Combining a piezoresistive pressure sensor with a microcomputer and digital signal processing yields a transducer that is both accurate and easy to use

hoosing a pressure transducer for a precision measurement system (one where the total error budget is less than 1 percent) has traditionally not been a simple matter of going to a catalogue and quickly finding a device to fit the needs of the application. The task of selection was complicated by many factors, among them specifications that were often deceptive, and hidden error sources both in the transducer and in its system interface. It was necessary not merely to select a single component—the transducer—but rather to define an entire measurement system made up of many parts, each with its own error contribution. As a result, the accuracy of the pressure-measurement system could not be determined easily from the implied accuracy of the transducer but had to be determined by time-consuming analysis and testing. Furthermore, the application often required a pressure measurement to be converted into other units for display, as when barometric pressure is used to measure the altitude of an aircraft. Such nonlinear conversions usually complicated the hardware and made it suitable for only a single function. The SMART pressure transducers described in this article provide a systems approach to precision pressure measurement; they free the systems designer from the tasks of researching, defining and verifying transducer performance. The SMART transducers result from merging three technologies: piezoresistive sensors, the microcomputer and digital signal processing. They provide compensation for temperature effects and nonlinearities, and they offer scaling and conversion to user units, all at the transducer end of a measurement system. They are characterized by a single accuracy number that defines in-system performance. What you see is what you get: There are no hidden error sources. The key benefits of the SMART transducers are superior performance over the environmental range, excellent longterm stability, freedom from interface-induced measurement errors, the ability to multiplex digital data from several transducers onto a common pair of wires, and the ability to simultaneously output multiple parameters, such as pressure and altitude.