Datasheet

OPA2690
SBOS238G –JUNE 2002–REVISED MARCH 2010 www.ti.com
BOARD LAYOUT GUIDELINES
0.2pF in shunt with the resistor. For resistor
Achieving optimum performance with a
values > 1.5kΩ, this parasitic capacitance can
high-frequency amplifier like the OPA2690 requires
add a pole and/or zero below 500MHz that can
careful attention to board layout parasitics and
affect circuit operation. Keep resistor values as
external component types. Recommendations that
low as possible consistent with load driving
will optimize performance include:
considerations. The 402Ω feedback used in the
a. Minimize parasitic capacitance to any ac
Electrical Characteristics is a good starting point
ground for all of the signal I/O pins. Parasitic
for design. Note that a 25Ω feedback resistor,
capacitance on the output and inverting input pins
rather than a direct short, is suggested for the
can cause instability: on the noninverting input, it
unity-gain follower application. This effectively
can react with the source impedance to cause
isolates the inverting input capacitance from the
unintentional bandlimiting. To reduce unwanted
output pin that would otherwise cause an
capacitance, a window around the signal I/O pins
additional peaking in the gain of +1 frequency
should be opened in all of the ground and power
response.
planes around those pins. Otherwise, ground and
d. Connections to other wideband devices on the
power planes should be unbroken elsewhere on
board may be made with short, direct traces or
the board.
through onboard transmission lines. For short
b. Minimize the distance (< 0.25") from the
connections, consider the trace and the input to
power-supply pins to high-frequency 0.1mF
the next device as a lumped capacitive load.
decoupling capacitors. At the device pins, the
Relatively wide traces (50mils or 1,27mm to
ground and power-plane layout should not be in
100mils or 2,54mm) should be used, preferably
close proximity to the signal I/O pins. Avoid
with ground and power planes opened up around
narrow power and ground traces to minimize
them. Estimate the total capacitive load and set
inductance between the pins and the decoupling
R
S
from the plot of Recommended R
S
vs
capacitors. The power-supply connections should
Capacitive Load (Figure 15 for ±5V and Figure 30
always be decoupled with these capacitors. An
for +5V). Low parasitic capacitive loads (< 3pF)
optional supply decoupling capacitor (0.1mF)
may not need an R
S
because the OPA2690 is
across the two power supplies (for bipolar
nominally compensated to operate with a 2pF
operation) will improve 2nd-harmonic distortion
parasitic load. Higher parasitic capacitive loads
performance. Larger (2.2mF to 6.8mF) decoupling
without an R
S
are allowed as the signal gain
capacitors, effective at lower frequencies, should
increases (increasing the unloaded phase
also be used on the main supply pins. These may
margin; see Figure 49). If a long trace is required,
be placed somewhat farther from the device and
and the 6dB signal loss intrinsic to a
may be shared among several devices in the
doubly-terminated transmission line is
same area of the PCB.
acceptable, implement a matched impedance
transmission line using microstrip or stripline
c. Careful selection and placement of external
techniques (consult an ECL design handbook for
components will preserve the high-frequency
microstrip and stripline layout techniques). A 50Ω
performance of the OPA2690. Resistors should
environment is normally not necessary on board,
be a very low reactance type. Surface-mount
and in fact, a higher impedance environment will
resistors work best and allow a tighter overall
improve distortion as shown in the distortion
layout. Metal film or carbon composition
versus load plots (Figure 7 for the ±5v and
axially-leaded resistors can also provide good
Figure 32 for the +5V). With a characteristic
high-frequency performance. Again, keep their
board trace impedance defined (based on board
leads and PCB traces as short as possible. Never
material and trace dimensions), a matching series
use wirewound type resistors in a high-frequency
resistor into the trace from the output of the
application. Since the output pin and inverting
OPA2690 is used as well as a terminating shunt
input pin are the most sensitive to parasitic
resistor at the input of the destination device.
capacitance, always position the feedback and
Remember also that the terminating impedance
series output resistor, if any, as close as possible
will be the parallel combination of the shunt
to the output pin. Other network components,
resistor and the input impedance of the
such as noninverting input termination resistors,
destination device; this total effective impedance
should also be placed close to the package. Even
should be set to match the trace impedance. The
with a low parasitic capacitance shunting the
high output voltage and current capability of the
external resistors, excessively high resistor
OPA2690 allows multiple destination devices to
values can create significant time constants that
be handled as separate transmission lines, each
can degrade performance. Good axial metal film
with their own series and shunt terminations. If
or surface-mount resistors have approximately
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