Measuring pressure with compensated silicon sensors and precision delta-sigma ADCs
Advent of the Modern Pressure Sensor
Many industrial, commercial, and medical applications require precision pressure measurements with ±1 to ± 0.1 percent or better accuracy over a wide dynamic range, with reasonable cost, and often with very low power consumption. The development of a silicon pressure sensor was the answer to these challenges.
The modern sensor age started in 1967 at the Honeywell Research Center where Art R. Zias and John Egan applied for patents on the edge-constrained silicon diaphragm.
Since the mid-1990s the piezoresistive, silicon-based pressure sensors called MEMS have been manufactured cost effectively in high volumes and, consequently, became the most popular pressure sensor. A MEMS device works for pressure ranges from 100mbar to 1500bar in the absolute, differential, and gauge pressure modes.
Piezoresistive silicon-based pressure sensors demonstrate substantially higher sensitivities than standard strain gauges; they have good linearity at constant temperature and acceptable hysteresis up to the destructive limit. These sensors also have some disadvantages, dictated by their “silicon” nature: strong nonlinear dependence of the fullscale signal on temperature, large initial offset, and large offset drift with temperature.
Many industrial and automotive applications require pressure measurements in the extended temperature range (–40°C to +125°C). To achieve precision pressure measurements with ±1 % or better accuracy in this wide temperature range, at least a first-order temperature compensation needs to be implemented:
VDIFF = VOS + TαVOS + P(S + TαS) (Eq. 1)
Where:
VDIFF is the differential voltage versus pressure, P, and temperature, T;
αS is the temperature coefficient of sensitivity;
αVOS is the temperature coefficient of offset.
An analog signal-conditioning approach for piezoresistive sensors is exemplified by the MAX1450. This signal conditioner can be applied to the uncompensated sensors and across -40°C to +125°C extended temperature ranges (Figure 3).
Click on image to enlarge.
Figure 3. The circuit's initial sensitivity (FSO) is adjusted at the FSOTRIM pin. The temp drift is adjusted by feeding back the sensor's drive voltage from the BDRIVE pin to the ISRC pin.
Compensation of offset and offset drift is accomplished with the programmable gain amplifier (PGA) and decoupled from the sensitivity compensation. The key function, however, is the controlled current source, which implements a unique algorithm for compensating the sensitivity drift.
A later generation IC for conditioning sensor signals (MAX1455) integrates programmable sensor excitation, a 16-step programmable-gain amplifier (PGA), 768-byte (6144 bits) internal EEPROM, and four 16-bit DACs used for FSO, offsets and span compensation.
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