THS4031 Datasheet, PDF (28/38 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031, THS4032
100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
SLOS224C – JULY 1999 – REVISED APRIL 2000
APPLICATION INFORMATION
circuit-layout considerations
In order to achieve the levels of high-frequency performance of the THS403x, it is essential that proper
printed-circuit board high-frequency design techniques be followed. A general set of guidelines is given below.
In addition, a THS403x evaluation board is available to use as a guide for layout or for evaluating the device
performance.
D Ground planes – It is highly recommended that a ground plane be used on the board to provide all
components with a low inductive ground connection. However, in the areas of the amplifier inputs and
output, the ground plane can be removed to minimize the stray capacitance.
D 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 between the device power terminals
and the ceramic capacitors.
D Sockets – Sockets are not recommended for high-speed operational amplifiers. The additional lead
inductance in the socket pins will often lead to stability problems. Surface-mount packages soldered directly
to the printed-circuit board is the best implementation.
D 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 will help to minimize stray
capacitance at the input of the amplifier.
D 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.
general PowerPAD™ design considerations
The THS403x is available in a thermally enhanced DGN package, which is a member of the PowerPAD™ family
of packages. This package is constructed using a downset leadframe upon which the die is mounted [see Figure
54(a) and Figure 54(b)]. This arrangement results in the leadframe being exposed as a thermal pad on the
underside of the package [see Figure 54(c)]. Because this thermal pad has direct thermal contact with the die,
excellent thermal performance can be achieved by providing a good thermal path away from the thermal pad.
The PowerPAD package allows for both assembly and thermal management in one manufacturing operation.
During the surface-mount solder operation (when the leads are being soldered), the thermal pad can also be
soldered to a copper area underneath the package. Through the use of thermal paths within this copper area,
heat can be conducted away from the package into either a ground plane or other heat-dissipating device.
The PowerPAD™ package represents a breakthrough in combining the small area and ease of assembly of
surface mount with the heretofore awkward mechanical methods of heatsinking.
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