一种无比较器的高精度RC振荡器设计

Design of a high precision RC oscillator without comparator

基于华虹0.18μm CMOS工艺,设计了一种无比较器的低温漂高精度RC振荡器。通过调整电流源的负温度系数电流补偿MOSFET阈值电压的温漂,保证输出频率在大温度范围内的高稳定性。通过提高电流源输出阻抗,提高振荡器的电压稳定性。采用数字修调技术矫正工艺偏差引起的频率误差。该振荡器由启动电路、CTAT电流源电路、电流镜电路、修调电路、竞争冒险消除电路和RC振荡电路六部分构成。因为没有采用比较器结构,所以在该振荡器中,不会出现由于比较器的传输延时与输入失调电压引起的非理想因素。采用Cadence进行电路仿真与验证,后仿真结果表明,该振荡器的典型频率为2 MHz,起振时间为5.1μs。在3~5 V电源电压变化范围内,频率偏差均在±0.55%以内;在-40~125℃温度范围内,输出频率随温度的变化率均在±1.2%以内,可适用于高精度的数模混合信号芯片。

Based on Huahong 0.18μm CMOS process,an RC oscillator was designed without comparators,which has both low temperature drift and high precision.By adjusting the negative temperature coefficient of electrical current of the current source,temperature drift of the threshold voltage of the MOSFET was compensated to achieve high stability of output frequency in a wide temperature range.Voltage stability of the oscillator was improved by increasing the output impedance of the current source.Digital trimming technology was used to correct frequency errors caused by process deviations.The oscillator is composed of a starting circuit,a CTAT current source circuit,a current mirror circuit,a trimming circuit,a competition risk elimination circuit,and an RC oscillator circuit.Without comparator structures,there are no non-ideal factors due to comparator transmission delay and input offset voltage in the oscillator.Cadence software was used for circuit simulation and verification.Simulation results show that the typical operating frequency of the oscillator is 2 MHz with start-up time of 5.1μs.For power supply from 3 V to 5 V,the frequency deviation is less than±0.55%between-40 and 125℃.The variation rate is less than±1.2%for output frequency as a function of temperature,which is suitable for high precision digital-analog mixed-signal chips.