AD8655/AD8656 Data Sheet
Rev. E | 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.
