This article describes a PNP-based temperature sensor that achieves both high energy efficiency and accuracy. Two resistors convert the CTAT and PTAT voltages generated by a PNP-based front-end into two currents whose ratio is then digitized by a continuous-time (CT) Δ Σ -modulator. Chopping and dynamic-element-matching (DEM) are used to mitigate the effects of component mismatch and 1/f noise, while the spread in V _BE and in the ratio of the two resistors is digitally trimmed at room temperature (RT). Fabricated in a 0.18μ m CMOS process, the sensor occupies 0.12 mm ², and draws 9.5μ A from a supply voltage ranging from 1.7 to 2.2 V. Measurements on 40 samples from one batch show that it achieves an inaccuracy of ± 0.1° C (3σ) from -55° C to 125° C, and a commensurate supply sensitivity of only 0.01° C/V. Furthermore, it achieves high energy efficiency, with a resolution Figure of Merit (FoM) of 0.85

BJT-based temperature sensors are widely used due to their high accuracy over a wide temperature range with a low-cost 1-point trim. Although resistor-based sensors can achieve better energy efficiency, they typically require a 2-point trim to achieve comparable accuracy, while thermal-diffusivity based sensors achieve superior accuracy at the cost of energy efficiency [1]. This paper presents a BJT-based temperature sensor that achieves both excellent accuracy and energy efficiency. To avoid the kTfC noise limitations of conventional discrete-time (OT) readout schemes [2], [3], it employs a compact continuous-time (CT) front-end. Component mismatch, which often limits the accuracy of CT front-ends [4], [5], is mitigated by a combination of dynamic element matching (OEM) and a low-cost resistor-ratio self-calibration scheme. As a result, the sensor achieves a resolution FoM of 0.85textpJcdotK 2, and a competitive inaccuracy of pm 0.1 circC (3sigma) from -55 circC tO 125 circC after a 1-point trim. This makes it 4times more energy-efficient than state-of-the-art BJT-based sensors with similar accuracy [2], [4], [5].

BJT-based temperature sensors are widely used because they can achieve excellent accuracy after 1-point calibration. However, they typically dissipate mu textWs of power and require supply voltages above 1V [1]. Although sensors based on DTMOSTs [2], [3], capacitively biased (CB) diodes and BJTs [4,5] have demonstrated sub-1V operation, this comes at the expense of accuracy. This paper presents a sub-1V CB BJT-based temperature sensor that achieves a 1-point-trimmed inaccuracy of 0.15°C (3σ) from -55 circC to 125 circC, which is 4times better than the CB BJT state-of-the-art [4]. It also achieves a resolution FoM of 0.34pJ.K2, which is 6.8 times better than that of state-of-the-art BJT-based sensors with a similar accuracy [1], [6], (Fig. 23.5.6).

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 mm2 and an absolute inaccuracy of ±2800 ppm from -45 °C to 125 °C with a fixed TC-trim and a one-point frequency trim. After one week of accelerated aging at 150 °C, however, significant drift (5000 ppm) was observed. The diffusion-based prototype exhibits greater inaccuracy (±14 400 ppm) but much less drift (600 ppm).

CMOS frequency references based on RC oscillators are usually preferred over bulky crystals in loT applications [1-5]. However, due to the process spread and finite temperature coefficient (TC) of most on-chip resistors, RC oscillators require trimming and temperature compensation to achieve decent accuracy. Enabled by high-resolution trimming techniques such as DeltaSigma [1], [2] or pulse-density [3] modulation, recent designs can obtain good accuracy (<0.1 %) at the expense of large chip area. However, existing compact (<0.02mm2) designs suffer from frequency errors in the order of 1% or more [4], [5]. Moreover, their temperature compensation schemes usually require the use of resistors with complementary TCs, which are not available in all CMOS technologies.

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.

This paper presents a 210nW BJT-based temperature sensor that achieves an inaccuracy of ±0.15°C (3s) from -15°C to 85°C. A dual-mode front-end (FE), which combines a bias circuit and a BJT core, halves the power needed to generate well-defined CTAT (VBE) and PTAT (?VBE) voltages. The use of a tracking ?S ADC reduces FE signal swing and further reduces system power consumption. In a 180-nm BCD process, the prototype achieves a 15mK resolution in 50ms conversion time, translating into a state-of-the-art FoM of 2.3pJK2.

This paper presents an overview of resistor-based sensors, with a focus on their energy efficiency. First, the theoretical energy efficiency limit of resistorbased sensors is determined and compared to that of traditional BJT-based sensors. This is followed by a review of the different types of resistor-based sensors. Finally, the design of a high-resolution Wheatstone bridge sensor is discussed in detail. Read out by a continuous-time Delta-Sigma modulator, the sensor achieves stateof-the-art energy efficiency, with a resolution FoM of 10 fJ·K2, which approaches the theoretical energy efficiency limit.

Flow sensors with high resolution (<200g/h/surdHz) and low offset drift (<pm 0.4mg/h) are essential in many microfluidic applications, such as flow cytometry and biological/chemical assays. Although thermal flow sensors can meet these specifications [1], [2], they measure flow velocity, so their calibration is fluid specific. Coriolis flow sensors [3]-[5] are a promising alternative because they measure mass flow and density regardless of fluid type, thus offering more flexibility. However, this has typically been at the expense of lower resolution, offset drift, and large footprint. This paper presents a mass-flow-to-digital converter (phi DC) based on a MEMS Coriolis mass flow sensor and a dedicated readout IC (ROIC). Compared to the state-of-the-art [5], it is more compact and has a digital output. Furthermore, it achieves a 3x improvement in resolution (100 g/h/ surd Hz) and a more than 2 ×improvement in zero stability (pm 0.35mg/h.

This letter describes an NPN-based temperature sensor that achieves a 1-point trimmed inaccuracy of ±0.15 °C (3σ) from -15 to 85 °C while dissipating only 210 nW. It uses a dual-mode frontend to roughly halve the power consumption of conventional frontends. First, two NPNs are used to generate a well-defined PTAT bias current, then this current is sampled and applied to the same NPNs to generate well-defined PTAT and CTAT voltages. These voltages are then applied to a low-power tracking ΔΣ modulator-based ADC, which employs a digital filter to efficiently generate a multibit representation of temperature. A prototype fabricated in a 180-nm BCD process achieves 15-mK resolution in a 50 ms conversion time, which translates into a state-of-the-art resolution FoM of 2.3 pJK2.

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].

Resistor-based temperature sensors can achieve higher resolution and energy-efficiency than traditional BJT-based sensors. To reach similar accuracy, however, they typically require 2-point (2-pt) calibration, compared to the low-cost 1-pt calibration required by BJT-based sensors. This paper presents a hybrid temperature sensor that uses an inherently accurate, but power-hungry, thermal-diffusivity (TD) sensor [1] to self-calibrate an inaccurate, but efficient, resistor-based sensor [2]. The use of an on-chip reference obviates the need for accurate temperature stabilized ovens or oil baths, drastically reducing calibration time and costs. Furthermore, by sharing most of the readout circuitry, the associated area overhead can be reduced. After self-calibration at room temperature (RT, \sim 25^{\circ}\mathrm{C}) and at an elevated temperature (\sim 85^{\circ}\mathrm{C}), the proposed hybrid temperature sensor achieves an inaccuracy of 0.25^{\circ}\mathrm{C} (3^{\sigma}) from -55^{\circ}\mathrm{C} to 125^{\circ}\mathrm{C}.

This article describes a hybrid temperature sensor in which an accurate, but energy-inefficient, thermal diffusivity (TD) sensor is used to calibrate an inaccurate, but efficient, resistor-based sensor. The latter is based on silicided polysilicon resistors embedded in a Wien-bridge (WB) filter, while the former is based on an electrothermal filter (ETF) made from a p-diffusion/metal thermopile and an n-diffusion heater. The use of an on-chip sensor for calibration obviates the need for an external temperature reference and a temperature-stabilized environment, thus reducing the cost. To mitigate the area overhead of the TD sensor, it reuses the WB filter's readout circuitry. Realized in a 180-nm CMOS technology, the hybrid sensor occupies 0.2 mm2. After calibration at room temperature (25 °C) and at an elevated temperature (85 °C), it achieves an inaccuracy of 0.25 °C (3 σ) from-55 °C to 125 °C. The WB sensor dissipates 66 μ W from a 1.8-V supply and achieves a resolution of 450 μ K rms in a 10-ms conversion time, which corresponds to a resolution figure-of-merit (FoM) of 0.13 pJ K2. The sensor also achieves a sub-10-mHz 1/f noise corner, which is comparable to that of bipolar junction transistor (BJT)-based temperature sensors.

This letter presents a compact, energy-efficient, and low-power Wheatstone-bridge temperature sensor for biomedical applications. To maximize sensitivity and reduce power dissipation, the sensor employs a high-resistance (600 kΩ ) bridge that consists of resistors with positive (silicided-poly) and negative ( n -poly) temperature coefficients. Resistor spread is then mitigated by trimming the n -poly arms with a 12-bit DAC, which consists of a 5-bit series DAC whose LSB is trimmed by a 7-bit PWM generator. The bridge is readout by a second-order delta-sigma modulator, which dynamically balances the bridge by tuning the resistance of the silicided-poly arms via a 1-bit series DAC. As a result, the modulator's bitstream average is an accurate and near-linear function of temperature, which does not require further correction in the digital domain. Fabricated in a 180-nm CMOS technology, the sensor occupies 0.12 mm2. After a 1-point trim, it achieves +0.2 °C/-0.1 °C ( 3σ ) inaccuracy in a ±10 °C range around body temperature (37.5 °C). It consumes 6.6 μW from a 1.6-V supply, and achieves 200- μK resolution in a 40-ms conversion time, which corresponds to a state-of-the-art resolution FoM of 11 fJ K2. Duty cycling the sensor results in even lower average power: 700 nW at 10 conversions/s.

This article describes a highly energy-efficient Wheatstone bridge temperature sensor. To maximize sensitivity, the bridge is made from resistors with positive (silicided diffusion) and negative (poly) temperature coefficients. The bridge is balanced by a resistive (poly) FIR-DAC, which is part of a 2nd-order continuous-time delta-sigma modulator (CT Δ Σ M). Each stage of the modulator is based on an energy-efficient current-reuse OTA. To efficiently suppress quantization noise foldback, the 1st stage OTA employs a tail-resistor linearization scheme. Sensor accuracy is enhanced by realizing the poly arms of the bridge and the DAC from identical unit elements. Fabricated in a 180-nm CMOS technology, the sensor draws 55 μ W from a 1.8-V supply and achieves a resolution of 150 μ K rms in an 8-ms conversion time. This translates into a state-of-the-art resolution figure-of-merit (FoM) of 10 fJ · K2. Furthermore, the sensor achieves an inaccuracy of ±0.4 °C (3 σ) from -55 °C to 125 °C after a ratio-based one-point trim and systematic non-linearity removal, which improves to ±0.1 °C (3 σ) after a 1st-order fit.

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].

Energy efficiency and accuracy are important specifications of CMOS temperature sensors. BJT -based sensors achieve state-of-the-art accuracy [1], while Wheatstone-bridge (WhB) sensors achieve lower accuracy but state-of-the-art energy efficiency [2], [3]. This paper presents a WhB sensor that is read out by an energy-efficient continuous-time delta-sigma modulator (CTDSM). Compared to [2], [3], the modulator achieves better energy efficiency with the help of a return-to-CM (RCM) DAC and an OTA with a tail-resistor linearization scheme. Moreover, better accuracy is achieved by embedding the DAC in the bridge and by using more sensitive silicided-diffusion resistors instead of silicided-poly resistors. Compared to the state-of-the-art [3], the proposed sensor achieves a 2× improvement in resolution FoM (10fJ.K2), and a 2× improvement in inaccuracy (0.4° C(3σ) from -55° C to 125°C after a 1-point trim).