Datasheet

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BOARD LAYOUT
Printed-Circuit Board Layout Techniques for
frequency
res
[
n (44GHz mm)
ȏ
where:
frequency
res
= the approximate power plane resonant
frequencies in GHz
= the length of the power plane dimensions in
millimeters
n = an integer (n > 1) related to the mode of the oscillation
ȏ
THS4302
SLOS403H – OCTOBER 2002 – REVISED AUGUST 2006
when they are not properly terminated to the 6. Recommended values for power supply
ground plane over the area and along the decoupling include a bulk decoupling capacitor
perimeter of the power plane by high frequency (22 μ F), a ferrite bead with a high self-resonant
capacitors. Doing so ensures that there are no frequency, a mid-range decoupling capacitor (0.1
power plane resonances in the needed frequency μ F) in series with a 30.1- Ω resistor, and a
range. Values used are in the range of 2 pF - 50 high-frequency decoupling capacitor (47 pF).
pF, depending on the frequencies to be
suppressed, with numerous vias for each. Using
0402 or smaller component sizes is
recommended. An approximate expression for
the resonant frequencies associated with a length
Optimal Performance
of one of the power plane dimensions is given in
Achieving optimum performance with a high
the following equation. Note that a power plane of
frequency amplifier like the THS4302 requires careful
arbitrary shape can have a number of resonant
attention to board layout parasitics and external
frequencies. A power plane without distributed
component types.
capacitors and with active parts near the center
of the plane usually has n even ( ≥ 2) due to the
Recommendations that optimize performance include:
half wave resonant nature of the plane.
1. Minimize parasitic capacitance to any ac
ground for all of the signal I/O pins. However,
if using a transmission line at the I/O, then place
the matching resistor as close to the part as
possible. Except for when transmission lines are
used, parasitic capacitance on the output and the
noninverting input pins can react with the load
and source impedances to cause unintentional
For guidance on capacitor spacing over the area
band limiting. To reduce unwanted capacitance, a
of the ground plane, specify the lowest resonant
window around the signal I/O pins should be
frequency to be tolerated, then solve using the
opened in all of the ground and power planes
equation above, with n = 2. Use this length for
around those pins. Otherwise, ground planes and
the capacitor spacing. It is recommended that a
power planes (if used) should be unbroken
power plane, if used, be either small enough, or
elsewhere on the board, and terminated as
decoupled as described, so that there are no
described in the Power Supply Decoupling
resonances in the frequency range of interest. An
section.
alternative is to use a ferrite bead outside the
2. Minimize the distance (< 0.25 ” ) from the
op-amp, high-frequency bypass capacitors to
power supply pins to high frequency 0.1-μ F
decouple the amplifier, and mid- and
decoupling capacitors. At the device pins, the
high-frequency bypass capacitors, from the
ground and power plane layout should not be in
power plane. When a trace is used to deliver
close proximity to the signal I/O pins. Avoid
power, its approximate self-resonance is given by
narrow power and ground traces to minimize
the equation above, substituting the trace length
inductance between the pins and the decoupling
for power plane dimension.
capacitors. Note that each millimeter of a line,
4. Bypass capacitors, because they have a
that is narrow relative to its length, has ~ 0.8 nH
self-inductance, resonate with each other. To
of inductance. The power supply connections
achieve optimum transfer characteristics through
should always be decoupled with the
2 GHz, it is recommended that the bypass
recommended capacitors. If not properly
arrangement employed in the prototype board be
decoupled, distortion performance is degraded.
used. The 30.1- Ω resistor in series with the
Larger (6.8- μ F to 22- μ F) decoupling capacitors,
0.1- μ F capacitor reduces the Q of the resonance
effective at lower frequency, should also be used
of the lumped parallel elements including the
on the main supply lines, preferably decoupled
0.1- μ F and 47-pF capacitors, and the power
from the amplifier and mid- and high-frequency
supply input of the amplifier. The ferrite bead
capacitors by a ferrite bead. See the Power
isolates the low-frequency 22- μ F capacitor and
Supply Decoupling Techniques section. The
power plane from the remainder of the bypass
larger caps may be placed somewhat farther from
network.
the device and may be shared among several
5. By removing the 30.1- Ω resistor and ferrite bead,
devices in the same area of the PC board. A very
the frequency response characteristic above 400
low inductance path should be used to connect
MHz may be modified. However, bandwidth,
the inverting pin of the amplifier to ground. A
distortion, and transient response remain optimal.
minimum of 5 vias as close to the part as
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