AD8091/AD8092

Rev. C | Page 12 of 20

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

POWER SUPPLY BYPASSING

Power supply pins are actually inputs, and care must be taken so

that a noise-free stable dc voltage is applied. The purpose of

bypass capacitors is to create low impedances from the supply

to ground at all frequencies, thereby shunting or filtering a

majority of the noise.

Decoupling schemes are designed to minimize the bypassing

impedance at all frequencies with a parallel combination of

capacitors. Chip capacitors of 0.01 μF or 0.001 μF (X7R or

NPO) are critical and should be as close as possible to the

amplifier package. Larger chip capacitors, such as the 0.1 μF

capacitor, can be shared among a few closely spaced active

components in the same signal path. A 10 μF tantalum

capacitor is less critical for high frequency bypassing and, in

most cases, only one per board is needed at the supply inputs.

GROUNDING

A ground plane layer is important in densely packed PC boards

to spread the current-minimizing parasitic inductances.

However, an understanding of where the current flows in a

circuit is critical to implementing effective high speed circuit

design. The length of the current path is directly proportional to

the magnitude of parasitic inductances and thus the high

frequency impedance of the path. High speed currents in an

inductive ground return create an unwanted voltage noise.

The lengths of the high frequency bypass capacitor leads are

most critical. A parasitic inductance in the bypass grounding

works against the low impedance created by the bypass

capacitor. Place the ground leads of the bypass capacitors at the

same physical location. Because load currents flow from the

supplies as well, the ground for the load impedance should be at

the same physical location as the bypass capacitor grounds. For

the larger value capacitors, which are intended to be effective at

lower frequencies, the current return path distance is less

critical.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can

be sensitive to parasitic capacitance between the inputs and

ground. A few pF of capacitance reduces the input impedance

at high frequencies, in turn increasing the amplifier’s gain and

causing peaking of the frequency response or even oscillations,

if severe enough. It is recommended that the external passive

components, which are connected to the input pins, be placed

as close as possible to the inputs to avoid parasitic capacitance.

The ground and power planes must be kept at a distance of at

least 0.05 mm from the input pins on all layers of the board.

INPUT-TO-OUTPUT COUPLING

The input and output signal traces should not be parallel to

minimize capacitive coupling between the inputs and output

and to avoid any positive feedback.