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.

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

6

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PRESSURE

PRESSURE

PRESSURE

TEMPERATURE

SMALL

NONLINEARITY ERROR

TEMPERATURE

SMALL

NONLINEARITY ERROR

TEMPERATURE

Figure 3. Multislope Temperature Compensation