BOARD LAYOUT GUIDELINES
OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008 ....................................................................................................................................................
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shunt with the resistor. For resistor values > 1.5k Ω,this parasitic capacitance can add a pole and/or zeroAchieving optimum performance with a
below 500MHz that can effect circuit operation. Keephigh-frequency amplifier like the OPA4830 requires
resistor values as low as possible consistent withcareful attention to board layout parasitics and
load driving considerations. The 750 Ωfeedback usedexternal component types. Recommendations that
in the Typical Characteristics is a good starting pointoptimize performance include:
for design.a) Minimize parasitic capacitance to any ac ground
d) Connections to other wideband devices on thefor all of the signal I/O pins. Parasitic capacitance on
board may be made with short direct traces orthe output and inverting input pins can cause
through onboard transmission lines. For shortinstability: on the noninverting input, it can react with
connections, consider the trace and the input to thethe source impedance to cause unintentional
next device as a lumped capacitive load. Relativelybandlimiting. To reduce unwanted capacitance, a
wide traces (50mils to 100mils) should be used,window around the signal I/O pins should be opened
preferably with ground and power planes opened upin all of the ground and power planes around those
around them. Estimate the total capacitive load andpins. Otherwise, ground and power planes should be
set R
S
from the typical characteristic curveunbroken elsewhere on the board.
Recommended R
S
vs Capacitive Load (Figure 15 ,Figure 38 , or Figure 63 ). Low parasitic capacitiveb) Minimize the distance ( < 0.25 ” ) from the
loads (< 5pF) may not need an R
S
because thepower-supply pins to high-frequency 0.1 µF
OPA4830 is nominally compensated to operate with adecoupling capacitors. At the device pins, the ground
2pF parasitic load. Higher parasitic capacitive loadsand power-plane layout should not be in close
without an R
S
are allowed as the signal gainproximity to the signal I/O pins. Avoid narrow power
increases (increasing the unloaded phase margin). Ifand ground traces to minimize inductance between
a long trace is required, and the 6dB signal lossthe pins and the decoupling capacitors. Each
intrinsic to a doubly-terminated transmission line ispower-supply connection should always be
acceptable, implement a matched impedancedecoupled with one of these capacitors. An optional
transmission line using microstrip or striplinesupply decoupling capacitor (0.1 µF) across the two
techniques (consult an ECL design handbook forpower supplies (for bipolar operation) improves
microstrip and stripline layout techniques). A 50 Ω2nd-harmonic distortion performance. Larger (2.2 µF
environment is normally not necessary onboard, andto 6.8 µF) decoupling capacitors, effective at lower
in fact, a higher impedance environment improvesfrequency, should also be used on the main supply
distortion as shown in the distortion versus load plots.pins. These may be placed somewhat farther from
With a characteristic board trace impedance definedthe device and may be shared among several
(based on board material and trace dimensions), adevices in the same area of the PCB.
matching series resistor into the trace from the outputc) Careful selection and placement of external
of the OPA4830 is used as well as a terminatingcomponents preserve the high-frequency
shunt resistor at the input of the destination device.performance. Resistors should be a very low
Remember also that the terminating impedance is thereactance type. Surface-mount resistors work best
parallel combination of the shunt resistor and theand allow a tighter overall layout. Metal film or carbon
input impedance of the destination device; this totalcomposition axially-leaded resistors can also provide
effective impedance should be set to match the tracegood high-frequency performance. Again, keep the
impedance. If the 6dB attenuation of aleads and PCB traces as short as possible. Never
doubly-terminated transmission line is unacceptable,use wire-wound type resistors in a high-frequency
a long trace can be series-terminated at the sourceapplication. Because the output pin and inverting
end only. Treat the trace as a capacitive load in thisinput pin are the most sensitive to parasitic
case and set the series resistor value as shown in thecapacitance, always position the feedback and series
typical characteristic curve Recommended R
S
vsoutput resistor, if any, as close as possible to the
Capacitive Load (Figure 15 ,Figure 38 , or Figure 63 ).output pin. Other network components, such as
This configuration does not preserve signal integritynoninverting input termination resistors, should also
as well as a doubly-terminated line. If the inputbe placed close to the package. Where double-side
impedance of the destination device is low, there willcomponent mounting is allowed, place the feedback
be some signal attenuation due to the voltage dividerresistor directly under the package on the other side
formed by the series output into the terminatingof the board between the output and inverting input
impedance.pins. Even with a low parasitic capacitance shuntingthe external resistors, excessively high resistor valuescan create significant time constants that candegrade performance. Good axial metal film orsurface-mount resistors have approximately 0.2pF in
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