes can form a parasitic capacitance at the amplifier’s 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.

EXPOSED PADDLE

The ADA4817-1/ADA4817-2 feature an exposed paddle, which

lowers the thermal resistance by 25% compared to a standard

SOIC plastic package. The exposed paddle of the ADA4817-1/

ADA4817-2 floats internally which provides the maximum

flexibility and ease of use. It can be connected to the ground

plane or to the negative power supply plane. In cases where

thermal heating is not an issue, the exposed pad could be left

floating.

The use of thermal vias or heat pipes can also be incorporated

into the design of the mounting pad for the exposed paddle.

These additional vias help to lower the overall junction-to-

the surface to which the amplifier’s exposed paddle is soldered

can greatly reduce the overall thermal resistance seen by the

ADA4817-1/ADA4817-2.