MAX1452_09 Datasheet, PDF (14/25 Pages) Maxim Integrated Products – Low-Cost Precision Sensor Signal Conditioner

Low-Cost Precision Sensor
Signal Conditioner
DRIVEN BY TESTER
THREE-STATE
NEED WEAK
PULLUP
THREE-STATE
2^ATIM +1 BYTE
TIMES
THREE-STATE
NEED WEAK
PULLUP
DIO _{1 1 1 1 1 0 1 0 0 1 1 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1}
HIGH IMPEDANCE
OUT
VALID OUT
Figure 6. Analog Output Timing
The analog signal driven onto the OUT pin is deter-
mined by the value in the ALOC register. The signals
are specified in Table 15.
Test System Configuration
The MAX1452 is designed to support an automated
production test system with integrated calibration and
temperature compensation. Figure 7 shows the imple-
mentation concept for a low-cost test system capable
of testing many transducer modules connected in par-
allel. The MAX1452 allows for a high degree of flexibili-
ty in system calibration design. This is achieved by use
of single-wire digital communication and three-state
output nodes. Depending upon specific calibration
requirements one may connect all the OUTs in parallel
or connect DIO and OUT on each individual module.
Sensor Compensation Overview
Compensation requires an examination of the sensor
performance over the operating pressure and tempera-
ture range. Use a minimum of two test pressures (e.g.,
zero and full-span) and two temperatures. More test
pressures and temperatures result in greater accuracy.
A typical compensation procedure can be summarized
as follows:
Set reference temperature (e.g., +25°C):
• Initialize each transducer by loading their respec-
tive registers with default coefficients (e.g., based
on mean values of offset, FSO and bridge resis-
tance) to prevent overload of the MAX1452.
• Set the initial bridge voltage (with the FSODAC) to
half of the supply voltage. Measure the bridge volt-
age using the BDR or OUT pins, or calculate based
on measurements.
• Calibrate the output offset and FSO of the transduc-
er using the ODAC and FSODAC, respectively.
• Store calibration data in the test computer or
MAX1452 EEPROM user memory.
Set next test temperature:
• Calibrate offset and FSO using the ODAC and FSO-
DAC, respectively.
• Store calibration data in the test computer or
MAX1452 EEPROM user memory.
• Calculate the correction coefficients.
• Download correction coefficients to EEPROM.
• Perform a final test.
Sensor Calibration and
Compensation Example
The MAX1452 temperature compensation design cor-
rects both sensor and IC temperature errors. This
enables the MAX1452 to provide temperature compen-
sation approaching the inherent repeatability of the
sensor. An example of the MAX1452’s capabilities is
shown in Figure 8.
A repeatable piezoresistive sensor with an initial offset
of 16.4mV and a span of 55.8mV was converted into a
compensated transducer (utilizing the piezoresistive
sensor with the MAX1452) with an offset of 0.5000V and
a span of 4.0000V. Nonlinear sensor offset and FSO
temperature errors, which were on the order of 20% to
30% FSO, were reduced to under ±0.1% FSO. The fol-
lowing graphs show the output of the uncompensated
sensor and the output of the compensated transducer.
Six temperature points were used to obtain this result.
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