The Readout Circuit Chip Design For CO₂Infrared Sensor

ABSTRACT:The readout circuit for CO2infrared sensor is mainly used in CO2gas concentration detecting apparatus which is based on infrared spectrum absorption. It simplifies the CO2gas concentration detecting circuit which is based on infrared spectrum absorption, makes it only consist of light source, sensor, readout circuit and MCU. It reduces discrete devices use and has many advantages like low power consumption, small size, simplifying the CO2gas concentration detecting whole circuit and so on. The readout circuit consists of filtering and amplifying circuit for sensor output voltage signal, low temperature drift Bandgap voltage reference circuit and low dropout linear regulator circuit. The power management and signal filtering processing is integrated, it reduces chips use,discrete devices use and the cost, improves the integration and the circuit is in line with portable and compact development of the carbon dioxide gas concentration detecting apparatus.This thesis first introduces the overall structure and functions of the CO2infrared sensor, and analyzes the function and circuit structure of each module. And then it elaborates each module circuit design of the readout circuit, including the circuit implementation of each module, layout design and simulation. Finally, measures the chip after taped out and analyzes the measurement results.The readout circuit chip design is fabricated in CSMC0.5μmDPTM (Double Poly Triple Metal) single-well process. After measurement all the functions is implemented, and the performance meets requirements. After measurement the whole circuit current consumption is1.52mA.The temperature coefficient of the Bandgap is less than10.15ppm/℃in the range-40℃-125℃, the output voltage is1.25V. The chip provides two external reference voltage1.28V and3.3V and one internal reference voltage1.67V. The readout circuit chip includes two LDO linear regulators which respectively the output voltage is2.5V, the maximum output current is30mA and the output voltage is3V, the maximum output current is100mA. MCU is supplied by LDO2.5V, the light source is supplied by LDO3V which has an enable port. MCU generates the control signal, the signal controls the source emitting light periodically through the port. Therefore, it can reduce power consumption. The two LDO modules have over current protection and short circuit protection, meanwhile, they also have thermal shutoff protection, while the chip temperature is too high, the LDOs will be off. The filtering and amplifying circuit filters and amplifies the output signals of the sensor respectively, the function is implemented.