AD8091/AD8092
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.
Input-to-Output Coupling
The input and output signal traces should not be parallel to mini-
mize capacitive coupling between the inputs and output, avoiding
any positive feedback.
DRIVING CAPACITIVE LOADS
A highly capacitive load will react with the output of the amplifiers,
causing a loss in phase margin and subsequent peaking or even
oscillation, as illustrated in Figures 2 and 3. There are two
methods to effectively minimize its effect.
Decoupling schemes are designed to minimize the bypassing imped-
ance at all frequencies with a parallel combination of capacitors.
0.01 µF or 0.001 µF (X7R or NPO) chip capacitors 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.
1. Put a small value resistor in series with the output to isolate
the load capacitor from the amps’ output stage.
2. Increase the phase margin with higher noise gains or by adding a
pole with a parallel resistor and capacitor from –IN to the output.
8
6
Grounding
4
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 will
create an unwanted voltage noise.
2
0
؊2
؊4
V
= +5V
S
؊6
؊8
G = +1
R
C
V
= 2k⍀
= 50pF
= 200mV p-p
L
L
O
؊10
The length of the high-frequency bypass capacitor leads are most
critical. A parasitic inductance in the bypass grounding will work
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.
500
0.1
1
10
100
FREQUENCY – MHz
Figure 2. Closed-Loop Frequency Response: CL = 50 pF
p
2.60V
Input Capacitance
2.55V
2.50V
2.45V
2.40V
Along with bypassing and ground, high-speed amplifiers can be
sensitive to parasitic capacitance between the inputs and ground.
A few pF of capacitance will reduce the input impedance at high
frequencies, in turn increasing the amplifier’s gain, 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.
Figure 3. 200 mV Step Response: CL = 50 pF
REV. A
–11–
ADI [ ADI ]
