ADA4895-2ARMZ-R7 datasheet(23/24 Pages) AD

23 / 24 page

Data Sheet

ADA4895-2

Rev. 0 | Page 23 of 24

LAYOUT CONSIDERATIONS

To ensure optimal performance, careful and deliberate attention

must be paid to the board layout, signal routing, power supply

bypassing, and grounding.

Ground Plane

It is important to avoid ground in the areas under and around the

input and output of the ADA4895-2. Stray capacitance created

between the ground plane and the input and output pads of a

device is detrimental to high speed amplifier performance. Stray

capacitance at the inverting input, along with the amplifier input

capacitance, lowers the phase margin and can cause instability.

Stray capacitance at the output creates a pole in the feedback

loop, which can reduce phase margin and can cause the circuit

to become unstable.

Power Supply Bypassing

Power supply bypassing is a critical aspect in the performance

of the ADA4895-2. A parallel connection of capacitors from each

power supply pin to ground works best. Smaller value capacitor

electrolytics offer better high frequency response, whereas larger

value capacitor electrolytics offer better low frequency

performance.

Paralleling different values and sizes of capacitors helps to ensure

that the power supply pins are provided with low ac impedance

across a wide band of frequencies. This is important for minimiz-

ing the coupling of noise into the amplifier—especially when the

amplifier PSRR begins to roll off—because the bypass capacitors

can help lessen the degradation in PSRR performance.

Place the smallest value capacitor on the same side of the board

as the amplifier and as close as possible to the amplifier power

supply pins. Connect the ground end of the capacitor directly to

the ground plane.

It is recommended that a 0.1 µF ceramic capacitor with a 0508

case size be used. The 0508 case size offers low series inductance

and excellent high frequency performance. Place a 10 µF electro-

lytic capacitor in parallel with the 0.1 µF capacitor. Depending

on the circuit parameters, some enhancement to performance

can be realized by adding additional capacitors. Each circuit

is different and should be analyzed individually for optimal

performance.

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