Design Of Readout Circuit For CO2 Concentration Detection Based On Infrared CO2 Gas Sensor

With the widely application of carbon dioxide concentration detection system, the detection system is developing towards the direction of high precision, small size and high stability. There are many methods to detect concentrations of CO2 gas and the readout circuit designed in this paper is based on infrared CO2 gas sensor. The circuit is mainly used to deal with the double-channel gas sensor output and supply power for the whole system.A power supply module and an active filter module, based on the Cadence full-custom IC design, are integrated in the readout circuit to reduce the volume. The circuit also has protection modules to enhance the stability. This thesis first describes the composition of carbon dioxide concentration detection system and the basic theory of infrared CO2 gas sensor, dividing the circuit into different modules based on functions. Secondly, it elaborates the basic theory of each module of the readout circuit, with which the design specifications are analyzed. Different modules schematics and pre simulation results are completed. Based on the design rules of layout, layout design and post simulation are given. Finally, measurement circuit is designed and measurement results are presented, with which the analysis is shown. The readout circuit mainly consists of active filter module and power supply module. There are two channel band pass filters for the double-channel gas sensor in the active filter module. The power supply module is composed of the bandgap voltage reference and two channel low-dropout regulators. The design requires the first channel filter center frequency is 10Hz with the gain of 66dB, the second channel filter center frequency is 10Hz with the gain of 77dB. The output of the bandgap voltage reference is 1.25± 0.12V, the temperature coefficient is 10ppm/℃, the power supply rejection is-70dB. The output voltage of first channel low-dropout regulator is 3±0.3V, the maximum output current is 90±9mA. The output voltage of second channel low-dropout regulator is 2.5±0.2V, the maximum output current is 27±3mA. The integral circuit static current consumption is less than 2mA.The schematics and layouts are designed with the Cadence EDA tools and the simulations are completed with the Spectre in this thesis. Through measurement and analysis of 16 taped out chips, it can be concluded that the design meets the requirements and the circuit has steady performance.