Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 65Other approaches for digital output sensors include electrical oscillatorbased(EOB) sensors and stochastic analog-to-digital (SAD) converters [15].An EOB sensor is designed to generate a periodic current or voltage signalwhen it is subjected to a measurand. Current-to-frequency or voltageto-frequencyconversion provides a digital signal. The current-to-frequencytechnique has been demonstrated on a piezoresistive silicon pressure sensor.The frequency changed from 200–230 kHz with a pressure change from0–750 mmHg.A different approach for an EOB sensor was used with a capacitive sensor.Two equal but opposite current sources applied a square wave to a capacitivesilicon pressure sensor. The zero pressure frequency was 155 kHz, and the sensitivitywas 25 kHz/450 mmHg for that design.A ring oscillator also has been used for EOB sensors. An odd number ofintegrated-injected logic (I 2 L) gates connected in a ring on a piezoresistive pressuresensor provided the ring oscillator. Sensitivities up to 10.6 kHz/bar wereobtained with a ring oscillator frequency of 667 kHz.SAD converters employ the noise in flip-flop circuits to generate a randomsignal and use the flip-flop as a comparator. A piezoresistive sensor elementin the flip-flop circuit is one way to implement this approach for a sensor.The number of 1s and 0s is a direct measure of the strain in a pressure sensorand a simple counting procedure for an MCU. A flip-flop sensor that can measurestresses as small as 8 kPa in a silicon cantilever beam has been reported.A final digital example is provided by arrays of sensing elements [17]. Apressure switch with four switch points was fabricated using silicon fusionbonding. Pressure-switch points of 1 4 (P1), 1 2 (P2), 3 4 (P3), and 1 (P4) atmospherewere designed using controlled thinning of the diaphragm. The switchesclose electrical contacts when their desired pressure threshold is exceeded.Table 3.2 shows the truth table for the design. The outputs can be directlyapplied to a logic control.3.5 Noise/Interference AspectsThe general model for a transducer shown in Figure 1.2 indicated the interferencesources that affect the low-level output of the sensor. An actual sensorsignal combined with noise and interference from other sources, such as temperature,humidity, or vibration, can be a major problem when dynamic signalsare measured. Compared to static signals where filtering can be used to minimizenoise, dynamic measurements can pose a challenge [18].Piezoresistive pressure sensors have two dominant types of noise [19].Shot noise is the result of the nonuniform flow of carriers across a junction and