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 (loT) applications require a stable on-chip frequency reference with low energy (<10 pJ / cycle) and high frequency stability (below \pm 300 ppm). 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 (<6 pJ / cycle), it only partially compensates for the resistors' high-order TCs, limiting the resulting accuracy to about ±500 ppm. Better accuracy (\pm 100 ppm [4]) can be achieved by using the output of a digital temperature sensor (TS) to perform a polynomial correction of the phase-shift \left(\mu_{p, T}\right) of an RC filter (Fig. 31.2.1). Alternatively, the phaseshifts \left(\mu_{p}\right.. and \left.\mu_{N}\right) of two RC filters with complementary TCs can be linearized \left(T_{p}\right.. and TN) and combined in the digital domain. Such dual-RC frequency references can also achieve good accuracy (\pm 200 ppm [5]). However, both architectures employ an analog phase-domain \Delta \Sigma modulator \left(\Phi-\Delta \Sigma M\right) for each RC filter, which consumes significant energy (25 pJ / cycle [4] and 107 pJ / cycle [5]) and area \left(0.3 mm^{2}[4]\right.. and \left.1.65 mm^{2}[5]\right).