MAX1457 Datasheet, PDF (6/12 Pages) Maxim Integrated Products – 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
FSO Calibration
Two adjustments are required for FSO calibration. First
set the coarse gain by digitally selecting the PGA gain.
Then calibrate the bridge current by writing a 16-bit
calibration coefficient word to the FSO DAC. These two
adjustments result in a calibration resolution of ±0.2mV
(±0.005% FSO).
Linear Temperature Compensation
Temperature errors are compensated by writing 16-bit
calibration coefficients into the OFFSET TC DAC and
the FSO TC DAC (changing the current-source value
through resistive feedback from the FSOTCDAC pin to
the ISRC pin). The piezoresistive sensor is powered by
a current source resulting in a temperature-dependent
bridge voltage. The reference inputs of the OFFSET TC
DAC and FSO TC DAC are connected to the bridge
voltage. For a fixed digital word, the DAC output volt-
ages track the bridge voltage as it varies with tempera-
ture (quasi-linearly).
Multislope Temperature Compensation
The MAX1457 utilizes multislope temperature compen-
sation, allowing for compensation of arbitrary error
curves restricted only by the available adjustment
range and the shape of the temperature signal.
The MAX1457 offers a maximum of 120 calibration
points (each consisting of one OFFSET TC coefficient
and one FSO TC coefficient) over the operating temper-
ature range. Each 16-bit calibration coefficient provides
compensation of the output (either offset or FSO) with
±0.2mV (0.005% FSO) resolution. A 12-bit ADC mea-
sures the temperature-dependent bridge voltage
(BDRIVE) and selects (by addressing the EEPROM) the
corresponding offset and FSO calibration data within a
specific narrow temperature span (e.g., ≅ 1°C). The
120-segment compensation enables the MAX1457 to
compensate temperature errors for a broad range of
sensors (Figure 2).
Calculate the correction coefficients by curve-fitting to
sensor-error test data. More test points allow for better
curve-fit accuracy but result in increased test over-
head. The remaining error is further affected by the
slope of the temperature errors. For example, correct-
ing a 6% nonlinearity over temperature with 60 seg-
ments (half of the available calibration points) with
perfect curve fitting yields an error on the order of 0.1%
(6%/60). Figure 3 illustrates this compensation.
PRESSURE
SMALL
NONLINEARITY ERROR
TEMPERATURE
a) UNCOMPENSATED SENSOR ERROR
PRESSURE
SMALL
NONLINEARITY ERROR
TEMPERATURE
b) RESULTANT ERROR AFTER LINEAR COMPENSATION
PRESSURE
TEMPERATURE
c) RESULTANT ERROR AFTER MULTISLOPE COMPENSATION
Figure 3. Multislope Temperature Compensation
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