THS4281 Datasheet, PDF (29/43 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

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9 Power Supply Recommendations
THS4281
SLOS432B – APRIL 2004 – REVISED OCTOBER 2015
9.1 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 operational amplifiers.
10 Layout
10.1 Layout Guidelines
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 76 to Figure 79) 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 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.
4. Connections to other wideband devices on the board should 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 (50 mils to 100 mils) should be used, preferably
with ground and power planes opened up around them. Low parasitic capacitive loads (< 4 pF) may not need
an R_(ISO), because the THS4281 is nominally compensated to operate at unity gain (+1 V/V) with a 2-pF
capacitive load. Higher capacitive loads without an R_(ISO) are allowed as the signal gain increases. If a long
trace is required, and the 6-dB 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 matching series resistor into the trace from
the output of the THS4281 is used as well as a terminating shunt resistor at the input of the destination
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