To achieve the levels of
high-frequency performance of the THS403x, follow
proper printed-circuit board (PCB), high-frequency design techniques. The following
is a general set of guidelines. In addition, a THS403x
evaluation board is available to use as a guide for layout or for evaluating the
device performance.
- Ground planes—make sure
that the ground plane used on the board provides 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 stray capacitance.
- Proper power-supply
decoupling—use a 6.8μF tantalum capacitor in parallel with a 0.1μF
ceramic capacitor on each supply pin. Sharing the tantalum capacitor among
several amplifiers is possible depending on the application, but always use a
0.1μF ceramic capacitor on the supply pin of every amplifier. In addition, place
the 0.1μF capacitor as close as possible to the supply pin. As this distance
increases, the inductance in the connecting trace makes the capacitor less
effective. Strive for distances of less than 0.1 inch (2.54mm) between the
device power pins and the ceramic capacitors.
- Short trace runs or compact
part placements—optimum high-frequency performance is achieved when
stray series inductance is minimized. To minimize stray inductance, make the
circuit layout as compact as possible, thereby minimizing the length of all
trace runs. Pay particular attention to the inputs of the amplifier, keeping the
trace lengths as short as possible. This layout helps to minimize stray
capacitance at the input of the amplifier.
- Sockets—TI does not recommend sockets for
high-speed operational amplifiers. The additional lead inductance in the socket
pins often leads to stability problems. Surface-mount packages soldered directly
to the printed-circuit board is the best implementation.
- Short trace runs and compact part
placements—Improved high-frequency performance is achieved when stray
series inductance is minimized. To reduce stray series inductance, the circuit
layout must be made as compact as possible, thereby minimizing the length of all
trace runs. Particular attention must be paid to the inverting input of the
amplifier. The length must be kept as short as possible to minimize stray
capacitance at the input of the amplifier.