Design techniques for low-voltage and low-power analog-to-digital converters

With the ever-increasing demand for portable devices used in applications such as wireless communication, mobile computing, consumer electronics, etc., the scaling of the CMOS process to deep submicron dimensions becomes more important to achieve low-cost, low-power and high-performance digital systems. However, this downscaling also requires similar shrinking of the supply voltage to insure device reliability. Even though the largest amount of signal processing is done in the digital domain, the on-chip analog-to-digital interface circuitry (analog-to-digital and digital-to-analog converters) is an important functional block in the system. These converters are also required to operate with low-voltage supply. In this thesis, design techniques for low-voltage and low-power analog-to-digital converters are proposed. The specific research contributions of this work include (1) introduction of a new low-voltage switching technique for switched- capacitor circuit design, (2) development of low-voltage and low-distortion delta- sigma modulator, (3) development of low-voltage switched-capacitor multiplying digital-to-analog converter (MDAC), (4) a new architecture for the low-power Nyquist rate pipelined ADC design. These design techniques enable the implementation of low-voltage and low-power CMOS analog-to-digital converters. To demonstrate the proposed design techniques, a 0.6 V, 82 dB, 2-2 cascaded audio delta-sigma ADC, a 0.9 V, 10-bit, 20MS/s CMOS pipelined ADC and a 2.4 V, 12-bit, 10MS/s CMOS pipelined ADC were implemented in standard CMOS processes.Keywords: ADC, delta-sigma, low-voltage, low-power, switched-R

Low Power CMOS Design : Exploring Radiation Tolerance in a 90 nm Low Power Commercial Process

This thesis aims to examine radiation tolerance of low power digital CMOS circuits in a commercial 90 nm low power triple-well process from TSMC. By combining supply voltage scaling and Radiation-Hardened By Design (RHBD) design techniques, the goal is to achieve low supply voltage, radiation tolerant, circuit behavior. The target circuit architecture for comparison between different radiation hardening techniques is a Successive Approximation Register (SAR) architecture comprising both combinational and sequential logic. The purpose of the SAR architecture is to emulate a larger system, since larger systems are usually composed of combinational and sequential building blocks. The method used for achieving low power operation is primarily voltage scaling, with the ultimate goal of reaching subthreshold operation, while maintaining radiation tolerant circuit behavior. Radiation hardening is performed on circuit-level by applying RHBD circuit topologies, as well as architectural-level mitigation techniques. This thesis includes three papers within the field of robust low power CMOS design. Two of the papers cover low power level shifter designs in 90 nm and 65 nm process from STMicroelectronics. The third paper examines memory element design using minority-3 gates and inverters for robust low voltage operation. Prototyping has been conducted on low power CMOS building blocks including level shifter and memory design, for potential use in future radiation tolerant designs. Prototyping has been conducted on two chips from two different 90 nm processes from STMicroelectronics and TSMC. A test setup for radiation induced errors has been developed. Experimental radiation tests of the SAR architectures were conducted at SAFE, revealing no radiation induced errors

Design techniques for low power mixed analog-digital circuits with application to smart wireless systems.

This dissertation presents and discusses new design techniques for mixed analog-digital circuits with emphases on low power and small area for standard low-cost CMOS VLSI technology. The application domain of the devised techniques is radio frequency identification (RFID) systems, however the presented techniques are applicable to wide range of mixed mode analog-digital applications. Hence the techniques herein apply to a range of smart wireless or mobile systems. The integration of both analog and digital circuits on a single substrate has many benefits such as reducing the system power, increasing the system reliability, reducing the system size and providing high inter-system communications speed - hence, a cost effective system implementation with increased performance. On the other hand, some difficulties arise from the fact that standard low-cost CMOS technologies are tuned toward maximising digital circuit performance and increasing transistor density per unit area. Usually these technologies have a wide spread in transistor parameters that require new design techniques that provide circuit characteristics based on relative transistor parameters rather than on the absolute value of these parameters. This research has identified new design techniques for mostly analog and some digital circuits for implementation in standard CMOS technologies with design parameters dependent on the relative values of process parameters, resulting in technology independent circuit design techniques. The techniques presented and discussed in this dissertation are (i) applied to the design of low-voltage and low-power controlled gain amplifiers, (ii) digital trimming techniques for operational amplifiers, (iii) low-power and low-voltage Schmitt trigger circuits, (iv) very low frequency to medium frequency low power oscillators, (v) low power Gray code counters, (vi) analog circuits utilising the neuron MOS transistor, (vii) high value floating resistors, and (viii) low power application specific integrated circuits (ASICs) that are particularly needed in radio frequency identification systems. The new techniques are analysed, simulated and verified experimentally via five chips fabricated through the MOSIS service.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 200

500mV low-voltage operational amplifier design

With the dramatic increase in the number of transistors on a chip and the increasing needs for battery-powered applications, low-voltage circuit design techniques have been widely studied in recent year. However, these low supply voltage research efforts have been focused mainly on digital circuits, especially on high density memory circuits. Reported success in achieved high performance low voltage operation in analog circuits lags far behind. Recent results have been presented on CMOS low-voltage operational amplifiers, where the supply voltage has been reduced to less than 2.5V in which the complementary input stages were used to keep the gm constant [SI95] [HL85]. Recently, the floating gate MOS transistor has attracted considerable interest as a nonvolatile analog storage device and as a precision analog trim element because it has threshold voltage programming ability [YU93] [RC95].;The particular focus of this work is on implementing very low voltage analog and mixed-signal integrated circuit in a standard CMOS process. As a proof-of-concept vehicle, this work concentrates on the design of very low voltage operational amplifiers in standard CMOS processes. By connecting a DC reference voltage source in series with the gate of all MOS transistors, the equivalent threshold voltage of all transistors can be electrically lowered. This technique makes it possible to decrease the power supply voltage. The DC reference voltage sources are realized by using a switched capacitor charged periodically and switched between the actual circuit and a reference precharge circuit. By extracting the reference voltage source directly from the threshold voltage itself, the threshold voltage variations due to the process and temperature variations can be compensated, since large threshold variations are intolerable for very low threshold voltage applications.;In a proof-of-concept two-stage operational amplifier designed to operate with a single 5OOmV power supply in a standard 2[Mu] process, the tail current is kept the same as in a 3.3V design, thus the key performance parameters are expected to be maintained at reasonable values. The dramatic decrease of the power supply possible with this approach is paralleled with a corresponding reduction in the power dissipation. Simulation results of this 5OOmV operational amplifier show a 7OdB DC gain, 7.8MHz unity gain bandwidth and a 650 phase margin. Power dissipation is reduced by more than 90% from that of the corresponding 3.3V design. Although the specific implementation is focused on the implementation of an operational amplifier with comparable performance parameters to those with larger supply voltage, the dominant applications of this technique are for designing a variety of analog and mixed-signal systems that operate at very low voltages and with low power dissipation

Circuit Modules for CMOS High-Power Short Pulse Generators

High-power short electrical pulses are important for high-performance functionality integration, such as the development of microelectromechanical/nanoelectromechanical systems (MEMS/NEMS), system on chip (SoC) and lab on chip (LoC). Many of these applications need high-power (low impedance load) short electrical pulses, in addition to CMOS digital intelligence. Therefore, it is of great interest to develop new circuit techniques to generate high-power high-voltage short electrical pulses on-chip. Results on pulse forming line (PFL) based CMOS pulse generator studies are reported. Through simulations, the effects of PFL length, switch speed and switch resistance on the output pulses are clarified. CMOS pulse generators are modeled and analyzed with on-chip transmission lines (TLs) as PFLs and CMOS transistors as switches. In the 0.13 um CMOS process with a 500 um long PFL, post layout simulations show that pulses of 10.4 ps width can be obtained. High-voltage and high-power outputs can be generated with other pulsed power circuits, such as Blumlein PFLs with stacked MOSFET switches. Thus, the PFL circuit significantly extends short and high-power pulse generation capabilities of CMOS technologies. A CMOS circuit with a 4 mm long PFL is implemented in the commercial 0.13 um technology. Pulses of ~ 160 ps duration and 110-200 mV amplitude on a 50 Ohms load are obtained when the power supply is tuned from 1.2 V to 2.0 V. Measurement Instruments limitations are probably the main reasons for the discrepancies among measurement and simulation results. A four-stage charge pump is presented as high voltage bias of the Blumlein PFLs pulse generator. Since Schottky diode has low forward drop voltage (~ 0.3V), using it as charge transfer cell can have high charge pumping gain and avoid additional control circuit for switch. A four-stage charge pump with Schottky diode as charge transfer cell is implemented in a commercial 0.13 um technology. Charge pump output and efficiency under different power supply voltages, load currents and clock frequencies are measured and presented. The maximum output voltage is ~ 6 V and the maximum efficiency is ~ 50%

Improving the power-delay performance in subthreshold source-coupled logic circuits

Subthreshold source-coupled logic (STSCL) circuits can be used in design of low-voltage and ultra-low power digital systems. This article introduces and analyzes new techniques for implementing complex digital systems using STSCL gates with an improved power-delay product (PDP) based on source-follower output stages. A test chip has been manufactured in a conventional digital 0.18$\mu$m CMOS technology to evaluate the performance of the proposed STSCL circuit, and speed and PDP improvements by a factor of up to 2.4 were demonstrated

Low-Voltage Bulk-Driven Amplifier Design and Its Application in Implantable Biomedical Sensors

The powering unit usually represents a significant component of the implantable biomedical sensor system since the integrated circuits (ICs) inside for monitoring different physiological functions consume a great amount of power. One method to reduce the volume of the powering unit is to minimize the power supply voltage of the entire system. On the other hand, with the development of the deep sub-micron CMOS technologies, the minimum channel length for a single transistor has been scaled down aggressively which facilitates the reduction of the chip area as well. Unfortunately, as an inevitable part of analytic systems, analog circuits such as the potentiostat are not amenable to either low-voltage operations or short channel transistor scheme. To date, several proposed low-voltage design techniques have not been adopted by mainstream analog circuits for reasons such as insufficient transconductance, limited dynamic range, etc. Operational amplifiers (OpAmps) are the most fundamental circuit blocks among all analog circuits. They are also employed extensively inside the implantable biosensor systems. This work first aims to develop a general purpose high performance low-voltage low-power OpAmp. The proposed OpAmp adopts the bulk-driven low-voltage design technique. An innovative low-voltage bulk-driven amplifier with enhanced effective transconductance is developed in an n-well digital CMOS process operating under 1-V power supply. The proposed circuit employs auxiliary bulk-driven input differential pairs to achieve the input transconductance comparable with the traditional gate-driven amplifiers, without consuming a large amount of current. The prototype measurement results show significant improvements in the open loop gain (AO) and the unity-gain bandwidth (UGBW) compared to other works. A 1-V potentiostat circuit for an implantable electrochemical sensor is then proposed by employing this bulk-driven amplifier. To the best of the author’s knowledge, this circuit represents the first reported low-voltage potentiostat system. This 1-V potentiostat possesses high linearity which is comparable or even better than the conventional potentiostat designs thanks to this transconductance enhanced bulk-driven amplifier. The current consumption of the overall potentiostat is maintained around 22 microampere. The area for the core layout of the integrated circuit chip is 0.13 mm2 for a 0.35 micrometer process

Noise optimization for low-voltage CMOS audio preamplifier systems

There is a large and growing market for portable consumer audio products with very small size. As the size of these products is reduced, the area occupied by batteries becomes significant and hence limits the number of batteries to one. In order to build such small products, high levels of integration are required to minimize both the number of integrated circuits and off-chip discrete components. With supply voltages limited to that supplied by one battery, special low-voltage and low-power circuit design techniques are required. For digital circuits integrated on a chip, the reduction in supply voltage directly translates to reduced power dissipation. However, for analog circuits on the same chip, the reduction in supply voltage can cause a significant increase in both required power dissipation and die area primarily due to the reduction in available signal swing and the limited selection of suitable low-voltage circuit architectures. The objective of this research was to provide analysis and optimization techniques to meet signal-to-noise ratio specifications with minimum power dissipation and die area. A 0.9V microphone preamplifier and programmable gain amplifier system was designed using these techniques and fabricated with a 0.35um CMOS process.Keywords: Optimization, Audio, Noise, CMOS, Preamplifier, Low-voltag