THS4001

270-MHz HIGH-SPEED AMPLIFIER

SLOS206A– DECEMBER 1997 – REVISED MARCH 1999

13

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

driving a capacitive load

Driving capacitive loads with high performance amplifiers is not a problem as long as certain precautions are

taken. The first is to realize that the THS4001 has been internally compensated to maximize its bandwidth and

slew rate performance. When the amplifier is compensated in this manner, capacitive loading directly on the

output will decrease the device’s phase margin leading to high frequency ringing or oscillations. Therefore, for

capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with the output of

the amplifier, as shown in Figure 22. A minimum value of 20

Ω should work well for most applications. For

example, in 75-

Ω transmission systems, setting the series resistor value to 75 Ω both isolates any capacitance

loading and provides the proper line impedance matching at the source end.

+

_

THS4001

CLOAD

1 k

Ω

Input

Output

1 k

Ω

20

Ω

Figure 22. Driving a Capacitive Load

circuit layout considerations

In order to achieve the levels of high frequency performance of the THS4001, 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 THS4001 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 inches between the device power

terminals and the ceramic capacitors.

D Sockets – Sockets are not recommended for high speed op amps. 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.