Design techniques for low-power SAR ADCs in nano-scale CMOS technologies

This thesis presents low power design techniques for successive approximation register (SAR) analog-to-digital converters (ADCs) in nano-scale CMOS technologies. Low power SAR ADCs face two major challenges especially at high resolutions: (1) increased comparator power to suppress the noise, and (2) increased DAC switching energy due to the large DAC size. To improve the comparator’s power efficiency, a statistical estimation based comparator noise reduction technique is presented. It allows a low power and noisy comparator to achieve high signal-to-noise ratio (SNR) by estimating the conversion residue. A first prototype ADC in 65nm CMOS has been developed to validate the proposed noise reduction technique. It achieves 4.5 fJ/conv-step Walden figure of merit and 64.5 dB signal-to-noise and distortion ratio (SNDR). In addition, a bidirectional single-side switching technique is developed to reduce the DAC switching power. It can reduce the DAC switching power and the total number of unit capacitors by 86% and 75%, respectively. A second prototype ADC with the proposed switching technique is designed and fabricated in 180nm CMOS technology. It achieves an SNDR of 63.4 dB and consumes only 24 Wat 1MS/s, leading to aWalden figure of merit of 19.9 fJ/conv-step. This thesis also presents an improved loop-unrolled SAR ADC, which works at high frequency with reduced SAR logic power and delay. It employs the bidirectional single-side switching technique to reduce the comparator common-mode voltage variation. In addition, it uses a Vcm-adaptive offset calibration technique which can accurately calibrate comparator’s offset at its operating Vcm. A prototype ADC designed in 40nm CMOS achieves 35 dB at 700 MS/s sampling rate and consumes only 0.95 mW, leading to a Walden figure of merit of 30 fJ/conv-step.Electrical and Computer Engineerin

Low-Power Energy Efficient Circuit Techniques for Small IoT Systems

Although the improvement in circuit speed has been limited in recent years, there has been increased focus on the internet of things (IoT) as technology scaling has decreased circuit size, power usage and cost. This trend has led to the development of many small sensor systems with affordable costs and diverse functions, offering people convenient connection with and control over their surroundings. This dissertation discusses the major challenges and their solutions in realizing small IoT systems, focusing on non-digital blocks, such as power converters and analog sensing blocks, which have difficulty in following the traditional scaling trends of digital circuits. To accommodate the limited energy storage and harvesting capacity of small IoT systems, this dissertation presents an energy harvester and voltage regulators with low quiescent power and good efficiency in ultra-low power ranges. Switched-capacitor-based converters with wide-range energy-efficient voltage-controlled oscillators assisted by power-efficient self-oscillating voltage doublers and new cascaded converter topologies for more conversion ratio configurability achieve efficient power conversion down to several nanowatts. To further improve the power efficiency of these systems, analog circuits essential to most wireless IoT systems are also discussed and improved. A capacitance-to-digital sensor interface and a clocked comparator design are improved by their digital-like implementation and operation in phase and frequency domain. Thanks to the removal of large passive elements and complex analog blocks, both designs achieve excellent area reduction while maintaining state-of-art energy efficiencies. Finally, a technique for removing dynamic voltage and temperature variations is presented as smaller circuits in advanced technologies are more vulnerable to these variations. A 2-D simultaneous feedback control using an on-chip oven control locks the supply voltage and temperature of a small on-chip domain and protects circuits in this locked domain from external voltage and temperature changes, demonstrating 0.0066 V/V and 0.013 °C/°C sensitivities to external changes. Simple digital implementation of the sensors and most parts of the control loops allows robust operation within wide voltage and temperature ranges.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138743/1/wanyeong_1.pd

Design and implementation of Radix-3/Radix-2 based novel hybrid SAR ADC in scaled CMOS technologies

This thesis focuses on low power and high speed design techniques for successive approximation register (SAR) analog-to-digital converters (ADCs) in nanoscale CMOS technologies. SAR ADCs’ speed is limited by the number of bits of resolution. An N-bit conventional SAR ADC takes N conversion cycles. To speed up the conversion process, we introduce a radix-3 SAR ADC which can compute 1:6 bits per cycle. To our knowledge, it is the first fully programmable and efficiently hardware controlled radix-3 SAR ADC. We had to use two comparators per cycle due to ADC architecture and we proposed a simple calibration scheme for the comparators. Also, as the architecture of the DAC array is completely different from the architecture of conventional radix-2 SAR ADC’s DAC arrays, we came up with an algorithm for calibration of capacitors of the DAC. Low power SAR ADCs face two major challenges especially at high resolutions: (1) increased comparator power to suppress the noise, and (2) increased DAC switching energy due to the large DAC size. Due to our proposed architecture,the radix-3 SAR ADC uses two comparators per cycle and two differential DACs. To improve the comparator’s power efficiency, an efficient and low cost calibration technique has been introduced. It allows a low power and noisy comparator to achieve high signal-to-noise ratio (SNR). To improve the DAC switching energy, we introduced a radix-3/radix-2 based novel hybrid SAR ADC. We use two single ended DACs for radix-3 SAR ADC and these two single ended DACs can be used as one differential DAC for radix-2 SAR ADC. So, overall, we only have a single DAC as conventional radix- 2 SAR ADC. In addition, a monotonic switching technique is adopted for radix-2 search to reduce the DAC capacitor size and hence, to reduce switching power. It can reduce the total number of unit capacitors by four times. Our proposed hybrid SAR ADC can achieve less DAC energy compared to radix-3 and radix-2 SAR ADCs. Also, to utilize technology scaling, we used the minimum capacitor size allowed by thermal noise limitations. To achieve high resolution, we introduced calibration algorithm for the DAC array. As mentioned earlier, the radix-3 SAR ADC offers higher power than conventional radix-2 SAR ADC because of simultaneous use of two comparators. In the proposed hybrid SAR ADC, we will be using radix-3 search for first few MSB bits. So, the resolution required for radix-3 comparators are much larger than the LSB value of 10-bit ADC. By implementing calibration of comparators, we can use low power, high input referred offset and high speed comparators for radix-3 search. Radix-2 search will be used for rest of the bits and the resolution of the radix-2 comparator has to be less than the required LSB value. So, a high power, low input referred offset and high speed comparator is used for radix-2 search. Also, we introduced clock gating for comparators. So, radix-3 comparators will not toggle during radix-2 search and the radix-2 comparators will be inactive during radix-3 search. By using the aforementioned techniques, the overall comparator power is definitely less than a radix-3 SAR ADC and comparable to a conventional radix-2 SAR ADC. A prototype radix-3/radix-2 based hybrid SAR ADC with the proposed technique is designed and fabricated in 40nm CMOS technology. It achieves an SNDR of 56.9 dB and consumes only 0.38 mW power at 30MS/s, leading to a Walden figure of merit of 21.5 fJ/conv-step.Electrical and Computer Engineerin

Design of Energy-Efficient A/D Converters with Partial Embedded Equalization for High-Speed Wireline Receiver Applications

As the data rates of wireline communication links increases, channel impairments such as skin effect, dielectric loss, fiber dispersion, reflections and cross-talk become more pronounced. This warrants more interest in analog-to-digital converter (ADC)-based serial link receivers, as they allow for more complex and flexible back-end digital signal processing (DSP) relative to binary or mixed-signal receivers. Utilizing this back-end DSP allows for complex digital equalization and more bandwidth-efficient modulation schemes, while also displaying reduced process/voltage/temperature (PVT) sensitivity. Furthermore, these architectures offer straightforward design translation and can directly leverage the area and power scaling offered by new CMOS technology nodes. However, the power consumption of the ADC front-end and subsequent digital signal processing is a major issue. Embedding partial equalization inside the front-end ADC can potentially result in lowering the complexity of back-end DSP and/or decreasing the ADC resolution requirement, which results in a more energy-effcient receiver. This dissertation presents efficient implementations for multi-GS/s time-interleaved ADCs with partial embedded equalization. First prototype details a 6b 1.6GS/s ADC with a novel embedded redundant-cycle 1-tap DFE structure in 90nm CMOS. The other two prototypes explain more complex 6b 10GS/s ADCs with efficiently embedded feed-forward equalization (FFE) and decision feedback equalization (DFE) in 65nm CMOS. Leveraging a time-interleaved successive approximation ADC architecture, new structures for embedded DFE and FFE are proposed with low power/area overhead. Measurement results over FR4 channels verify the effectiveness of proposed embedded equalization schemes. The comparison of fabricated prototypes against state-of-the-art general-purpose ADCs at similar speed/resolution range shows comparable performances, while the proposed architectures include embedded equalization as well

A Resolution-Reconfigurable 5-to-10-Bit 0.4-to-1 V Power Scalable SAR ADC for Sensor Applications

A power-scalable SAR ADC for sensor applications is presented. The ADC features a reconfigurable 5-to-10-bit DAC whose power scales exponentially with resolution. At low resolutions where noise and linearity requirements are reduced, supply voltage scaling is leveraged to further reduce the energy-per-conversion. The ADC operates up to 2 MS/s at 1 V and 5 kS/s at 0.4 V, and its power scales linearly with sample rate down to leakage levels of 53 nW at 1 V and 4 nW at 0.4 V. Leakage power-gating during a SLEEP mode in between conversions reduces total power by up to 14% at sample rates below 1 kS/s. Prototyped in a low-power 65 nm CMOS process, the ADC in 10-bit mode achieves an INL and DNL of 0.57 LSB and 0.58 LSB respectively at 0.6 V, and the Nyquist SNDR and SFDR are 55 dB and 69 dB respectively at 0.55 V and 20 kS/s. The ADC achieves an optimal FOM of 22.4 fJ/conversion-step at 0.55 V in 10-bit mode. The combined techniques of DAC resolution and voltage scaling maximize efficiency at low resolutions, resulting in an FOM that increases by only 7x over the 5-bit scaling range, improving upon a 32x degradation that would otherwise arise from truncation of bits from an ADC of fixed resolution and voltage.United States. Defense Advanced Research Projects AgencyNatural Sciences and Engineering Research Council of Canad

Energy efficient hybrid computing systems using spin devices

Emerging spin-devices like magnetic tunnel junctions (MTJ\u27s), spin-valves and domain wall magnets (DWM) have opened new avenues for spin-based logic design. This work explored potential computing applications which can exploit such devices for higher energy-efficiency and performance. The proposed applications involve hybrid design schemes, where charge-based devices supplement the spin-devices, to gain large benefits at the system level. As an example, lateral spin valves (LSV) involve switching of nanomagnets using spin-polarized current injection through a metallic channel such as Cu. Such spin-torque based devices possess several interesting properties that can be exploited for ultra-low power computation. Analog characteristic of spin current facilitate non-Boolean computation like majority evaluation that can be used to model a neuron. The magneto-metallic neurons can operate at ultra-low terminal voltage of ∼20mV, thereby resulting in small computation power. Moreover, since nano-magnets inherently act as memory elements, these devices can facilitate integration of logic and memory in interesting ways. The spin based neurons can be integrated with CMOS and other emerging devices leading to different classes of neuromorphic/non-Von-Neumann architectures. The spin-based designs involve `mixed-mode\u27 processing and hence can provide very compact and ultra-low energy solutions for complex computation blocks, both digital as well as analog. Such low-power, hybrid designs can be suitable for various data processing applications like cognitive computing, associative memory, and currentmode on-chip global interconnects. Simulation results for these applications based on device-circuit co-simulation framework predict more than ∼100x improvement in computation energy as compared to state of the art CMOS design, for optimal spin-device parameters