ADA4817-1ACPZ-R7 datasheet(22/29 Pages) AD

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Data Sheet

ADA4817-1/ADA4817-2

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

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Rev. H | 22 of 29

Laying out the PCB is usually the last step in the design process

and often proves to be one of the most critical. A good design

can be rendered useless because of poor layout. Because the

ADA4817-1/ADA4817-2 can operate into the radio frequency (RF)

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 a 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 amplifi-

er, it is recommended to place a short but wide trace between the

dedicated feedback pins and the inverting input to the amplifier to

minimize stray parasitic inductance.

To minimize parasitic inductances, use ground planes under high

frequency signal traces. However, remove the ground plane from

under the input and output pins to minimize the formation of para-

sitic capacitors, which degrades phase margin. Run signals are

susceptible to noise pickup 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, properly bypass the ADA4817-1/

ADA4817-2 power supply pins.

A parallel connection of capacitors from each of the power supply

pins to ground works best. Paralleling different values and sizes of

capacitors helps ensure that the power supply pins see a low ac

impedance across a wide band of frequencies, which is important

for minimizing the coupling of noise into the amplifier. Starting

directly at the power supply pins, place the smallest value and sized

component on the same side of the board as the amplifier, and

as close as possible to the amplifier, and connect it to the ground

plane. Repeat this process for the next largest value capacitor.

It is recommended to use a 0.1 µF ceramic, 0508 case for the

ADA4817-1/ADA4817-2.

The 0508 case offers low series inductance and excellent high

frequency performance. The 0.1 µF provides low impedance at high

frequencies. Place a 10 µF electrolytic capacitor in parallel with the

0.1 µF. The 10 µF electrolytic 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) is also important.

Returning the grounds of the capacitor close to the amplifier load

is critical for distortion performance. Keeping the distance of the ca-

pacitors 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. Bypassing is another option to improve perform-

ance.

Minimizing the trace length and widening the trace from the capac-

itors to the amplifier reduces the trace inductance. A series induc-

tance 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. Minimize the use of vias 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.

Do not use ground and power planes under any of the pins. The

mounting pads and the ground or power planes can form a parasitic

capacitance at the input of the amplifier. 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 PAD

The ADA4817-1/ADA4817-2 feature an exposed pad, which lowers

the thermal resistance by 25% compared to a standard SOIC

plastic package. The exposed pad 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 can 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 pad. These

additional vias help to lower the overall junction to ambient temper-

ature (θJA). Using a heavier weight copper on the surface to which

the exposed paddle of the amplifier is soldered can greatly reduce

the overall thermal resistance seen by the ADA4817-1/ADA4817-2.

LEAKAGE CURRENTS

Poor PCB layout, contaminants, and the board insulator material

can create leakage currents that are much larger than the input

bias current of the ADA4817-1/ADA4817-2. Any voltage differential

between the inputs and nearby runs sets up leakage currents

through the PCB insulator, for example, 1 V/ 100 GΩ = 10 pA.

Similarly, any contaminants, such as skin oils on the board, can

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