Datasheet

ADA4895-1/ADA4895-2 Data Sheet
Rev. A | Page 22 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 ADA4895-1/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 ADA4895-1/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 minimizing
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 electrolytic
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.