Understanding Smart Sensors - Nomads.usp

110 Understanding Smart SensorsRamp generatorU2MDC4010CT1Q1MMBT3904LT1R44.75 kΩR522.1 kΩC13.3 µ FC21.0 µ F+5.0 VPulse trainfrom microPWM outputto microX1MPX5100DPR110 kΩR210 kΩ−+R34.75 kΩU1LM311DComparator stagePressure sensorFigure 5.8 PWM output pressure sensor schematic.Autoreferencing can be performed by the MCU to correct for commonmodeerrors, especially with low-level signals. Autoreferencing uses the MCUlogic and clock signal combined with a DAC and counter. A signal from theMCU initiates the counter. The DAC provides a sample-and-hold and a programmablevoltage source. The sensor output is summed with the autoreferencecorrection at the input of an amplifier to obtain a corrected output to the system.A calibration-free method for pressure sensors has been designed using acalibrated pressure sensor, an MCU with integral ADC, and two additional ICs[14]. As shown in Figure 5.9, two input channels and one output port are usedin the calibration portion of the system. The analog switches provide voltagesV1 and V2, which are converted and stored in registers in the MCU. Theswitches are then put in the opposite direction and the new values are stored.The MCU adds the differential results that become an input to the ADC. Allerrors from the instrument amplifier are canceled in this circuit using the differentialconversion. For a measurement that requires only a full-scale accuracy of±2.5%, the offset of the pressure sensor can be neglected and the system does