A CMOS Temperature Sensor for MEMS-based Frequency Source

Abstract

This paper presents a precision CMOS temperature-to-digital converter (TDC), which sensing the temperature-dependent base-emitter voltage of substrate PNPs. The TDC is applied to temperature compensation of FBAR oscillator. The proposed TDC adopts the bipolar transistors as the core device, and use Zoom analog-to-digital converter (Zoom ADC) to read temperature information. The PTAT bias circuit is used in the temperature frontend to correct the nonlinearity of the $V_{B E}$. The mismatch of current sources and the offset of operational amplifiers are reduced by dynamically element matched (DEM) and chopping techniques, respectively. With these techniques, the current gain $\beta_{F}$ of BJT is compensated and the accuracy of circuit is improved. The Zoom ADC is composed of SAR ADC and $\Sigma-\Delta$ ADC. The SAR ADC is used to complete rough quantization and narrow the quantization interval, meanwhile, the fine quantization is realized by $\Sigma-\Delta$ ADC. By this method, Zoom ADC takes full advantages of the fast speed of the SAR ADC and the high accuracy of the $\Sigma-\Delta$ ADC, which significantly reduces the ENOB requirement for the $\Sigma-\Delta$ ADC and design difficulty. The control clock circuit of the ADC is integrated in the circuit to simplify system application. This circuit is designed in TSMC $0.18 \mu \mathrm{m}$ CMOS processes. The simulation results show that the static current of the circuit is $131 \mu \mathrm{A}\left(\text{@} 27^{\circ} \mathrm{C}\right)$ under the supply voltage of 1.8 V. After two-point calibration correction, the temperature measurement accuracy achieved $\pm 0.5^{\circ} \text{C}$ in the range of $-55^{\circ} \text{C} \sim 125^{\circ} \text{C}$. The core area of the circuit is $560 \mu m \times 330 \mu m$.