Issue link: https://te.mouser.com/i/1385324
mouser.com/te-connectivity 27 FIGURE 4: Wheatstone bridge. (TE Connectivity) Pressure sensors are in various markets such as automotive, industrial, medical, and consumer products. Our world is becoming more connected and smarter. Sensors are also in applications and environments that are not so friendly. Many of these applications require sensors designed to withstand exposure to corrosive and hostile chemicals. Often these chemicals exist in the medium that the sensor is trying to measure. The chemicals will have direct contact with the sensor itself. Modern sensors must withstand and resist attacks by these chemicals to provide reliability and long service life. One example is the use of a pressure sensor in a swimming watch. These watches are regularly exposed to water containing chlorine as either a dissolved gas (swimming pool) or in ionic form (ocean water). Chlorine is a strong oxidizer that quickly corrodes most metals and does irreversible damage to the product. Operation of a MEMS Pressure Sensor A typical structure of a piezoresistive pressure sensor is a planar silicon diaphragm formed by chemical or dry etching. Piezoresistors are placed near the edge of the membrane and within the sensor's linear operating range (Figure 1). When applying pressure to the micro-electromechanical system (MEMS) element, it deflects much like a trampoline (Figure 2). The deflection causes a strain in the piezoresistive elements, and they change their resistance value in proportion to the deflection (Figure 3). In practice, four piezoresistors are used, arranged in a Wheatstone bridge configuration, to maximize signal levels and FIGURE 3: Die reaction to pressure. (TE Connectivity) TE Connectivity M12 X-Code Field- Installable Connectors provide a degree of common-mode rejection to undesirable noise (Figure 4). One way to detect problems with the sensor is to look at the output signal offsets with no pressure applied. If operating correctly, the differential output should be 0V. If the output is not 0V, it's an indication of a problem with the sensing element. Corrosion from aggressive chemicals can affect how the piezoresistive components, interconnects, and wire bond pads function. The corrosion can introduce offsets in the output signal and erroneous data from the sensor. The MEMS sensor element signal is connected to a complementary metal-oxide-semiconductor (CMOS) application-specific integrated circuit (ASIC) where it is compensated for temperature linearity and other errors, then amplified and digitized. This digital data is then formatted for accessible communication to a master microprocessor via I 2 C or SPI protocol.