Precision amplifier for bridge circuits
AM467
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 (R
1
to R
4
) are required (see
Figure 2).
The values of the two resistors R
2
and R
4
are fixed for the network and do not have to be adjusted
during calibration. Like capacitors C
1
to C
4
they can be mounted on the circuit board at the outset.
Resistors R
1
and R
3
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 (R
1
to R
4
and C
1
to C
4
) 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 R
1
and R
3
are
calculated separately for each individual system. These then replace the measuring resistors and
have to be soldered onto the circuit board.
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: http://www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
+49 (0)6131/91 073 – 30
info@analogmicro.de
February 2007
6/9
Rev. 1.0
AME [ ANALOG MICROELECTRONICS ]
