OPA3684 Datasheet, PDF (21/26 Pages) Burr-Brown (TI) – Low-Power, Triple Current-Feedback OPERATIONAL AMPLIFIER With Disable

BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency am-
plifier like the OPA3684 requires careful attention to board
layout parasitics and external component types. Recommen-
dations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for
all of the signal I/O pins. Parasitic capacitance on the
output and inverting input pins can cause instability; on
the noninverting input, it can react with the source
impedance to cause unintentional bandlimiting. To re-
duce unwanted capacitance, a window around the sig-
nal I/O pins should be opened in all of the ground and
power planes around those pins. Otherwise, ground and
power planes should be unbroken elsewhere on the
board.
b) Minimize the distance (< 0.25") from the power-supply
pins to high-frequency 0.1µF decoupling capacitors. At
the device pins, the ground and power-plane layout
should not be in close proximity to the signal I/O pins.
Avoid narrow power and ground traces to minimize
inductance between the pins and the decoupling capaci-
tors. The power-supply connections should always be
decoupled with these capacitors. An optional supply de-
coupling capacitor (0.01µF) across the two power sup-
plies (for bipolar operation) will improve 2nd-harmonic
distortion performance. Larger (2.2µF to 6.8µF)
decoupling capacitors, effective at lower frequencies,
should also be used on the main supply pins. These may
be placed somewhat farther from the device and may be
shared among several devices in the same area of the
PC board.
c) Careful selection and placement of external compo-
nents will preserve the high-frequency performance
of the OPA3684. Resistors should be a very low reac-
tance type. Surface-mount resistors work best and allow
a tighter overall layout. Metal film and carbon composi-
tion axially-leaded resistors can also provide good high-
frequency performance. Again, keep their leads and PC-
board trace length as short as possible. Never use
wirewound type resistors in a high-frequency applica-
tion. Since the output pin and inverting input pin are the
most sensitive to parasitic capacitance, always position
the feedback and series output resistor, if any, as close
as possible to the output pin. The quad amplifier pinout
allows each output and inverting input to be connected
by the feedback element with virtually no trace length.
Other network components, such as noninverting input
termination resistors, should also be placed close to the
package. The frequency response is primarily deter-
mined by the feedback resistor value as described
previously. Increasing its value will reduce the peaking
at higher gains, while decreasing it will give a more
peaked frequency response at lower gains. The 800Ω
feedback resistor used in the Typical Characteristics at
a gain of +2 on ±5V supplies is a good starting point for
design. Note that a 800Ω feedback resistor, rather than
a direct short, is required for the unity-gain follower
application. A current-feedback op amp requires a feed-
back resistor even in the unity-gain follower configura-
tion to control stability.
d) Connections to other wideband devices on the board
may be made with short direct traces or through onboard
transmission lines. For short connections, consider the
trace and the input to the next device as a lumped
capacitive load. Relatively wide traces (50mils to 100mils)
should be used, preferably with ground and power
planes opened up around them. Estimate the total ca-
pacitive load and set R_S from the plot of recommended
“R_S vs C_LOAD”. Low parasitic capacitive loads
(< 5pF) may not need an R_S since the OPA3684 is
nominally compensated to operate with a 2pF parasitic
load. If a long trace is required, and the 6dB signal loss
intrinsic to a doubly-terminated transmission line is ac-
ceptable, implement a matched impedance transmis-
sion line using microstrip or stripline techniques (consult
an ECL design handbook for microstrip and stripline
layout techniques). A 50Ω environment is normally not
necessary on board, and in fact a higher impedance
environment will improve distortion, see the distortion
versus load plots. With a characteristic board trace
impedance defined based on board material and trace
dimensions, a matching series resistor into the trace
from the output of the OPA3684 is used, as well as a
terminating shunt resistor at the input of the destination
device. Remember also that the terminating impedance
will be the parallel combination of the shunt resistor and
the input impedance of the destination device; this total
effective impedance should be set to match the trace
impedance. The high output voltage and current capabil-
ity of the OPA3684 allows multiple destination devices to
be handled as separate transmission lines, each with
their own series and shunt terminations. If the 6dB
attenuation of a doubly-terminated transmission line is
unacceptable, a long trace can be series-terminated at
the source end only. Treat the trace as a capacitive load
in this case and set the series resistor value as shown
in the plot of “R_S vs C_LOAD”. This will not preserve signal
integrity as well as a doubly-terminated line. If the input
impedance of the destination device is LOW, there will
be some signal attenuation due to the voltage divider
formed by the series output into the terminating imped-
ance.
e) Socketing a high-speed part like the OPA3684 is not
recommended. The additional lead length and pin-to-
pin capacitance introduced by the socket can create an
extremely troublesome parasitic network which can make
it almost impossible to achieve a smooth, stable fre-
quency response. Best results are obtained by soldering
the OPA3684 onto the board.
OPA3684
21
SBOS241A
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