To comply with wired communication standards such as USB, SATA and PCI/PCI-E, systems-on-chip require frequency references with better than 300ppm accuracy. LC-based references achieve 100ppm accuracy [1], but suffer from high power consumption (∼20mW). Thermal diffusivity (TD) references require less power (∼2mW), at the expense of less accuracy (1000ppm) [2]. RC-based references offer the lowest power consumption, but their accuracy is typically limited to ∼0.1% [3]. In RC relaxation oscillators, comparator offset and delay are the major sources of inaccuracy [4,5]. References based on frequency-locked loops (FLLs) circumvent these by locking an oscillator's frequency to the time-constant of an RC filter, but their accuracy is then limited by the nonlinear temperature dependency of on-chip resistors [3,6].

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This paper presents a 7-MHz CMOS RC frequency reference. It consists of a frequency-locked loop in which the output frequency of a digitally controlled oscillator (DCO) is locked to the combined phase shifts of two independent RC (Wien bridge) filters, each employing resistors with complementary temperature coefficients. The filters are driven by the DCO's output frequency and the resulting phase shifts are digitized by high-resolution phase-to-digital converters. Their outputs are then combined in the digital domain to realize a temperature-independent frequency error signal. This digitally assisted temperature compensation scheme achieves an inaccuracy of ±200 ppm from -45 °C to 85 °C after a two-point trim. The frequency reference draws 430 μA from a 1.8-V supply, while achieving a supply sensitivity of 0.18%/V and a 330-ppb Allan deviation floor in 3 s of measurement time.@en

The thermal diffusivity of silicon DSi has been used to realize fully-CMOS frequency references. However, due to the temperature dependence of DSi, the accuracy of such frequency references is limited to about 1000 ppm (−55 °C to 125 °C, one-point trim) due to the inaccuracy of the on-chip temperature compensation circuitry. As an alternative, we propose a frequency reference based on the thermal diffusivity of silicon dioxide DOx. Since the temperature dependence of DOx is much less than that of DSi, the resulting frequency reference will be much more stable over temperature. To investigate this idea, a thermal-diffusivity-based frequency-locked loop (FLL) was realized in 0.18-μm CMOS. With ideal temperature compensation, the proposed frequency reference achieves an inaccuracy of 90 ppm (−45 °C to 85 °C, two-point trim). Even with 0.1 °C inaccuracy, which can be achieved by BJT-based temperature sensors, 200 ppm can still be achieved. This demonstrates the feasibility of high-accuracy oxide-based frequency references in standard CMOS.@en

Due to their relatively stable phase shift over temperature, electrothermal filters (ETFs) with an oxide heat path have been used as on-chip phase references, e.g. for thermal diffusivity (TD) temperature sensors. However, previous oxide ETFs were limited to SOI processes, whose deep-trench isolation could be used to create an oxide-dominated heat path. This paper describes, for the first time, an oxide ETF realized in a bulk CMOS process. It achieves a phase spread of 0.6 % (3 sigma, no trim) from -40 °C to 125 °C. When used as a reference for a TD temperature sensor, this translates into a temperature sensing spread of ±2.7 °C (3 sigma, no trim). This is 1.8 times less than the spread reported for SOI implementations, making the CMOS variant not only feasible, but also competitive.@en