642 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 60, NO. 10, OCTOBER 2013 A ±0.5% Precision On-Chip Frequency Reference With Programmable Switch Array for Crystal-Less Applications Yan Lu, Student Member, IEEE, Gang Yuan, Lawrence Der, Wing-Hung Ki, Member, IEEE, and C. Patrick Yue, Senior Member, IEEE Abstract—An on-chip frequency reference is designed for low- power, low-cost, and fully integrated system-on-chip designs. In this relaxation oscillator, pseudodifferential architecture is used to eliminate frequency variation caused by bias current, and inter- leaving capacitors are implemented to extend its discharge time. A low-leakage programmable switch array (PSA) trimming method is proposed to calibrate the first- and second-order temperature coefficients (TCs) of the composite resistor. The oscillator was fabricated in a 0.35-μm 2P4M CMOS process with an area of 0.162 mm 2 . The oscillator operates at 130 kHz, and measurement results show that it achieves a frequency variation of less than ±0.5% over a temperature range of −20 ◦ C–100 ◦ C and less than ±0.4% over a supply voltage range of 1–3 V. Index Terms—Crystal-less clock, frequency reference, low-power radio, relaxation oscillator, temperature compensation, wireless sensor networks. I. I NTRODUCTION A CRYSTAL-LESS receiver is on high demand for low-cost and/or small-form-factor applications, such as AM/FM radios [1] and wireless sensor nodes [2]. The frequency ref- erences are usually obtained from quartz crystal oscillators (XOs) that provide stable and low-noise output frequencies or, alternatively, from microelectromechanical system (MEMS)- based oscillators that give comparable temperature coefficients (TCs) and phase-noise performance [3]. However, both XO and MEMS oscillators need additional printed-circuit-board space or system-in-package technology that will increase the volume and the cost. From the energy-efficiency perspective, on-chip solutions should have higher efficiency compared with XO and MEMS oscillators because there is no need to convert between electrical and mechanical energy that would induce extra power loss. Requirements for the frequency reference could be relaxed with novel receiver architectures. For example, in [1], the highly integrated radio receiver with an LC -based voltage-controlled oscillator and a frequency synthesizer can accept a wide range of reference clocks (from 31.13 to 40 kHz with ±100-ppm stability tolerance). A second example allows an even more Manuscript received March 13, 2013; revised May 23, 2013; accepted July 14, 2013. Date of publication August 22, 2013; date of current ver- sion October 14, 2013. This brief was recommended by Associate Editor S. Levantino. Y. Lu, W.-H. Ki, and C. P. Yue are with the Department of Electronic and Computer Engineering, The Hong Kong University of Science and Tech- nology, Kowloon, Hong Kong (e-mail: yanlu@ece.ust.hk; eeki@ece.ust.hk; eepatrick@ust.hk). G. Yuan and L. Der are with Silicon Laboratories, Austin, TX 78701 USA. Color versions of one or more of the figures in this brief are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCSII.2013.2277984 TABLE I COMPARISON OF DIFFERENT TYPES OF OSCILLATORS relaxed frequency variation (±1%) for communication links such as the universal asynchronous receiver/transmitter [4]. It means that the absolute value of the reference clock is not as important as its immunity to temperature and supply voltage variations. Thus, accuracy could be traded off for better stability for on-chip frequency reference designs. Relaxation oscillators are good candidates for low-power operation with tolerable ac- curacy [5]. The frequency spread within ±0.5% after two-point trim is achieved in [2], consuming a current of 42.6 μA, and frequency variations of ±0.68% and ±0.82% with respect to temperature and supply voltage, respectively, are accomplished with only 280 nW in [4]. Moreover, a relaxation oscillator has the advantages of large tuning range and small area [6]. Table I summarized and compared the general performance of different types of reference generators. The MEMS os- cillator is not power efficient as it consumes over 5 mA for submegahertz-range oscillation frequencies [3]. A TC- compensated on-chip LC oscillator could achieve an accuracy better than ±100 ppm; however, it still dissipates large power since its core oscillation frequency is usually in the gigahertz range [7]. On the other hand, a relaxation oscillator consumes the lowest power with relatively low oscillation frequency and acceptable frequency stability, indicating that it could be the best choice for ultralow-power, low-cost, and compact-size communication applications. The frequency drift of a relaxation oscillator is mainly caused by the variations of polyresistors and bias currents due to process, voltage, and temperature. On-chip resistors have rela- tively large TCs compared with capacitors; hence, a composite resistor with two types of resistors (positive- and negative-TC resistors) is commonly employed to achieve a zero/low-TC resistor [4]. However, process variations of those two resistors 1549-7747 © 2013 IEEE