THS4151 Datasheet, PDF (19/35 Pages) National Semiconductor (TI) – HIGH-SPEED DIFFERENTIAL I/O AMPLIFIERS

THS4150
THS4151
www.ti.com........................................................................................................................................................... SLOS321G – MAY 2000 – REVISED MARCH 2009
The general transfer function for this circuit is:
ǒ Ǔ ^VOD
VIN1 – VIN2
+
Rf
R(g)
1
)
2R2
R1
V_IN1
THS4012
+
R_(g)
R_f
_
R2
_
R1
+
THS415x
R2
_
V_IN2
+ _THS4012
R_(g)
R_f
Figure 41. Fully Differential Instrumentation Amplifier
CIRCUIT LAYOUT CONSIDERATIONS
To achieve the levels of high frequency performance of the THS415x, follow proper printed-circuit board high
frequency design techniques. A general set of guidelines is given below. In addition, a THS415x 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-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.
• 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.
• 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.
• 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.
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