Precision amplifier for bridge circuits

AM467

analog microelectronics

February 2011

Analog Microelectronics GmbH

Phone:

+49 (0)6131/91 073 – 0

7/9

An der Fahrt 13, D – 55124 Mainz

Fax:

+49 (0)6131/91 073 – 30

Rev. 1.1

Internet: http://www.analogmicro.de

Email:

info@analogmicro.de

Once these resistors have been mounted the calibration procedure is complete. Depending on the

accuracy of the resistors used an offset value of 0.5 V and a full scale signal of 4.5 V should have

been obtained.

Should the expected final accuracy require it, in an additional second stage (Stage 2) the offset of

the output voltage can be corrected using resistor R1. To this end the AM467 output must again be

measured at P = 0 bar. If at 0 bar the output is too high or too low by a few millivolts, for example,

the Excel program then calculates the necessary second correction of R1 in ohms.

The offset error which is then obtained if a different resistance is used in place of calculated

resistance R1 can also be determined by the Excel sheet. The sensitivity in mV/ohm is instrumental

here, providing information as to by how many millivolts the offset shifts when the used resistor R1

has a delta R (in ohms) to the calculated value. The given sensitivity value (mV/ohm) is multiplied

by the delta R of resistor R1 to obtain a change in offset which affects both the offset (0.5 V) and the

full scale signal (4.5 V).

Figure 2: Ceramic sensing element with AM467 and a calibration network

with external resistors.

DIMENSIONING

In the combination of ceramic sensing element and AM467 the operating point (the initial value of

the four resistors R1 to R4) was determined for all sensing elements in a specific sensing element

category. In the case of piezoceramic sensors the term "sensing element category" is used to denote

sensing elements with the following characteristics:

Bridge resistance: 11 kOhm ± 20%

Sensitivity: 2.4 ± 1 mV/V

IN+

IN-

AM467

3

2

AMP

VCC

8

OUT

1

GND

4

Vout

GND

C3

C2

R2

R1

R4

R3

Output stage

with short

circuit proofing

C4

C1