This paper presents a high-accuracy, low-drift 16 MHz RC frequency reference. It is based on a Wien bridge filter that incorporates silicided n-poly resistors and MIM capacitors, whose temperature coefficient is compensated by a PNP-based temperature sensor. After a 2-point trim, it achieves ± 350 ppm inaccuracy from -45°C to 85° C, which increases to only ± 450 ppm after accelerated aging. This represents competitive accuracy and state-of-the-art stability for RC-based frequency references, approaching that of their LC-based counterparts while dissipating lower power and occupying less area.

This article presents a 16-MHz RC frequency reference implemented in a standard 180-nm CMOS process. It consists of a frequency-locked loop (FLL) in which the output frequency of a digitally controlled oscillator (DCO) is locked to the frequency-phase characteristic of a Wien bridge RC filter. Since it is made from on-chip resistors and capacitors, the filter's characteristic is temperature dependent. To compensate for this, the control signal of the DCO is derived by digitizing the filter's output phase and combining it with the digital output of a Wheatstone bridge temperature sensor. After a two-point trim, this digital temperature compensation scheme achieves an inaccuracy of ±90 ppm from -45 °C to 85 °C. The frequency reference draws 220 $\mu \text{A}$ from a 1.8-V supply, with a supply sensitivity of 0.12%/V and a 320-ppb Allan Deviation floor for a 10-s stride.

Recently, rapid strides have been made in improving the accuracy of RC-based frequency references [1 -3]. Inaccuracies better than \pm 500ppm from -45^{\circ}C to 85^{\circ}C have been achieved, but typically at the expense of a costly and time-consuming 2-point trim to compensate for RC spread and temperature dependence. This paper describes a 16MHz RC-based frequency reference that achieves \pm 400ppm inaccuracy over the industrial temperature range with a single room-temperature (RT) trim. The prototype draws 88\muA from a1.8V supply and occupies 0.14mm^{2}, which represents a 2\times improvement in both power and area compared to the state of the art [2].

This article presents a low-power eddy-current sensor interface for touch applications. It is based on a bang-bang digital phase-locked loop (DPLL) that converts the displacement of a metal target into digital information. The PLL consists of a digitally controlled oscillator (DCO) built around a sensing coil and a capacitive DAC, a comparator-based bang-bang phase/frequency detector (PFD), and a digital loop filter (DLF). The PLL locks the DCO to a reference frequency, making its digital input a direct representation of the sensing coil inductance. To compensate for the coil inductance tolerances, the DCO's center frequency can be trimmed by a second capacitive DAC. This approach obviates the need for a reference coil. When combined with a 5-mm-diameter sensing coil located 500μm from a metal target, the interface achieves a displacement resolution of 6.7 nm (rms) in a 3-kHz bandwidth. It consumes 200μW from a 1.8-V power supply, which represents the best-reported tradeoff between power consumption, bandwidth, and resolution.

Systems-on-chip traditionally rely on bulky quartz crystals to comply with wired communication standards like CAN or USB 2.0. Integrated frequency references with better than 500ppm inaccuracy could meet this need, resulting in higher integration and lower cost. Candidate architectures have employed RC-, LC- or TD (thermal diffusivity)-based time constants, all of which can be realized in standard CMOS. Compared to LC (sim 20mathrm{mW}, sim 100mathrm{ppm}) [1] or TD (sim 2mathrm{mW},sim 1000mathrm{ppm}) [2] references, RC references offer the lowest power consumption and competitive accuracy (< 1mathrm{mW}, 200mathrm{ppm})[3]. However, due to the nonlinear temperature dependence of on-chip resistors, such references require complex temperature-compensation schemes based on higher-order correction polynomials and extensive calibration [3], [4], or complicated analog compensation networks [5].

This paper describes a low-power eddy current displacement sensor intended for safety-critical touch applications. A sensing inductor is incorporated into a digital PLL to efficiently digitize the displacement of a flexible metal foil. At a stand-off distance of 500μm, the sensor achieves 6.7nm resolution in a 3kHz bandwidth over a 43μm range. It consumes 200μW from a 1.8V supply, which represents a 35× improvement on the state of the art.

Resistor-based temperature sensors can achieve much higher resolution and energy efficiency than conventional BJT-based sensors [1], but they typically occupy more area (> 0.25 mm ² ) and have lower operating temperatures (le 125 {circ} {C}) [2]-[4]. This work describes a 0.12mm ² resistor-based sensor that uses a Wien-bridge (WB) filter to achieve 0.1 {circ} {C} (3 sigma) inaccuracy from - 40 {circ} {C} to 180 {circ} {C}. Compared to a state-of-the-art WB sensor [4], it occupies 6 × less area and achieves comparable relative accuracy over a 76% wider operating range.

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].