Subthreshold Source-coupled Logic Research Articles

An ultra low-power current-mode clock and data recovery design with input bit-rate adaptability for biomedical applications in CMOS 90 nm

This paper presents a clock and data recovery (CDR) topology that can adjust the power consumption with its input bit-rate. Current-mode structures are utilized to control the power consumption. The system is comprised of a phase detector, a charge pump, a second-order filter, a voltage-to-current converter, a bias controller, and a current controlled oscillator. In order to attain an ultra-low power profile, subthreshold source-coupled logic (STSCL) is used to implement the system blocks. A modified version of low-voltage current-mirror is proposed and utilized to improve the thermal stability of the CDR structure. The proposed current mirror improves the oscillator frequency variation more than 20 times. The CDR is simulated in 90 nm standard CMOS process with VDD = 1 V. It can recover clock frequencies between 1 MHz and 25 MHz. The corresponding power consumption is between 195 nW and 450 nW, and the measured deterministic-jitter is less than 100 ns.

Power-Gating Sub-Threshold Source-Coupled Logic (PG-STSCL) circuits for ultra-low-power applications

The sub-threshold circuit design is regarded as a promising technique to provide considerable power reduction for ultra-low-power applications under tight energy constraints. This paper presents Power-Gating Sub-Threshold Source-Coupled Logic (PG-STSCL), which employs the fine-grain power gating at the gate level. It introduces isolation and retention circuits to ensure reliable propagation of data along a pipeline of power gated circuits, called a micro-pipeline. While the conventional STSCL circuits can considerably cut down the active power consumption, they have the drawback of continuous static current flow. To overcome this drawback, the proposed architecture shuts off the static current by utilizing the fine-grain power-gating technique. We have designed a 32-bit adder based on the proposed PG-STSCL gates in a 65 nm CMOS technology. The adder was simulated and compared to reference adders using standard CMOS gates, and conventional STSCL gates. Simulations demonstrated that the proposed gates provide a power reduction of 89.56% and 99.78% when compared to the standard CMOS and STSCL gates, respectively.

Analysis and Characterization of Variability in Subthreshold Source-Coupled Logic Circuits

This article explores the effect of device parameter variations on the performance of subthreshold source-coupled logic (STSCL) circuits. A test chip has been fabricated in a standard CMOS 90 nm technology to study the matching properties of STSCL circuits. Both process variations and device mismatch have been included in this study. The performed analysis shows that while the STSCL topology is very robust against global variations mainly thanks to the adoption of an on-chip bias generator circuit, special design techniques are required to compensate for the effect of device mismatch. Proper device sizing as well as creating intentional mismatch in the biasing condition of STSCL gates are two effective approaches that have been investigated to overcome the variation related issues. Both die-to-die (D2D) and within-die (WID) variations in the delay of STSCL gates have been characterized and validated through measurements. A comprehensive analysis of timing jitter in STSCL-based ring oscillators is also presented.

Leakage Reduction Using DTSCL and Current Mirror SCL Logic Structures for LP-LV Circuits

This paper presents a novel approach to design robust Source Coupled Logic (SCL) for implementing ultra low power circuits. In this paper, we propose two different source coupled logic structures and analyze the performance of these structures with STSCL (Sub-threshold SCL). The first design under consideration is DTPMOS as load device which analyses the performance of Dynamic Threshold SCL (DTSCL) Logic with previous source coupled logic for ultra low power operation. DTSCL circuits exhibit a better power-delay Performance compared with the STSCL Logic. It can be seen that the proposed circuit provides 56% reduction in power delay product. The second design under consideration uses basic current mirror active load device to provide required voltage swing. Current mirror source coupled logic (CMSCL) can be used for high speed operation. The advantage of this design is that it provides 54% reduction in power delay product over conventional STSCL. The main drawback of this design is that it provides a higher power dissipation compared to other source coupled logic structures. The proposed circuit provides lower sensitivity to temperature and power supply variation, with a superior control over power dissipation. Measurements of test structures simulated in 0.18 μm CMOS technology shows that the proposed DTSCL logic concept can be utilized successfully for bias currents as low as 1 pA. Measurements show that existing standard cell libraries offer a good solution for ultra low power SCL circuits. Cadence Virtuoso schematic editor and Spectre Simulation tools have been used.

Wide-Range Dynamic Power Management in Low-Voltage Low-Power Subthreshold SCL

Power-frequency scaling in subthreshold source-coupled logic (STSCL) systems has been studied and analyzed. It is shown that the operating frequency of such systems can be adjusted over about three decades with linearly proportional power dissipation. The heart of such a system is a phase-locked loop (PLL)-based clock generator (CG) with a very wide tuning range controlling the dynamics of the STSCL system. The design of a wide tuning range PLL utilizing a novel self-adjustable loop filter that generates the reference clock as well as the bias current for the STSCL system is described. The PLL-based CG exhibits linear power-frequency characteristics in order to minimize its power consumption overhead (7 pJ with 350 nA standby current). Implemented in 0.13 μm CMOS, the CG occupies 0.06 mm2 with a supply voltage that can be reduced down to VDD = 0.9 V.

Nanowatt Range Folding-Interpolating Analog-to-Digital Converter Using Subthreshold Source-Coupled Circuits

A very low power mixed-signal design methodology based on subthreshold source-coupled circuits is presented, and a nano-Watt range analog-to-digital converter (ADC) circuit based on folding-interpolating topology is proposed as a complete design example. To reduce the power dissipation to sub-µW level, subthreshold source-coupled circuit family has been developed for both analog and digital parts. As all the devices are biased in subthreshold, the sampling frequency and power consumption of the ADC can be adjusted over a very wide range. Using pipelined subthreshold source-coupled logic (STSCL) circuits renders the power dissipation of the digital part scalable, and at the same time negligible with respect to the analog part. Implemented in 0.18µm CMOS technology, the active area of the circuit is 0.6mm2. Measured integral nonlinearity (INL), and differential nonlinearity (DNL) of the ADC are 1.0 and 0.4 LSB, respectively, while sampling frequency can be adjusted from 500S/s (17nW) to 80kS/s (1.9μW).

Leakage Current Reduction Using Subthreshold Source-Coupled Logic

The performance of subthreshold source-coupled logic (STSCL) circuits for ultra-low-power applications is explored. It is shown that the power consumption of STSCL circuits can be reduced well below the subthreshold leakage current of static CMOS circuits. STSCL circuits exhibit a better power-delay performance compared with their static CMOS counterparts in situations where the leakage current constitutes a significant part of the power dissipation of static CMOS gates. The superior control on power consumption, in addition to the lower sensitivity to the process and supply voltage variations, makes the STSCL topology very suitable for implementing ultra-low-power low-frequency digital systems in modern nanometer-scale technologies. An analytical approach for comparing the power-delay performance of these two topologies is proposed.

Ultra-low power 32-bit pipelined adder using subthreshold source-coupled logic with 5 fJ/stage PDP

This article presents a new approach for improving the power-delay performance of subthreshold source-couple logic (STSCL) circuits. Using a simple two-phase pipelining technique, it is possible to increase the activity rate of STSCL gates with negligible additional cost, and hence reduce the total system energy consumption per operation. In the proposed pipelined topology, each STSCL gate is followed by a simple cross-coupled differential pair operating as a state keeper with a very low power consumption and small area overhead. Measurement results on a 32-bit pipelined adder chain fabricated with 0.18μm CMOS technology show that the proposed approach can achieve a significant reduction in power-delay product (PDP) down to 5fJ/stage.

Improving Power-Delay Performance of Ultra-Low-Power Subthreshold SCL Circuits

This brief presents a technique for improving the power-delay performance of subthreshold source-coupled logic (SCL) circuits. Based on the proposed approach, a source-follower buffer stage is used at the output of each SCL stage. Analytical results confirmed by measurements in 0.18-mum CMOS technology show an improvement by a factor of as high as 2.4 in power-delay product (PDP). It is also shown that the proposed technique can be used for implementing subthreshold ultra-low power SCL logic gates with a better power and area efficiency, compared to the traditional SCL subthreshold circuits. An optimized approach is proposed to improve the power efficiency of ultra-low power STSCL library cells.