Understanding Smart Sensors - Nomads.usp

Using MCUs/DSPs to Increase Sensor IQ 1095.04.0++++Linearity (%FS)3.02.01.00−1.00 10 20 30 40 50 60 70 80 90Span (mV)Figure 5.7 Span versus linearity for pressure sensor output.environments [12]. Figure 5.8 shows the simple, inexpensive circuitry used tocreate a duty cycle that is linear to the applied pressure. The MCU-generatedpulse train is applied to a ramp generator. The frequency and duration of thepulse can be accurately controlled in software. The MCU requires inputcaptureand output-compare timer channels. The output-capture pin is programmedto output the pulse train that drives the ramp generator, while theinput-capture pin detects edge transitions to measure the PWM output pulsewidth. The pulse width changes from 50 to almost 650 ms for zero to full-scaleoutput for this sensor.5.5.3 Autozero and AutorangeCombining a sensor and an MCU to perform a measurement that otherwisewould be less accurate or more costly than other available alternatives is feasible.The cost of many MCUs is comparable or lower than the micromachinedsensors that provide their input signal. For example, a signal conditioned pressuresensor has been combined with a 68HC05 MCU to measure 1.5 inches orless of water with an accuracy of 1% of the full-scale reading [13]. The MCUprovides software calibration, software temperature compensation, anddynamic-zero capability. Also, a digital output compatible with the SPI protocolis provided for the pressure measurement.