AD8655/AD8656

Rev. A | Page 18 of 20

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

POWER SUPPLY BYPASSING

Power supply pins can act as inputs for noise, so care must be

taken to apply a noise-free, stable dc voltage. The purpose of

bypass capacitors is to create low impedances from the supply

to ground at all frequencies, thereby shunting or filtering most

of the noise. Bypassing schemes are designed to minimize the

supply impedance at all frequencies with a parallel combination

of capacitors with values of 0.1 μF and 4.7 μF. Chip capacitors

of 0.1 μF (X7R or NPO) are critical and should be as close as

possible to the amplifier package. The 4.7 μF tantalum capacitor

is less critical for high frequency bypassing, and, in most cases,

only one is needed per board at the supply inputs.

GROUNDING

A ground plane layer is important for densely packed PC

boards to minimize parasitic inductances. This minimizes

voltage drops with changes in current. However, an under-

standing 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, therefore, the high frequency

impedance of the path. Large changes in currents in an

inductive ground return create unwanted voltage noise.

The length of the high frequency bypass capacitor leads is

critical, and, therefore, surface-mount capacitors are recom-

mended. A parasitic inductance in the bypass ground trace

works against the low impedance created by the bypass

capacitor. Because load currents flow from the supplies, the

ground for the load impedance should be at the same physical

location as the bypass capacitor grounds. For larger value

capacitors intended to be effective at lower frequencies, the

current return path distance is less critical.

LEAKAGE CURRENTS

Poor PC board layout, contaminants, and the board insulator

material can create leakage currents that are much larger than

the input bias current of the AD8655/AD8656. Any voltage

differential between the inputs and nearby traces creates leakage

currents through the PC board insulator, for example, 1 V/100

GΩ = 10 pA. Similarly, any contaminants on the board can

create significant leakage (skin oils are a common problem).

To significantly reduce leakage, put a guard ring (shield) around

the inputs and input leads that are driven to the same voltage

potential as the inputs. This ensures there is no voltage potential

between the inputs and the surrounding area to create any

leakage currents. To be effective, the guard ring must be driven

by a relatively low impedance source and should completely

surround the input leads on all sides, above and below, by using

a multilayer board.

The charge absorption of the insulator material itself can also

cause leakage currents. Minimizing the amount of material

between the input leads and the guard ring helps to reduce the

absorption. Also, using low absorption materials, such as

Teflon® or ceramic, may be necessary in some instances.