Datasheet

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BOARD LAYOUT
Printed-Circuit Board Layout Techniques for Opti-
frequency
res

n  (44GHz mm)

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

THS4303
SLOS421B – NOVEMBER 2003 – REVISED JANUARY 2005
to the caps, and 3 or more vias to connect the power plane from the remainder of the bypass
caps to the ground plane. network.
2. Placement priority should put the smallest valued 2. By removing the 30.1- Ω resistor and ferrite bead,
capacitors closest to the device. the frequency response characteristic above 400
MHz may be modified. However, bandwidth, dis-
3. Solid power planes can lead to PCB resonances
tortion, and transient response remain optimal.
when they are not properly terminated to the
ground plane over the area and along the per- 3. Recommended values for power supply decoup-
imeter of the power plane by high frequency ling include a bulk decoupling capacitor (22 µF),
capacitors. Doing so assures that there are no a ferrite bead with a high self-resonant frequency,
power plane resonances in the needed frequency a mid-range decoupling capacitor (0.1 µF) in
range. Values used are in the range of 2 pF - 50 series with a 30.1- Ω resistor, and a high fre-
pF, depending on the frequencies to be sup- quency decoupling capacitor (47 pF).
pressed, with numerous vias for each.
4. Using 0402 or smaller component sizes is rec-
ommended. An approximate expression for the
resonate frequencies associated with a length of
mal Performance
one of the power plane dimensions is given in
equation (1). Note that a power plane of arbitrary
Achieving optimum performance with a high fre-
shape can have a number of resonant fre-
quency amplifier like the THS4303 requires careful
quencies. A power plane without distributed ca-
attention to board layout parasitics and external
pacitors and with active parts near the center of
component types.
the plane usually has n even ( ≥ 2) due to the half
Recommendations that optimize performance include:
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
band limiting. To reduce unwanted capacitance, a
• For guidance on capacitor spacing over the area
window around the signal I/O pins should be
of the ground plane, specify the lowest resonant
opened in all of the ground and power planes
frequency to be tolerated, then solve for in
around those pins. Otherwise, ground planes and
equation (1) above, with n = 2. Use this length for
power planes (if used) should be unbroken else-
the capacitor spacing. It is recommended that a
where on the board, and terminated as described
power plane, if used, be either small enough, or
in the Power Supply Decoupling section.
decoupled as described, so that there are no
2. Minimize the distance (< 0.25”) from the
resonances in the frequency range of interest. An
power supply pins to high frequency 0.1-µF
alternative is to use a ferrite bead outside of the
decoupling capacitors. At the device pins, the
opamp high frequency bypass caps to decouple
ground and power plane layout should not be in
the amplifier, and mid and high frequency bypass
close proximity to the signal I/O pins. Avoid
capacitors, from the power plane. When a trace is
narrow power and ground traces to minimize
used to deliver power, its self-resonance is given
inductance between the pins and the decoupling
approximately by equation (1), substituting the
capacitors. Note that each millimeter of a line,
trace length for power plane dimension.
that is narrow relative to its length, has ~ 0.8 nH
1. Bypass capacitors, since they have a
of inductance. The power supply connections
self-inductance, resonate with each other. To
should always be decoupled with the rec-
achieve optimum transfer characteristics through
ommended capacitors. If not properly decoupled,
2 GHz, it is recommended that the bypass
distortion performance is degraded. Larger
arrangement employed in the prototype board be
(6.8-µF to 22-µF) decoupling capacitors, effective
used. The 30.1- Ω resistor in series with the
at lower frequency, should also be used on the
0.1-µF capacitor reduces the Q of the resonance
main supply lines, preferably decoupled from the
of the lumped parallel elements including the
amplifier and mid and high frequency capacitors
0.1-µF and 47-pF capacitors, and the power
by a ferrite bead. Reference the Power Supply
supply input of the amplifier. The ferrite bead
Decoupling Techniques section. The larger caps
isolates the low frequency 22-µF capacitor and
may be placed somewhat farther from the device
17