OPA2890_08 Datasheet, PDF (27/35 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
www.ti.com.................................................................................................................................................... SBOS364B – DECEMBER 2007 – REVISED MAY 2008
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a
high-frequency amplifier such as the OPA2890
requires careful attention to board layout parasitics
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
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. Otherwise, ground and power planes should be
unbroken elsewhere on the board.
b) Minimize the distance (< 0.25in, or 6.35mm) 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 should always be
decoupled with these capacitors. An optional supply
decoupling capacitor (0.1µF) across the two power
supplies (for bipolar operation) improves
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 capacitors may be placed somewhat
farther from the device and may be shared among
several devices in the same area of the printed circuit
board (PCB).
c) Careful selection and placement of external
components preserves the high-frequency
performance of the OPA2890. 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 performance.
Again, keep the leads and PCB traces as short as
possible. Never use wirewound type resistors in a
high-frequency application. Because 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. Even with a low
parasitic capacitance shunting the external resistors,
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 > 1.5kΩ, 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
considerations. The 750Ω feedback used in the
Electrical Characteristics is a good starting point for
design. Note that a 0Ω feedback resistor is suggested
for the unity-gain follower application.
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 plots of
Figure 15 and Figure 36. Low parasitic capacitive
loads (< 3pF) may not need an R_S because the
OPA2890 is nominally compensated to operate with a
2pF parasitic load. Higher parasitic capacitive loads
without an R_S are allowed as the signal gain
increases (increasing the unloaded phase margin;
see Figure 61). 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 on board, and
in fact, a higher impedance environment improves
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 OPA2890 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.
e) Socketing a high-speed part such as the
OPA2890 is not recommended. The additional lead
length and pin-to-pin capacitance introduced by the
socket can create an extremely troublesome parasitic
network that can make it almost impossible to
achieve a smooth, stable frequency response. Best
results are obtained by soldering the OPA2890 onto
the board.
Copyright © 2007–2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2890
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