31.2 A 0.9V 28MHz Dual-RC Frequency Reference with 5pJ/Cycle and ±200 ppm Inaccuracy from -40°C to 85°C

Wireless sensor nodes in battery-powered internet-of-things (IoT) applications require a stable on-chip frequency reference with low energy (<; 10pJ/cycle) and high frequency stability (below ±300ppm). CMOS RC frequency references are promising due to their low-cost integration and high energy efficiency [1-5]. Conventional RC references, however, achieve only moderate accuracy (a few %) due to the large temperature coefficient (TC) of on-chip resistors [3]. First-order TC compensation can be achieved by combining resistors with complementary TCs [1,2]. Although this is energy efficient (<; 6pJ/cycle), it only partially compensates for the resistors' high-order TCs, limiting the resulting accuracy to about ±500ppm. Better accuracy (±100ppm [4]) can be achieved by using the output of a digital temperature sensor (TS) to perform a polynomial correction of the phase-shift (μp,T) of an RC filter (Fig. 31.2.1). Alternatively, the phaseshifts (μP and μN) of two RC filters with complementary TCs can be linearized (TP and TN) and combined in the digital domain. Such dual-RC frequency references can also achieve good accuracy (±200ppm [5]). However, both architectures employ an analog phase-domain ΔΣ modulator (Φ-ΔΣM) for each RC filter, which consumes significant energy (25pJ/cycle [4] and 107pJ/cycle [5]) and area (0.3mm
²
[4] and 1.65mm2 [5]).