AD8045(RevB) 查看數據表（PDF） - Analog Devices
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AD8045 Datasheet PDF : 26 Pages
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AD8045
PRINTED CIRCUIT BOARD LAYOUT
Laying out the printed circuit board (PCB) is usually the last step
in the design process and often proves to be one of the most
critical. A brilliant design can be rendered useless because of a
poor or sloppy layout. Since the AD8045 can operate into the RF
frequency spectrum, high frequency board layout considerations
must be taken into account. The PCB layout, signal routing,
power supply bypassing, and grounding all must be addressed
to ensure optimal performance.
SIGNAL ROUTING
The AD8045 LFCSP features the new low distortion pinout with
a dedicated feedback pin and allows a compact layout. The
dedicated feedback pin reduces the distance from the output to
the inverting input, which greatly simplifies the routing of the
feedback network.
When laying out the AD8045 as a unity-gain amplifier, it is
recommended that a short, but wide, trace between the dedicated
feedback pin and the inverting input to the amplifier be used to
minimize stray parasitic inductance.
To minimize parasitic inductances, ground planes should be used
under high frequency signal traces. However, the ground plane
should be removed from under the input and output pins to
minimize the formation of parasitic capacitors, which degrades
phase margin. Signals that are susceptible to noise pickup
should be run on the internal layers of the PCB, which can
provide maximum shielding.
POWER SUPPLY BYPASSING
Power supply bypassing is a critical aspect of the PCB design
process. For best performance, the AD8045 power supply pins
need to be properly bypassed.
A parallel connection of capacitors from each of the power supply
pins to ground works best. Paralleling different values and sizes
of capacitors helps to ensure that the power supply pins see a
low ac impedance across a wide band of frequencies. This is
important for minimizing the coupling of noise into the amplifier.
Starting directly at the power supply pins, the smallest value
and sized component should be placed on the same side of the
board as the amplifier, and as close as possible to the amplifier,
and connected to the ground plane. This process should be
repeated for the next larger value capacitor. It is recommended
for the AD8045 that a 0.1 µF ceramic 0508 case be used. The
0508 offers low series inductance and excellent high frequency
performance. The 0.1 µF case provides low impedance at high
frequencies. A 10 µF electrolytic capacitor should be placed in
parallel with the 0.1 µF. The 10 µf capacitor provides low ac
impedance at low frequencies. Smaller values of electrolytic
capacitors may be used depending on the circuit requirements.
Additional smaller value capacitors help to provide a low
impedance path for unwanted noise out to higher frequencies
but are not always necessary.
Data Sheet
Placement of the capacitor returns (grounds), where the capacitors
enter into the ground plane, is also important. Returning the
capacitors grounds close to the amplifier load is critical for
distortion performance. Keeping the capacitors distance short,
but equal from the load, is optimal for performance.
In some cases, bypassing between the two supplies can help
to improve PSRR and to maintain distortion performance in
crowded or difficult layouts. It is brought to the attention of the
designer here as another option to improve performance.
Minimizing the trace length and widening the trace from the
capacitors to the amplifier reduce the trace inductance. A series
inductance with the parallel capacitance can form a tank circuit,
which can introduce high frequency ringing at the output. This
additional inductance can also contribute to increased distortion
due to high frequency compression at the output. The use of vias
should be minimized in the direct path to the amplifier power
supply pins since vias can introduce parasitic inductance, which
can lead to instability. When required, use multiple large diameter
vias because this lowers the equivalent parasitic inductance.
GROUNDING
The use of ground and power planes is encouraged as a method
of proving low impedance returns for power supply and signal
currents. Ground and power planes can also help to reduce stray
trace inductance and to provide a low thermal path for the
amplifier. Ground and power planes should not be used under
any of the pins of the AD8045. The mounting pads and the
ground or power planes can form a parasitic capacitance at the
amplifiers input. Stray capacitance on the inverting input and
the feedback resistor form a pole, which degrades the phase
margin, leading to instability. Excessive stray capacitance on
the output also forms a pole, which degrades phase margin.
Rev. B | Page 22 of 24
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