Understanding Smart Sensors - Nomads.usp

Getting Sensor Information Into the MCU 77InputsignalBandwidth,loop gainDriveamplifierChanging input impedance,current spikesAccuracy,Outputsampling rateimpedanceAnalogINADCDigitalOUTSampleddataA/D highreferenceFull-scaleoutput voltageZero pressureoffset voltageA/D low referenceSensor’sfull-scalevoltagespanA/DdynamicrangeOverall noise, distortion,effective number of bits(a)(b)Figure 4.4 (a) Drive amplifier and ADC block diagram, and (b) effect of rail-to-rail operationon A/D performance. (After: [9].)requirements of high-performance analog functions. For the amplifier to contributeno additional error to the ADC, the amplifier’s signal-to-noise ratioshould be better than the theoretical best-case dynamic range of the ADC.Depending on the amount of noise present, the combination of the driveamplifier and the ADC will determine the effective number of bits of resolution.The A/D dynamic range is the full-scale value (high reference minus theA/D low reference). The normal output from the sensor must be inside theA/D dynamic range to ensure proper operating headroom.A single-pole (or multiple-pole) resistance-capacitance (RC) filter can beused to minimize the sensor noise that otherwise would be passed to an ADC[10]. In applications in which the ADC is sensitive to high source impedance, abuffer should be used. A rail-to-rail buffer amplifier is used in Figure 4.5 withan integrated pressure sensor (IPS) that also has a rail-to-rail output swing. Therail-to-rail input and output capability of the op amp avoids saturating thebuffer.4.2.4 Switched-Capacitor AmplifierCMOS is preferred for signal conditioning sensors with capacitive outputbecause of the high input impedance of CMOS. Another advantage of CMOSis low-power-supply current requirements. Switched-capacitor techniques