Understanding Smart Sensors - Nomads.usp

66 Understanding Smart SensorsTable 3.2Truth Table for Array of Four Pressure SwitchesPressure (P) S1 S2 S3 S4 Out1 Out2 Out3 Out4P < P1 Open Open Open Open 0 0 0 0P1 ≤ P < P2 Closed Open Open Open 1 0 0 0P2 ≤ P < P3 Closed Closed Open Open 1 1 0 0P3 ≤ P < P4 Closed Closed Closed Open 1 1 1 0P4 ≤ P Closed Closed Closed Closed 1 1 1 1is independent of temperature. Shot noise increases in sensors with increasedlevels of integration. The second type of noise, flicker noise, or 1/f, results fromcrystal defects and wafer processing. Flicker noise is proportional to the inverseof frequency and is more dominant at lower frequencies. Decoupling and filteringcan reduce these types of noise. Thermal or Johnson noise produced bythermal agitation of electronics in conductors and semiconductors is usuallysmall in those devices.3.6 Low-Power, Low-Voltage SensorsPortable sensing applications, including portable data loggers and data acquisitionsystems, require sensors to operate at low power levels and low battery supplyvoltages. For low-power pressure measurements, high-impedance pressuresensors and pulse-width modulation (PWM) techniques have been developedto reduce power drain. Depending on the type of measurement (static ordynamic) and how frequently it must be measured, the sensing system can be ina sleep mode and wake up to make periodic readings that can be transmitted todistant recording instruments. This technique is useful in process controls, hazardousmaterial monitoring systems, and a variety of data acquisition applicationsthat would have been more time consuming, dangerous, prohibitivelyexpensive, or too heavy with previous technology. Monitoring pressure is oneof the more frequent measurements that must be made in systems, rangingfrom tire pressure on vehicles to leakage monitoring in tanks, blood pressure inportable healthcare monitors, and barometric pressure in weather sondes. Thecombination of higher impedance sensors with new design approaches, such asPWM input and other power management techniques, is increasing the applicationsin this area of portable equipment.