V

OD

V

IN1

– V

IN2

+

R

f

R

(g)

1 )

2R2

R1

_

+

_

+

_

+

THS4012

THS4012

R2

R1

R2

THS415x

CIRCUIT LAYOUT CONSIDERATIONS

THS4150

THS4151

The general transfer function for this circuit is:

Figure 41. Fully Differential Instrumentation Amplifier

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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Product Folder Link(s): THS4150 THS4151