Precision amplifier for bridge circuits

AM457

analog microelectronics

May 2005

Analog Microelectronics GmbH

Phone: +49 (0)6131/91 073 – 0

7/10

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

EXAMPLE APPLICATION

Offset and span compensation for a Wheatstone bridge with discrete adjusting resistors.

AM457 is suitable for the amplification of all resistance bridge circuits (four-resistor bridges)

with low sensitivity.

To compensate for or calibrate the offset and span and to amplify the signal as necessary a network

was developed (see Figure 2) for a four-resistor resistance bridge circuit (such as a DMS sensing

element, for example) consisting of the resistance bridge, an AM457 IC, four resistors and a

maximum of six capacitors (see [1]). AM457's characteristics define the network and the IC

amplifies the bridge output signal to the required output values.

Figure 2: Schematic circuit diagram for resistance bridges with low sensitivity

The formulae used here to calibrate the offset and span of a bridge circuit are calculated using a

model which is based on the classic Wheatstone bridge circuit of four resistors.

On the basis of this model three simple equations were approximated for the setting of the resistors;

they are also suitable for calibration using a minimum of data pertinent to the sensing element and

without any need for further correction.

OP

AMP

Temp

V

CC

AM457

1

Diagnostic

unit

2

3

4

5

6

7

8

OUTOP

IN+

INOP

GND

OUT

DIAG

VCC

1k

9k

R

4

R

1

R

3

V=5V

CC

V

DIAG

V

OUT

GND

C

4

C

2

C

6

C

5

C

1

IN-