Understanding Smart Sensors - Nomads.usp

78 Understanding Smart Sensors+5V3 1750 Ω−+MC33502A/D1.0 mF0.01 mFIntegratedpressuresensor20.33 mFFigure 4.5 A rail-to-rail buffer used to reduce the output impedance of an RC filter [10].using CMOS circuitry have been developed to detect changes as small as 0.5 fF.A switched capacitor filter is an analog sampled-data circuit. The componentsof a basic circuit are an op amp, a capacitor, and an analog switch, all driven bya clock [11]. The filter’s performance is determined by the ratio of the capacitorvalues and not the absolute values. Switching the capacitor makes it behave as aresistor with an effective resistance that is proportional to the switching frequency.Figure 4.6 shows a switched-capacitor filter that has been developedfor a surface micromachined accelerometer with differential capacitive sensing[12]. This type of circuit minimizes effects of parasitic capacitance and has avery high input impedance for minimal signal loading.V REF C 1−C 3C 2+GroundV REF/2Figure 4.6 Switched-capacitor filter circuit.