11

FN6219.2

July 27, 2006

Differential Output Instrumentation Amplifier

The addition of a third amplifier to the conventional three

amplifier instrumentation amplifier introduces the benefits of

differential signal realization, specifically the advantage of

using common-mode rejection to remove coupled noise and

ground potential errors inherent in remote transmission. This

configuration also provides enhanced bandwidth, wider

output swing and faster slew rate than conventional three

amplifier solutions with only the cost of an additional

amplifier and few resistors.

Strain Gauge

The strain gauge is an ideal application to take advantage of

the moderate bandwidth and high accuracy of the ISL55004.

The operation of the circuit is very straightforward. As the

strain variable component resistor in the balanced bridge is

subjected to increasing strain, its resistance changes,

resulting in an imbalance in the bridge. A voltage variation

from the referenced high accuracy source is generated and

translated to the difference amplifier through the buffer

stage. This voltage difference as a function of the strain is

converted into an output voltage.

+

-

-

+

-

+

+

-

REF

+

-

FIGURE 30. DIFFERENTIAL OUTPUT AMPLIFIER

e

o3

12R

⁄

+

() e

1

e

2

–

()

–

=

e

o4

12R

⁄

+

() e

1

e

2

–

()

=

e

o

21

2R

⁄

+

() e

1

e

2

–

()

–

=

BW

2f

C1 2

,

A

Di

------------------

=

A

Di

21

2R

⁄

+

()

–

=

+

-

V+

V-

5V

1nF

(V1+V2+V3+V4)

1k

Ω

5V

1k

Ω

1nF

1k

Ω

+

-

1k

Ω

+

-

+

-

VARIABLE SUBJECT

TO STRAIN

1k

Ω

1k

Ω

1k

Ω

1k

Ω

0V

FIGURE 31. STRAIN GAUGE

ISL55004