Datasheet

OPA690
SBOS223F –DECEMBER 2001–REVISED FEBRUARY 2010 www.ti.com
BOARD LAYOUT GUIDELINES
external resistors, excessively high resistor
Achieving optimum performance with a
values can create significant time constants that
high-frequency amplifier like the OPA690 requires
can degrade performance. Good axial metal film
careful attention to board layout parasitics and
or surface-mount resistors have approximately
external component types. Recommendations that
0.2pF in shunt with the resistor. For resistor
will optimize performance include:
values > 1.5kΩ, this parasitic capacitance can
a. Minimize parasitic capacitance to any ac
add a pole and/or zero below 500MHz that can
ground for all of the signal I/O pins. Parasitic
affect circuit operation. Keep resistor values as
capacitance on the output and inverting input pins
low as possible consistent with load driving
can cause instability: on the noninverting input, it
considerations. The 402Ω feedback used in the
can react with the source impedance to cause
Electrical Characteristics is a good starting point
unintentional bandlimiting. To reduce unwanted
for design. Note that a 25Ω feedback resistor,
capacitance, a window around the signal I/O pins
rather than a direct short, is suggested for the
should be opened in all of the ground and power
unity-gain follower application. This effectively
planes around those pins. Otherwise, ground and
isolates the inverting input capacitance from the
power planes should be unbroken elsewhere on
output pin that would otherwise cause an
the board.
additional peaking in the gain of +1 frequency
response.
b. Minimize the distance (< 0.25") from the
power-supply pins to high-frequency 0.1mF
d. Connections to other wideband devices on the
decoupling capacitors. At the device pins, the
board may be made with short, direct traces or
ground and power-plane layout should not be in
through onboard transmission lines. For short
close proximity to the signal I/O pins. Avoid
connections, consider the trace and the input to
narrow power and ground traces to minimize
the next device as a lumped capacitive load.
inductance between the pins and the decoupling
Relatively wide traces (50mils or 1,27mm to
capacitors. The power-supply connections should
100mils or 2,54mm) should be used, preferably
always be decoupled with these capacitors. An
with ground and power planes opened up around
optional supply decoupling capacitor (0.1mF)
them. Estimate the total capacitive load and set
across the two power supplies (for bipolar
R
S
from the plot of Recommended R
S
vs
operation) will improve 2nd-harmonic distortion
Capacitive Load (Figure 15 for ±5V and Figure 30
performance. Larger (2.2mF to 6.8mF) decoupling
for +5V). Low parasitic capacitive loads (< 5pF)
capacitors, effective at lower frequencies, should
may not need an R
S
because the OPA690 is
also be used on the main supply pins. These may
nominally compensated to operate with a 2pF
be placed somewhat farther from the device and
parasitic load. Higher parasitic capacitive loads
may be shared among several devices in the
without an R
S
are allowed as the signal gain
same area of the PCB.
increases (increasing the unloaded phase
margin). If a long trace is required, and the 6dB
c. Careful selection and placement of external
signal loss intrinsic to a doubly-terminated
components will preserve the high-frequency
transmission line is acceptable, implement a
performance of the OPA690. Resistors should
matched impedance transmission line using
be a very low reactance type. Surface-mount
microstrip or stripline techniques (consult an ECL
resistors work best and allow a tighter overall
design handbook for microstrip and stripline
layout. Metal film or carbon composition
layout techniques). A 50Ω environment is
axially-leaded resistors can also provide good
normally not necessary on board, and in fact, a
high-frequency performance. Again, keep their
higher impedance environment will improve
leads and PCB traces as short as possible. Never
distortion as shown in the distortion versus load
use wirewound type resistors in a high-frequency
plots. With a characteristic board trace
application. Since the output pin and inverting
impedance defined (based on board material and
input pin are the most sensitive to parasitic
trace dimensions), a matching series resistor into
capacitance, always position the feedback and
the trace from the output of the OPA690 is used
series output resistor, if any, as close as possible
as well as a terminating shunt resistor at the input
to the output pin. Other network components,
of the destination device. Remember also that the
such as noninverting input termination resistors,
terminating impedance will be the parallel
should also be placed close to the package.
combination of the shunt resistor and the input
Where double-side component mounting is
impedance of the destination device; this total
allowed, place the feedback resistor directly
effective impedance should be set to match the
under the package on the other side of the board
trace impedance. The high output voltage and
between the output and inverting input pins. Even
current capability of the OPA690 allows multiple
with a low parasitic capacitance shunting the
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