A steady trend towards the design of mostly-digital and digital-friendly analog circuits, suitable to integration in mainstream nanoscale CMOS by a highly automated design flow, has been observed in the last years to address the requirements of the emerging Internet of Things (IoT) applications. In this context, this tutorial brief presents an overview of concepts and design methodologies that emerged in the last decade, aimed to the implementation of analog circuits like Operational Transconductance Amplifiers, Voltage References and Data Converters by digital circuits. The current design challenges and application scenarios as well as the future perspectives and opportunities in the field of digital-based analog processing are finally discussed.

Re-Thinking Analog Integrated Circuits in Digital Terms: A New Design Concept for the IoT Era

An improved digital interface circuit for resistive sensors is proposed in this paper. The circuit offers many positive features, such as (1) use of single-reference voltage in its architecture, (2) facility to tune/preset the sensor-excitation current, even for different values of resistive sensors, (3) minimal dependence on many circuit non-ideal parameters. The working of the circuit is mathematically described in this paper, followed by its extensive error evaluation. Detailed simulation and experimental studies are performed to verify the feasibility of the interface for three different types of sensors. These studies show that the circuit can efficiently be used in temperature-monitors, magnetometers, and displacement-sensing. The maximum experimental nonlinearity observed from all of these sensor configurations does not exceed 0.06 %. Then, the extensive studies are carried out by simulation as well as experimentation to show the effect of environmental temperature changes on the proposed circuit. Finally, the proposed circuit is compared with some of the recent resistive digital interfaces.

Performance Verification of a Digital Interface Suitable for a Broad Class of Resistive Sensors

This paper develops a new detection system, for the first time, to detect the gas concentrations of the SF6/N2 mixture in extra/ultra-high voltage power transmission systems. The concentrations of SF6 and N2 are calculated from the thermal conductivity function of the mixture gas. The main contribution of this work is that a specially-designed thermostatic chamber with adaptive temperature controller is developed to ensure constant pressure of the gas flowed through the thermal conductivity sensor. Another contribution is the combination of multiple sensors (e.g. humidity and electrochemical sensors), which enables the detector to address the penetration effects of H2O and O2 in the SF6/N2 mixture. Experimental evaluation results using the prototype demonstrated that satisfactory accuracy (±1% of the measurement error) has been achieved for the concentration detection of the SF6/N2 mixture under variable operation conditions. Compared with existing detection techniques, the proposed detector not only can detect the SF6/N2 concentration by taking the air infiltration effect into account, but also reduce the cost.

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A New Concentration Detection System for SF 6 /N 2 Mixture Gas in Extra/Ultra High Voltage Power Transmission Systems

This paper presents a readout circuit for a carbon dioxide (CO2) sensor that measures the CO2-dependent thermal time constant of a hot-wire transducer. The readout circuit periodically heats up the transducer and uses a phase-domain $\Delta \Sigma $ modulator to digitize the phase shift of the resulting temperature transients. A single resistive transducer is used both as a heater and as a temperature sensor, thus greatly simplifying its fabrication. To extract the transducer’s resistance, and hence its temperature, in the presence of large heating currents, a pair of transducers is configured as a differentially driven bridge. The transducers and the readout circuit have been implemented in a standard 0.16- $\mu \text{m}$ CMOS technology, with an active area of 0.3 and 3.14 mm2, respectively. The sensor consumes 6.8 mW from a 1.8-V supply, of which 6.3 mW is dissipated in the transducers. A resolution of 94-ppm CO2 is achieved in a 1.8-s measurement time, which corresponds to an energy consumption of 12 mJ per measurement, >10 $\times $ less than prior CO2 sensors in CMOS technology.

/pdf/a-phase-domain-readout-circuit-for-a-cmos-compatible-hot-2j7apytfxl.pdf

A Phase-Domain Readout Circuit for a CMOS-Compatible Hot-Wire CO 2 Sensor

This paper presents the operation principle and the silicon characterization of a power efficient ultra-low voltage and ultra-low area fully-differential, digital-based Operational Transconductance Amplifier (OTA), suitable for microscale biosensing applications (BioDIGOTA). Measured results in 180nm CMOS prototypes show that the proposed BioDIGOTA is able to work with a supply voltage down to 400 mV, consuming only 95 nW. Owing to its intrinsically highly-digital feature, the BioDIGOTA layout occupies only 0.022 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of total silicon area, lowering the area by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.22\times $ </tex-math></inline-formula> times compared to the current state of the art, while keeping reasonable system performance, such as 7.6 NEF with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.25~\mu V_{\mathrm {RMS}}$ </tex-math></inline-formula> input referred noise over a 10 Hz bandwidth, 1.8% of THD, 62 dB of CMRR and 55 dB of PSRR. 

/pdf/low-voltage-low-area-nw-power-cmos-digital-based-biosignal-38l51qq5.pdf

Low-Voltage, Low-Area, nW-Power CMOS Digital-Based Biosignal Amplifier

This paper reports a readout circuit capable of accurately measuring not only the resistance of a resistive transducer, but also the power dissipated in it, which is a critical parameter in thermal flow sensors or thermal-conductivity sensors. A front-end circuit, integrated in a standard CMOS technology, sets the voltage drop across the transducer, and senses the resulting current via an on-chip reference resistor. The voltages across the transducer and the reference resistor are digitized by a time-multiplexed high-resolution analog-to-digital converter (ADC) and post-processed to calculate resistance and power dissipation. To obtain accurate resistance and power readings, a voltage reference and a temperature-compensated reference resistor are required. An accurate voltage reference is constructed algorithmically, without relying on precision analog signal processing, by using the ADC to successively digitize the base–emitter voltages of an on-chip bipolar transistor biased at several different current levels, and then combining the results to obtain the equivalent of a precision curvature-corrected bandgap reference with a temperature coefficient of 18 ppm/°C, which is close to the state-of-the-art. We show that the same ADC readings can be used to determine die temperature, with an absolute inaccuracy of ±0.25 °C (5 samples, min–max) after a 1-point trim. This information is used to compensate for the temperature dependence of the on-chip polysilicon reference resistor, effectively providing a temperature-compensated resistance reference. With this approach, the resistance and power dissipation of a 100 $\Omega $ transducer have been measured with an inaccuracy of less than $\pm 0.55~\Omega $ and ±0.8%, respectively, from −40 °C to 125 °C.

/pdf/a-cmos-readout-circuit-for-resistive-transducers-based-on-25u5onqbs9.pdf

Luis E. Rueda Guerrero

Papers

A CMOS Readout Circuit for Resistive Transducers Based on Algorithmic Resistance and Power Measurement