Understanding Smart Sensors - Nomads.usp

Getting Sensor Information Into the MCU 814.2.8 Inherent Power-Supply RejectionIf a high-voltage power supply with poor regulation is used to provide power tothe sensor and the ADC input to the MCU, a software/hardware scheme canbe used to reduce the errors caused by supply voltage fluctuation [16]. Thescheme relies on the ratiometric properties of the sensor and the ADCs. Asshown in Figure 4.9, a programmable resistor-divider array is used to determinethe amount of attenuation of the unregulated 24V supply. A software algorithmrelies on a calibration procedure that stores data in EEPROM. The resistorvalues are chosen based on the following:• The expected range of unregulated voltages;• The resistor array resolution necessary to attenuate the range ofunregulated voltages;• Power consumption considerations in the digital switch and programmableresistor-divider array.Unregulatedsupply(24V ±20%)1V INV OUT3MC7805CT 128R264RPA 032RV DIV16RR PU8R4R2RRV DDPA 1PA 2PA 3PA 4PA 5PA 6PA 7A/D Ref HighA/D INPA 8MC68HC705B163112−+345R BMC33072Q 1MPSA06V SS2Pressure sensorFigure 4.9 An inherent power-supply rejection circuit for a high-voltage unregulatedsupply.