Understanding Smart Sensors - Nomads.usp

72 Understanding Smart Sensorsthe task of signal conditioning a broad range of sensor and application circuitsdeveloped for piezoresistive pressure sensors with specific performancecharacteristics.4.2 Amplification and Signal ConditioningMicromachining is used to manufacture a diaphragm or beam thickness tonominal targets. Microelectronics is used to provide the precision for semiconductorsensors. Accuracy combined with ease of interface, cost, power consumption,printed circuit board space, and power supply voltage can be amongthe considerations in the selection of a signal conditioning IC. The semiconductortechnologies used for the modular amplifiers and ICs in this sectionhave a major impact on those criteria.Sensor signal conditioning circuits are based on two fundamental technologies:bipolar and CMOS. However, those two technologies have a vastnumber of derivatives. The requirements of the application should be used todetermine which technology or derivative is appropriate. A performance comparisonbetween bipolar and CMOS for signal conditioning circuits is shownin Table 4.1 [1]. The comparison was performed in the early 1990s. Processtechnology improvements in continuously and rapidly shrinking CMOSprocesses have changed some of the parameters in favor of CMOS, especiallyfor the most advanced processes. At any point in time, the two technologies canbe combined to obtain BiCMOS and the best features of bipolar and CMOS atthe expense of a more complex process, going from a 10-mask bipolar processto one requiring 14/15 mask steps. The BiCMOS process has lower yieldsbecause of the added processing complexity so applications must utilize the performanceimprovements it provides.A 5V supply is commonly used for MCUs and DSPs. Consequently,common signal conditioned outputs for sensors are 0.5–4.5V or 0.25–4.75V.Other standard industrial outputs are 1–6V, 1–5V, and 0–6V. The supplyvoltage for those units can range from 7 to 30V. Data acquisition systems typicallyhave used ±15V, ±12V, and ±5V supplies.Signal conditioners for sensor outputs range from basic low-gain dcamplifiers to specialized amplifiers such as phase-sensitive demodulators [2].Linear, logarithmic, high-gain, and dc-bridge amplifiers all fall in the followingcategories: differential amplifier, ac-coupled amplifier, chopper-stabilizedamplifier, carrier amplifier, dc-bridge amplifier, and ac-level amplifier. Anumber of specialized amplifiers are also used for sensors, including log-linearamplifiers, frequency-to-voltage converters, integrator amplifiers, and differentiatoramplifiers. Those packaged solutions take the low-level output from