THS4281 Datasheet, PDF (21/37 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

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Power-Supply Decoupling Techniques and
Recommendations
Power-supply decoupling is a critical aspect of any
high-performance amplifier design. Careful
decoupling provides higher quality ac performance.
The following guidelines ensure the highest level of
performance.
1. Place decoupling capacitors as close to the
power-supply inputs as possible, with the goal of
minimizing the inductance.
2. Placement priority should put the smallest valued
capacitors closest to the device.
3. Use of solid power and ground planes is
recommended to reduce the inductance along
power-supply return current paths (with the
exception of the areas underneath the input and
output pins as noted below).
4. A bulk decoupling capacitor is recommended (6.8
μF to 22 μF) within 1 inch, and a ceramic (0.1 μF)
within 0.1 inch of the power input pins.
NOTE
The bulk capacitor may be
shared by other op amps.
BOARD LAYOUT
Achieving optimum performance with a
high-frequency amplifier like the THS4281 requires
careful attention to board layout parasitics and
external component types. See the EVM layout
figures (Figure 78 to Figure 81) in the Design Tools
section.
Recommendations that optimize performance include:
1. 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 and on the noninverting
input, it can react with the source impedance to
THS4281
SLOS432A – APRIL 2004 – REVISED NOVEMBER 2009
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.
2. Minimize the distance (< 0.1 inch) from the
power-supply pins to high-frequency, 0.1-μF
decoupling capacitors. Avoid narrow power and
ground traces to minimize inductance. The
power-supply connections should always be
decoupled as described above.
3. Careful selection and placement of external
components preserves the high-frequency
performance of the THS4281. Resistors should
be a low reactance type. Surface-mount resistors
work best and allow a tighter overall layout.
Metal-film, axial-lead 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. 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. Excessively high resistor values
can create significant phase lag that can degrade
performance. Keep resistor values as low as
possible, consistent with load-driving
considerations. It is suggested that a good
starting point for design is to set the R_f to 2 kΩ for
low-gain, noninverting applications. Doing this
automatically keeps the resistor noise terms
reasonable and minimizes the effect of parasitic
capacitance.
Copyright © 2004–2009, Texas Instruments Incorporated
Product Folder Link(s): THS4281
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