OPA2673 Datasheet, PDF (31/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
www.ti.com..................................................................................................................................................... SBOS382A – JUNE 2008 – REVISED OCTOBER 2008
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a
high-frequency amplifier such as the OPA2673
requires careful attention to board layout parasitic and
external component types. Recommendations that
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 band
limiting. 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.
Otherwise, ground and power planes should be
unbroken elsewhere on the board.
b) Minimize the distance (< 0.25in, or 6,350mm)
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 capacitors. The
power-supply connections (on pins 7 and 14 for a
QFN package) should always be decoupled with
these capacitors. An optional supply decoupling
capacitor across the two power supplies (for bipolar
operation) improves second-harmonic distortion
performance. Larger (2.2µF to 6.8µF) decoupling
capacitors, effective at a lower frequency, should also
be used on the main supply pins. These can be
placed somewhat farther from the device and may be
shared among several devices in the same area of
the PCB.
c) Careful selection and placement of external
components preserve the high-frequency
performance of the OPA2673. Resistors should be
of a very low reactance type. Surface-mount resistors
work best and allow a tighter overall layout. Metal film
and carbon composition axially-leaded resistors can
also provide good high-frequency performance.
Again, keep the leads and PCB trace length as short
as possible. Never use wire-wound type resistors in a
high-frequency application. Although 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. Other network components, such as
noninverting input termination resistors, should also
be placed close to the package. Where double-side
component mounting is allowed, place the feedback
resistor directly under the package on the other side
of the board between the output and inverting input
pins. The frequency response is primarily determined
by the feedback resistor value as described
previously. Increasing the value reduces the
bandwidth, whereas decreasing it gives a more
peaked frequency response. The 402Ω feedback
resistor used in the Typical Characteristics at a gain
of +4V/V on ±6V supplies is a good starting point for
design. Note that a 511Ω feedback resistor, rather
than a direct short, is recommended for the unity-gain
follower application. A current-feedback op amp
requires a feedback resistor even in the unity-gain
follower configuration 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, or 1,27mm to
2,54mm) should be used, preferably with ground and
power planes opened up around them. Estimate the
total capacitive load and set R_S from the plot of
Differential R_S vs Capacitive Load (Figure 27). Low
parasitic capacitive loads (< 5pF) may not need an
R_S because the OPA2673 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 acceptable,
implement a matched impedance transmission line
using microstrip or stripline techniques (consult an
ECL design handbook for microstrip and stripline
layout techniques). A 50Ω environment is normally
not necessary onboard. In fact, a higher impedance
environment improves 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 OPA2673 is used, as well
as a terminating shunt resistor at the input of the
destination device. Remember also that the
terminating impedance is 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 capability of the OPA2673 allows multiple
destination devices to be handled as separate
transmission lines, each with respective 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 Differential R_S vs Capacitive Load
(Figure 27). However, this approach does not
preserve signal integrity as well as a
doubly-terminated line. If the input impedance of the
destination device is low, there is some signal
attenuation because of the voltage divider formed by
the series output into the terminating impedance.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2673
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