Precision amplifier for bridge circuits

AM457

analog microelectronics

May 2005

Analog Microelectronics GmbH

Phone: +49 (0)6131/91 073 – 0

8/10

An der Fahrt 13, D – 55124 Mainz

Fax:

+49 (0)6131/91 073 – 30

Rev. 2.2

Internet: www.analogmicro.de

Email:

info@analogmicro.de

The formulae used are:

RS

B

2

1

d

5

R

2

R

R

⋅

⋅

=

=

Span adjustment

(1)

100

R

R

1

Auxiliary resistor (2)

RS

RO

4

RS

RO

3

d

9

d

8

R

)

d

d

8

(

R

⋅

+

⋅

⋅

+

⋅

−

=

Offset adjustment (3)

whereby the following applies to the individual sensing elements:

resistance of the sensing element to be calibrated are first determined. The sensing element is then

connected up to an AM457 to form the circuit illustrated in Figure 2. The resistors are dimensioned

and applied according to the given formulae. The values of the capacitors are selected according to

Using the calculated resistances and the given capacitors the offset and span (0.5...4.5V) should

now be set. Discrepancies between the resistors used and the theoretical value enter the offset error

proportionally.

The calibration suggested in this article is suitable and has a sufficient accuracy for all sensing

elements

• which consist of four bridge resistors

• which permit calibration with discrete resistors

• which have a sensitivity of ±5…±100mV FS

• where the span signal is greater than the offset.

Depending on the level of accuracy required, when using sensing elements which have additional

resistors (such as ceramic sensing elements for offset correction, for example) the discrete adjusting

resistors must be corrected due to the discrepancy between the model and the sensing element (see

[2]).