Precision amplifier for bridge circuits
AM467
Resistors R
1
and R
3
are described as calibration resistors. These stipulate the final operating point.
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 R
1
. 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 R
1
in ohms.
The offset error which is then obtained if a different resistance is used in place of calculated
resistance R
1
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 R
1
has a delta R (in ohms) to the calculated value. The given sensitivity value (mV/ohm) is multiplied
by the delta R of resistor R
1
to obtain a change in offset which affects both the offset (0.5 V) and the
full scale signal (4.5
V).
AM467
R3
8
VCC
C1
IN+
C4
IN-
3
R1
AMP
2
Output stage
with short
circuit proofing
4
GND
1
OUT
R2
R4
Vout
C3
C2
GND
Figure 3:
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:
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
7/9
Rev. 1.0
AME [ ANALOG MICROELECTRONICS ]
