OPA3680 Burr-Brown, OPA3680 Datasheet - Page 20

• Manufacturer Part Number: OPA3680
• Description: Triple / Wideband / Voltage-Feedback OPERATIONAL AMPLIFIER With Disable
• Manufacturer: Burr-Brown

This worst-case condition is still well within rated maximum
T
the 175 C maximum junction temperature rating. Careful
attention to internal power dissipation is required and per-
haps airflow considered under extreme conditions.
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high frequency
amplifier like the OPA3680 requires careful attention to
board layout parasitics and external component types. Rec-
ommendations 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 non-invert-
ing input, it can react with the source impedance to cause
unintentional bandlimiting. To reduce unwanted capacitance,
a window around the signal I/O pins should be opened in all
of the ground and power planes around those pins. Other-
wise, ground and power planes should be unbroken else-
where on the board.
b) Minimize the distance (< 0.25") from the power supply
pins to high frequency 0.1 F 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 capacitors. The power supply
connections should always be decoupled with these capaci-
tors. An optional supply decoupling capacitor (0.1 F) across
the two power supplies (for bipolar operation) will improve
2nd harmonic distortion performance. Larger (2.2 F to 6.8 F)
decoupling capacitors, effective at lower frequency, 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 OPA3680. Resistors should be a very low reactance
type. Surface-mount resistors work best and allow a tighter
overall layout. Metal film or carbon composition axially-
leaded resistors can also provide good high frequency per-
formance. Again, keep their leads and PC board traces as
short as possible. Never use wirewound type resistors in a
high frequency application. Since the output pin and invert-
ing 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. Other network
components, such as non-inverting input termination resis-
tors, should also be placed close to the package. Where
double-side component mounting is allowed, place the feed-
back resistor directly under the package on the other side of
the board between the output and inverting input pins. Even
with a low parasitic capacitance shunting the external resis-
J
for this 100
®
OPA3680
load. Heavier loads may, however, exceed
20
tors, excessively high resistor values can create significant
time constants that can degrade performance. Good axial
metal film or surface-mount resistors have approximately
0.2pF in shunt with the resistor. For resistor values > 1.5k ,
this parasitic capacitance can add a pole and/or zero below
500MHz that can effect circuit operation. Keep resistor
values as low as possible consistent with load driving con-
siderations. The 250
mance specifications is a good starting point for design.
Note that a 25 feedback resistor, rather than a direct short,
is suggested for the unity gain follower application. This
effectively isolates the inverting input capacitance from the
output pin that would otherwise cause an additional peaking
in the gain of +1 frequency response.
d) Connections to other wideband devices on the board
may be made with short direct traces or through on-board
transmission lines. For short connections, consider the trace
and the input to the next device as a lumped capacitive load.
Relatively wide traces (50 to 100mils) should be used,
preferably with ground and power planes opened up around
them. Estimate the total capacitive load and set R
plot of Recommended R
capacitive loads (< 5pF) may not need an R
OPA3680 is nominally compensated to operate with a 2pF
parasitic load. Higher parasitic capacitive loads without an
R
unloaded phase margin) If a long trace is required, and the
6dB signal loss intrinsic to a doubly terminated transmission
line is acceptable, implement a matched impedance trans-
mission line using microstrip or stripline techniques (consult
an ECL design handbook for microstrip and stripline layout
techniques). A 50
on board, and in fact, a higher impedance environment will
improve distortion as shown in 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 OPA3680
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 capa-
bility of the OPA3680 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 Recommended
R
rity as well as a doubly terminated line. If the input imped-
ance 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 impedance.
S
S
are allowed as the signal gain increases (increasing the
vs Capacitive Load. This will not preserve signal integ-
environment is normally not necessary
feedback used in the typical perfor-
S
vs Capacitive Load. Low parasitic
S
S
since the
from the