Design And Implementation Of 12bit SAR ADC For High Temperature MEMS Accelerometer

In the field of petroleum drilling,the micro-machined accelerometer installed near the drill is used to measure the attitude,position and other information of the drill.The working environment temperature of the accelerometer usually reaches 175℃.The commonly used servo ASIC circuit of the MEMS accelerometer in China is usually applied within 125℃,which can not meet the needs of oil drilling.Therefore,it is necessary to design a servo ASIC circuit which can operate normally at 175 C.In this servo ASIC circuit,the analog-to-digital converter(ADC)is of great significance as a module to digitize the acceleration information.Among many types of ADCs,successive approximation register analog-to-digital converter(SAR ADC)is widely used because of its low power consumption,small area,high accuracy and speed.The purpose of this paper is to design a 12 bit SAR ADC for servo ASIC circuit of high temperature MEMS accelerometer.Its reference voltage is provided by a specially designed low noise reference voltage generator to meet the low noise requirement of ADC for reference voltage.The main work of this paper is as follows:1.High temperature SAR ADC circuit design and pre-simulation.The main work includes the design of low noise reference voltage generator,full differential digital-to-analog converter(DAC)with piecewise capacitor array,preamplifier latch comparator,digital logic control circuit and SAR ADC whole circuit.The circuit parameters are modified to meet the design targets based on the previous simulation results.2.Layout design and post-simulation of high temperature SAR ADC.The main work includes the layout design of low noise reference voltage generator,DAC,comparator,digital logic control circuit and the whole circuit.According to the simulation results,the placing and routing are adjusted to minimize the parasitic parameters of the layout on the circuit performance.3.Testing of high temperature SAR ADC chip.The temperature and noise characteristics of the low noise reference voltage generator chip are tested,and compared with the temperature coefficient and noise curve obtained by simulation.The layout of the low noise reference voltage generator is modified accordingly.In this paper,the design,simulation and fabrication of high temperature SAR ADC circuit based on X-FAB 0.18μm XH018 high temperature and high voltage CMOS process are presented.The simulation results show that when the power supply voltage of analog module is-5-+5V,the power supply voltage of digital module is-5～0V,and the sampling rate is 0.8MS/s,the temperature coefficients of the positive and negative reference voltages are 18.04 ppm/℃ and 13.34 ppm/℃ respectively.The maximum low frequency noise of the positive and negative reference voltages is basically the same as that of the previous simulation results,and both are less than 3μV/(?).The effective digit(ENOB)of SAR ADC is 11.28 bit at 25℃;the non-spurious dynamic range(SFDR)is 83.26 dBc,the ENOB of SAR ADC is 11.18 bit at-45℃;the SFDR is 82.74 dBc,and the ENOB of SAR ADC is 10.74 bit at 175℃,and the SFDR is 81.14 dBc,which all meet the design requirements.