Understanding Smart Sensors - Nomads.usp

Getting Sensor Information Into the MCU 794.2.5 Barometer Application CircuitSensor applications that use only a limited portion of the available output froma sensor can provide interesting challenges to sensor manufacturers and circuitdesigners. Barometric measurements, for example, use only a pressure variationof a few inches of mercury. In addition, that variation is superimposed on thealtitude pressure reading, which can vary from 29.92 inches of mercury at sealevel to 16.86 inches of mercury at 15,000 feet. A circuit that interfaces atemperature-compensated and calibrated 100-kPa (29.92 inches of mercury)absolute pressure sensor to a microcontroller has been designed based on avariation of Figure 4.2 [13]. The circuit has a gain of 187 and obtains a resolutionof 0.1 inch of mercury when it is interfaced to an 8-bit MCU with an integralADC.4.2.6 4- to 20-mA Signal TransmitterThe use of monolithic two-wire transmitters can provide a low-cost, easy-tointerfacesignal conditioning solution for 4- to 20-mA current loops to transmitsensor readings in many industrial applications. The current loop avoids theproblems of voltage drop when transmitting an analog signal over a long distanceand the supply voltage, which can range from 9 to 40V. The circuitry inthese devices can also compensate for nonlinearity in a resistance temperaturedetector (RTD) or resistance bridge sensor and therefore improve the overallperformance of the sensor–signal conditioning combination. Furthermore, theavailability of additional features, such as a precision current source or a 5Vshuntregulator, can simplify the task of the circuit designer and allow use ofthose ICs for several types of sensors. Figure 4.7 shows a high-impedance pressuresensor interfaced with a transmitter IC [14]. With zero pressure applied,the output is 4 mA; with full pressure, it is 20 mA. A 240Ω resistor referencedto ground at the receiving end provides a 0.96–4.8V signal that can be interfacedto an ADC.4.2.7 Schmitt TriggerOne final circuit to be considered is the Schmitt trigger, which turns the pulsedoutput from a sensor such as an optodetector or phototransistor into a puredigital signal. Figure 4.8 shows the opto input and output of the Schmitt trigger[15]. The lower and upper thresholds in the trigger remove the linear transitionregion between the on and off states. This hysteresis filters electrical noisethat can cause the output to change state when it is close to the threshold of a