AD8067

Rev. 0 | Page 19 of 24

GROUNDING

To minimize parasitic inductances and ground loops in high speed,

densely populated boards, a ground plane layer is critical.

Understanding where the current flows in a circuit is critical in the

implementation of high speed circuit design. The length of the

current path is directly proportional to the magnitude of the

parasitic inductances and thus the high frequency impedance of the

path. Fast current changes in an inductive ground return will create

unwanted noise and ringing.

The length of the high frequency bypass capacitor leads is critical.

A parasitic inductance in the bypass grounding will work against

the low impedance created by the bypass capacitor. Because load

currents flow from supplies as well as ground, the load should be

placed at the same physical location as the bypass capacitor ground.

For large values of capacitors, which are intended to be effective at

lower frequencies, the current return path length is less critical.

POWER SUPPLY BYPASSING

Power supply pins are actually inputs and care must be taken to

provide a clean, low noise dc voltage source to these inputs. The

bypass capacitors have two functions:

1.

Provide a low impedance path for unwanted frequencies

from the supply inputs to ground, thereby reducing the

effect of noise on the supply lines

2.

Provide localized charge storage—this is usually

accomplished with larger electrolytic capacitors

Decoupling methods are designed to minimize the bypassing

impedance at all frequencies. This can be accomplished with a

combination of capacitors in parallel to ground. Good quality

ceramic chip capacitors (X7R or NPO) should be used and always

kept as close to the amplifier package as possible. A parallel

combination of a 0.1 µF ceramic and a 10 µF electrolytic, covers a

wide range of rejection for unwanted noise. The 10 µF capacitor is

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

per supply line is sufficient.