Precision amplifier for bridge circuits

AM457

analog microelectronics

May 2005

Analog Microelectronics GmbH

Phone: +49 (0)6131/91 073 – 0

6/10

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

DESCRIPTION OF FUNCTIONS

AM457 is an integrated precision circuit for the high-impedance amplification and conditioning of

signals and monitoring of sensor bridges with low sensitivity, particularly for ceramic or DMS

sensing elements. The IC generates a rail-to-rail output signal of 0.2V to Vcc-0.2V. With suitable

dimensioning an output voltage of 0.5...4.5V can also be provided. As these values are standard the

following description refers to this output voltage.

Besides amplifying and conditioning signals the IC can also display a range of system errors

("system" in this context refers to the sensor bridge and the IC plus the voltage supply). AM457

works on the principle of ratiometry with a supply voltage of 5V

±5%.

The IC is distinguished by its low offset, extremely low thermal offset drift and its ability to cope

with a wide range of temperature, enabling it to be classified as a precision amplifier.

FUNCTIONS

AM457 is made up of three modular function blocks (see Figure 1) which can be accessed

individually or operated in suitable combinations. These are:

1. A precision amplifier (AMP) with a sink/source facility for the amplification and conditioning

of bridge signals within a range of

±5...±100mV FS including the output stage. The output stage

is protected against short circuits. The amplification and offset can be fixed using external

resistors.

2. An operational amplifier (OP) whose positive input is set internally to reference point 0.1

The negative input is accessible externally, with which the OP is set as an inverter. The OP may

be used as a current source to power the bridge, for example, as an output line driver or to

record temperature when a sensing element is supplied by voltage. The latter application allows

the temperature dependency of the sensing element bridge resistors to be exploited, for example.

3. A diagnostic unit which monitors system features. With a signal source in bridge configuration

this displays the following faults and errors:

• Lack of bridge voltage

• Lack of bridge ground

• Missing signal connection

• Two missing signal connections

• At least one of the four bridge resistors is of high impedance

• Malfunction of the signal conditioning unit due to an AMP overload

• Malfunction of the signal conditioning unit due to undervoltage

• Detection of excessive temperature by the signal conditioning unit.

In the event of one or more disruptions output DIAG (pin 5) switches to a logic high (see the

electrical specifications). A low is otherwise indicated.

The precision amplifier (AMP) and additional operational amplifier (OP) can be operated

independently of one another without the diagnostic unit.