THS4011_16 Datasheet, PDF (17/35 Pages) Texas Instruments – 290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS

www.ti.com
THS4011
THS4012
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.
C1
V_I
R1
+
R2
_
C2
R_F
R_G
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f–3dB
1
2 RC
( ) ^{RG =}
R_F
2−
1
Q
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
Product Folder Link(s): THS4011 THS4012
Submit Documentation Feedback
17