A Compact 10-MHz RC Frequency Reference With a Versatile Temperature Compensation Scheme

• Published in: IEEE journal of solid-state circuits Vol. 58; no. 12; pp. 1 - 9
• Main Authors: Pan, Sining, An, Xiaomeng, Yu, Zheru, Jiang, Hui, Makinwa, Kofi A. A.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.12.2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Capacitors; CMOS; CMOS frequency reference; Drift; Frequency locked loops; Frequency locking; on-chip trimming; Polysilicon; Prototypes; resistor aging; Resistors; Silicides; Temperature compensation; Temperature dependence; Time constant; Time-frequency analysis; Voltage; Voltage controlled oscillators; Zero temperature coefficient
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2023.3322307

This article presents the design and implementation of a compact CMOS RC frequency reference. It consists of a frequency-locked loop (FLL) that locks the period of a voltage-controlled oscillator (VCO) to the time an RC network takes to charge to a reference voltage. Conventionally, an RC time constant with a near-zero temperature coefficient (TC) is realized by using a trimmed network of resistors with different TCs. In this work, such a network is used to realize a temperature-dependent reference voltage whose TC cancels that of a single-resistor RC time constant. Compared with the conventional approach, which requires resistors with TCs of opposite polarity, the proposed approach can be implemented with resistors with TCs of similar polarity, and so it can be implemented in most CMOS processes. To compensate for RC spread, a trimmed capacitor is used to adjust the nominal frequency. Two prototype chips were made, one based on p-/n-polysilicon resistors and other based on silicided/p-diffusion resistors. Fabricated in a standard 180-nm CMOS technology, the polysilicon-based prototype has an active area of 0.01 mm<inline-formula> <tex-math notation="LaTeX">^{2}</tex-math> </inline-formula> and an absolute inaccuracy of <inline-formula> <tex-math notation="LaTeX">\pm</tex-math> </inline-formula>2800 ppm from <inline-formula> <tex-math notation="LaTeX">-</tex-math> </inline-formula>45 <inline-formula> <tex-math notation="LaTeX">^{\circ}</tex-math> </inline-formula>C to 125 <inline-formula> <tex-math notation="LaTeX">^{\circ}</tex-math> </inline-formula>C with a fixed TC-trim and a one-point frequency trim. After one week of accelerated aging at 150 <inline-formula> <tex-math notation="LaTeX">^{\circ}</tex-math> </inline-formula>C, however, significant drift (5000 ppm) was observed. The diffusion-based prototype exhibits greater inaccuracy (<inline-formula> <tex-math notation="LaTeX">\pm</tex-math> </inline-formula>14 400 ppm) but much less drift (600 ppm).