Data Sheet AD8067
Rev. B | Page 17 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
creates unwanted noise and ringing.
The length of the high frequency bypass capacitor leads is
critical. A parasitic inductance in the bypass grounding works
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:
• Provide a low impedance path for unwanted frequencies
from the supply inputs to ground, thereby reducing the effect
of noise on the supply lines
• 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.
