THS4021_16 Datasheet, PDF (14/29 Pages) Texas Instruments – 350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4021
THS4022
SLOS265C – SEPTEMBER 1999 – REVISED JULY 2007
www.ti.com
Circuit Layout Considerations
To achieve the levels of high-frequency performance of the THS402x, follow proper printed-circuit board
high-frequency design techniques. A general set of guidelines is given as follows. In addition, a THS402x
evaluation board is available to use as a guide for layout or for evaluating the device performance.
• Ground planes—It is highly recommended that a ground plane be used on the board to provide all
components with a low-inducance ground connection. However, in the areas of the amplifier inputs and
output, the ground plane can be removed to minimize the stray capacitance.
• Proper power-supply decoupling—Use a 6.8-μF tantalum capacitor in parallel with a 0.1-μF ceramic capacitor
on each supply terminal. It may be possible to share the tantalum among several amplifiers depending on the
application, but a 0.1-μF ceramic capacitor should always be used on the supply terminal of every amplifier.
In addition, the 0.1-μF capacitor should be placed as close as possible to the supply terminal. As this distance
increases, the inductance in the connecting trace makes the capacitor less effective. The designer should
strive for distances of less than 0.1 inch (2.54 mm) between the device power terminals and the ceramic
capacitors.
• Sockets—Sockets are not recommended for high-speed operational amplifiers. The additional lead
inductance in the socket pins often produces stability problems. Surface-mount packages soldered directly to
the PCB is the best implementation.
• Short trace runs/compact part placements—Optimum high-frequency performance is achieved when stray
series inductance has been minimized. To realize this, the circuit layout should be made as compact as
possible, thereby minimizing the length of all trace runs. Particular attention should be paid to the inverting
input of the amplifier. Its length should be kept as short as possible. This helps to minimize stray capacitance
at the input of the amplifier.
• Surface-mount passive components—Using surface-mount passive components is recommended for
high-frequency amplifier circuits for several reasons. First, because of the extremely low lead inductance of
surface-mount components, the problem with stray series inductance is greatly reduced. Second, the small
size of surface-mount components naturally leads to a more compact layout, thereby minimizing both stray
inductance and capacitance. If leaded components are used, it is recommended that the lead lengths be kept
as short as possible.
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