Precision amplifier for bridge circuits

AM467

analog microelectronics

February 2011

Analog Microelectronics GmbH

Phone:

+49 (0)6131/91 073 – 0

6/9

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

based on a piezoceramic sensing element such as the above and is to be calibrated to an output

signal of 0.5...4.5 V (sensor system).

The basic calibration procedure

The sensor system is calibrated in two stages. Using a mathematical algorithm (Excel sheet:

Cali_AM467_rev1.xls) and taking AM467's measured output voltage values and the individual

sensor bridge values as a basis, the values for the two resistors (calibration resistors) are calculated.

In the calibration of the system all effective errors (parasitic effects and component tolerances) are

taken into account.

The sensing element is first measured and secondly the electrical characteristics of the sensor using

predefined precision measuring resistors. This information is then processed in an Excel program to

calculate the setpoint for two of the calibration resistors which are then swapped with the given

measuring resistors and added to the circuit accordingly.

If necessary, in a second stage the offset of the output voltage can be corrected using the

information given in the Excel sheet.

The procedure in detail (for calibration with discrete resistors)

In order to evaluate the sensor system signal four resistors (R1 to R4) are required (see Figure 2).

The values of the two resistors R2 and R4 are fixed for the network and do not have to be adjusted

during calibration. Like capacitors C1 to C4 they can be mounted on the circuit board at the outset.

Resistors R1 and R3 act as measuring resistors and are assembled on the measuring apparatus for all

sensor systems in one sensing element category (see Dimensioning). Their values must be as close

as possible to those calculated by Analog Microelectronics for the relevant sensing element

category. With the described components (R1 to R4 and C1 to C4) the sensor signal is at its operating

point which then enables calibration.

For this purpose a few of the electrical characteristics of the sensing element are first determined

without the evaluation circuitry being connected (Stage 1). Using the Excel calibration software

(Kali_AM467.xls) the necessary calibration parameters are: the sensing element resistance (RBR),

average output voltage of the sensing element (VBR) and the system's supply voltage (VCC).

Once these three values have been determined the sensing element is connected up to the evaluation

circuit. The output signal at the AM467 IC (Out1 and Out2) is measured at zero and full pressure

(offset and full scale signal) and entered into the Excel program with the bridge values including the

values for the given maximum pressure of the sensing element (Pmax).

Taking these values and applying them to a calibration algorithm the two resistors R1 and R3 are

calculated separately for each individual system. These then replace the measuring resistors and

have to be soldered onto the circuit board.

Resistors R1 and R3 are described as calibration resistors. These stipulate the final operating point.