C2

R2

R1

C1

R1=R2=R

C1=C2=C

Q=PeakingFactor

(ButterworthQ=0.707)

(

=

1

Q

2 −

)

_

+

f

–3dB

1

2 RC

THS4011

THS4012

www.ti.com

SLOS216E – JUNE 1999 – REVISED APRIL 2010

If even more attenuation is needed, a multiple-pole filter is required. The Sallen-Key filter can be used for this

task. For best results, the amplifier should have a bandwidth that is 8 to 10 times the filter frequency bandwidth.

Failure to do this can result in phase shift of the amplifier.

Figure 32. 2-Pole Low-Pass Sallen-Key Filter

CIRCUIT LAYOUT CONSIDERATIONS

To achieve the high-frequency performance levels of the THS401x, follow proper printed circuit board (PCB)

high-frequency design techniques. A general set of guidelines is given in the following paragraphs. In addition, a

THS401x 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 inductive 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-mF tantalum capacitor in parallel with a 0.1-mF 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-mF ceramic capacitor should always be used on the supply terminal of every amplifier.

In addition, the 0.1-mF 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 in 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 leads to stability problems. Surface-mount packages soldered directly to

the PCB are 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 minimizes 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.

GENERAL PowerPAD™ DESIGN CONSIDERATIONS

The THS401x is available packaged 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 33(a) and Figure 33(b)]. This arrangement results in the lead frame being exposed as a

thermal pad on the underside of the package [see Figure 33(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.

Copyright © 1999–2010, Texas Instruments Incorporated

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