Figure 6a shows the schematic for the SOIC-package, 800-MHz AD8001 op-amp-evaluation board.
Figures 6b and 6c, respectively, show the top and bottom sides of the pc board. The circuit connects the
amplifier in the noninverting mode. The top (Figure 6b) shows the SOIC package along with input and
output SMA connectors. The ground plane is cut around the SOIC to minimize parasitic capacitance.
The bottom of the board (Figure 6c) shows the surface-mount resistors and capacitors that comprise the
op-amp gain-setting and power-supply decoupling circuits, respectively.
In high-speed, high-precision ICs, pay special attention to power-supply decoupling. For example,
fast-slewing signals into relatively low-impedance loads produce high-speed transient currents at an op
amp's power-supply pins. These transient currents produce corresponding voltages across any parasitic
impedance that exists in the power-supply traces. These voltages can couple to the amplifier output
because of the op amp's finite power-supply rejection at high frequencies.
A three-capacitor decoupling scheme for the AD8001 evaluation board ensures a low-impedance path to
ground at all transient frequencies. The 1000-pF and 0.01-£gF ceramic capacitors shunt the highest
frequency transients to ground. These capacitors sit as close to the power-supply pins as possible to
minimize any series inductance and resistance. Because these components are surface-mount, minimum
stray inductance and resistance exist in the path to the ground plane. The 10-£gF tantalum capacitors
shunt the lower frequency transient currents to ground.
The input and output signal traces of the AD8001 evaluation board are 50 Ohm microstrip transmission
lines. A considerable amount of continuous ground-plane area exists on both sides of the pc board.
Plated-through holes connect the top and bottom ground planes at several points to maintain the
continuity of this low-impedance ground at high frequencies. |
