Understanding Smart Sensors - Nomads.usp

76 Understanding Smart SensorsFractionalload currentdetector%ofI LCurrentthresholddetectorAwake tosleep modedelay circuitI HysteresisBufferBufferI EnableC StorageBiasBiasboostI LInputshapeInterfaceshapeV inI BiasI SleepI AwakeEnableOutputstageV outR LOverdrivecorrectionSleep modecurrentregulatorAwake modecurrentregulatorFigure 4.3 Block diagram of SLEEPMODE operational amplifier.4.2.3 Rail-to-Rail Operational SimplifiersBattery-powered circuits are requiring system operation at progressively lowervoltages. An operational amplifier has been designed that avoids the highthreshold voltage of CMOS amplifiers and operates with a single-sided supplyat voltages as low as 1V or a dual supply voltage of ±0.5V [8]. The rail-to-raildesign has an output stage that is current-boosted to deliver at least 10 mA tothe load within 50 mV of the supply rails at 1V. The circuit can operate at voltagesup to 7V. The input stage is a folded cascode that uses variable-thresholddepletion mode MOSFETs. Specialized circuits of this type can be useful inproviding performance for sensor signal conditioning in specific applications.Figure 4.4 shows the effect of rail-to-rail operation [9]. Traditionally,a CMOS process has had difficulty achieving low-drift and low-noise