Piezoresistive pressure sensors
Digital signal processing
Total accuracy / Total error band
Ratiometric output signal
Silicon-based piezoresistive pressure sensors are made from a thin diaphragm in which resistors are embedded to form a Wheatstone bridge. When pressure is applied to the diaphragm, the electrical resistivity changes due to the mechanical stress (piezoresistive effect). If the bridge circuit is supplied with a voltage, a sensor output signal proportional to pressure is generated.
Silicon offers special benefits when it comes to manufacturing piezoresistive pressure sensor chips. Due to its single crystal structure, it shows no plastic deformation but returns to its original state without deformation after pressure loading. Material fatigue and hysteresis effects are therefore virtually eliminated. The semiconductor resistors implanted in the silicon sensor diaphragm are highly sensitive to even the smallest pressures and allow for full scale ranges of only a few millibars.
The temperature effect specifies the maximum deviation of the output signal due to temperature changes over the sensor's operating temperature range relative to a reference temperature (e.g. 25 °C). A distinction is made between the temperature effect of offset and the temperature effect of span. The temperature effect is often specified as a temperature coefficient expressed as % per °C. Temperature effects may be caused by changes in the temperature of the medium as well as changes in the ambient temperature.
The analog mV pressure signal and the corresponding temperature information from the measuring bridge are amplified and digitally converted. A microcontroller calculates the corrected pressure value using a special mathematical algorithm and sensor specific compensation coefficient swhich have been stored in the microcontroller memory. These exact coefficients have been determined by the temperature and pressure cycling of each device during production. The final sensor signal is available via a digital bus interface (e.g. I²C, SPI) and as an analog voltage output signal.
The sensor's maximum total error over the compensated temperature range. With First Sensor pressure sensors, total accuracy includes all errors from offset and span calibration, temperature effects, non-linearity and hysteresis. Total accuracy is also referred to as Total Error Band (TEB).
The maximum deviation between output readings when the same pressure value is applied with increasing and decreasing pressure under the same operating conditions.
Non-linearity refers to the maximum deviation of the sensor output from a straight line over the specified operating pressure range.The straight line can be determined using different methods such as Best Fit Straight Line (BFSL) or Terminal Base Line (TBL). The Best Fit Straight Line is fitted such that the positive and negative distances to the actual sensor output are minimised (least squares method). The Terminal Base Line runs through the start and end point of the sensor output. Non-linearity according to TBL can be much larger as if using BFSL.
The maximum deviation between repeated output readings when the same pressure value is approached from the same direction (either increasing or decreasing pressure) under the same operating conditions.
For ratiometric pressure sensors the output signal behaves proportional to the supply voltage. If for example the supply voltage for a sensor with 0.5...4.5 V output at 5 V supply increases by 10 % to 5.5 V, the output signal also increases by 10 % to 0.55...4.95 V. In contrast, sensors with an internal reference voltage have a non-ratiometric output signal.
Proof pressure is the maximum pressure which may be applied without causing durable shifts of the electrical parameters of the sensing element. Exposure to pressures above the proof pressure may lead to the sensor permanently not complying with its specification.
Burst pressure is the maximum pressure which may be applied without causing damage to the sensing element or leaks to the housing.
Piezoresistive silicon pressure sensors with extremely low pressure ranges of only a few millibars feature a very thin diaphragm which may also show sensitivity to forces which are not caused by the applied pressure but by the sensor moving and changing position.
Note: First Sensor LDE/LME/LMI ultra low pressure sensors are based on thermal flow measurement of gas through a micro-flow channel integrated within the sensor chip and show no position sensitivity.
Absolute pressure is referred to the vacuum of free space (zero pressure). In practice, absolute piezoresistive pressure sensors measure the pressure relative to a high vacuum reference sealed behind its sensing diaphragm. The vacuum has to be negligible compared to the pressure to be measured.
Differential pressure is the difference between any two process pressures. Differential pressure sensors have two separate pressure ports and can be calibrated to measure both positive and negative differential pressures (bidirectional pressure measurement).
Gage pressure is measured relative to the ambient atmospheric pressure. The average atmospheric pressure at sea level is 1013.25 mbar. Changes of the atmospheric pressure due to weather conditions or altitude directly influence the output of a gage pressure sensor and are therefore compensated.
Pressure below atmospheric pressure is called negative or vacuum gage pressure. In general a vacuum is a volume of space that is essentially empty of matter. According to its quality vacuum is divided into different ranges such as low, high and ultra high vacuum.
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