Reduced comparator high speed low power ADC using 90 nm CMOS technology

The increasing demand of high speed and low power ADC in serial links, gigabit ethernet, high speed instruments in general and communication technologies such as ultra wide band systems in particular has put tremendous pressure on efficient design of data converters. Presently flash ADC is the architecture of choice with sampling frequency ranging from 2 to 40 GS/s with 4–6 bit resolution, where speed and low resolution is required. However we are forced to compromise between performance and complexity when such ADC is used. In this paper a single channel high speed low power CMOS based 4-bit ADC using reduced comparator and multiplexer based architecture is presented. For improving the conversion rate, both the analog (comparator array) and the digital (encoder) parts of the proposed ADC are fully modified and the architecture uses only 4 comparators instead of 15 as used in conventional flash ADC, thereby saving considerable amount of power. The proposed 4-bit 2 GS/s ADC is designed and simulated in Tanner tools with 1.2 V supply voltage using 90 nm CMOS technology. HSpice simulation result of proposed architecture shows a power dissipation of 23 mW with INL and DNL errors between ±0.4 LSB and ±0.34 LSB respectively. ENOB and SNDR for the proposed architecture are 3.72 and 24.2 respectively.     

An 8-bit 1 GS/s folding and interpolating ADC with a base-4 architecture

A base-4 architecture for folding and interpolating ADC is proposed. It employs cascaded folding and interpolating topology with both the folding factors and interpolating factors of 4. Duo to that the base-4 folding and interpolating has an intrinsic relationship with the quantization process which is base-2, the architecture requires only 2 × N + 6 comparators for an N-bit ADC. What’s more, the coarse flash ADC can be eliminated because all the most significant bits can be conveniently extracted from the intermediate signals as the “byproduct” of the folding amplifiers. In addition, the base-4 architecture can be extended to higher resolution easily because of the modularized and unified configuration. This architecture is implemented with a 1 GS/s 8-bit ADC in 0.35 μm SiGe BiCMOS process. Measurement results reveal the chip exhibits DNL of 0.30/?0.26 LSB and INL of 0.80/?0.80 LSB. The ENOB is 6.9 LSB at 10.1 MHz input. The SNDR is above 42 dB over the first and the second Nyquist zone. The SFDR is above 45 dB over the first Nyquist zone and the second Nyquist zone. The ERBW is over 1.2 GHz.     

A variability tolerant system-on-chip ready nano-CMOS analogue-to-digital converter

As integrated circuit technologies progress to nanoscale, process variations become relatively large and significantly impact circuit performance. The proactive management of process variation during the design process is critical to ensure effective device yield and to keep manufacturing costs down. In the present scenario, designers are searching for analogue-to-digital converter (ADC) architectures which are nanoscale CMOS processes tolerant. Expectations of the performance of ADCs are continuously increasing along with the progress of digital systems. A process and supply variation tolerant, System-on-Chip (SoC) ready, 1 GS/s, 6-bit flash ADC suitable for integration into nanoscale digital CMOS technologies is presented. The physical design of the ADC has been done using a generic 90 nm Salicide 1.2 V/2.5 V 1 Poly 9 Metal process design kit. Baseline post layout simulation results at nominal supply and threshold voltages are presented. The parasitic-extracted physical design of the ADC is then subjected to a corner-based methodology of process variation. The results show that process variation causes a maximum variation of 10.5% in the integral non-linearity (INL) and 5.7% in the differential non-linearity (DNL), with both INL and DNL being less than 0.5 LSB. The 90 nm ADC consumes a peak power of 5.794 mW and an average power of 3.875 mW. The comparators for the ADC have been designed using the threshold inverting technique. To show technology scalability of the design, the ADC has also been presented using a 45 nm Predictive Technology Models (PTM). At 45 nm, INL = 0.46 LSB, DNL = 0.7 LSB and a sampling rate of 100 MS/s were obtained. The 45 nm ADC consumes a peak power of 45.42 μW, and average power of 8.8 μW.     

A noise-shaping SAR ADC for energy limited applications in 90 nm CMOS technology

In this paper, an ultra-low-power successive approximation register analog-to-digital converter (ADC) for energy limited applications is presented. The ADC resolution is enhanced by using a noise-shaping technique which does not need any integrator and only uses a finite impulse response (FIR) filter. To provide a first-order noise-shaping, the quantization error is firstly extracted by using the digital-to-analog converter (DAC) dummy capacitor and it is then employed in the error feedback scheme. The proposed structure employs a low-gain and low-swing operational transconductance amplifier (OTA) to realize the FIR filter which operates only at the sampling phase. To minimize the power consumption of the ADC analog part, the OTA is powered off during the conversion phase. The proposed ADC is designed and simulated in a 90 nm CMOS technology using Spectre with a 0.5 V single power supply. The simulated ADC uses a fully-differential 8-bit charge redistribution DAC with an oversampling ratio of 8 and achieves 10.7-bit accuracy. The simulated average power consumption is 4.53 μW and the achieved maximum SNDR and SFDR are 66.1 and 73.1 dB, respectively, resulting in a figure of merit of 27.6 fJ/conversion-step.     

A 12 b 8 kS/s 16 μW 0.35 μm CMOS algorithmic ADC for sensor interface in ubiquitous environments

This work proposes a 12 b 8 kS/s ultra-low-power CMOS algorithmic analog-to-digital converter (ADC) for sensor interface applications such as accelerometers and gyro sensors requiring high-resolution, low-power, and small size simultaneously. The proposed ADC employs switched-bias power reduction and bias sharing circuits to minimize chip area and power dissipation. A signal-insensitive all directionally symmetric layout technique based on a double-poly CMOS process reduces capacitor mismatch in the multiplying D/A converter for 12 b-level high accuracy without additional conventional calibration schemes. Two independently generated currents with the same negative temperature coefficient are subtracted from each other to implement temperature- and supply-insensitive current and voltage references on-chip. The prototype ADC in a 0.35 μm 2P4M CMOS technology demonstrates a measured differential non-linearity and integral non-linearity within 0.15 and 0.56 LSB at 12 b and shows a maximum signal-to-noise-and-distortion ratio and spurious-free dynamic range of 68 and 77 dB at 8 kS/s, respectively. The ADC with an active die area of 0.70 mm² consumes 16 μW at 8 kS/s and 2.5 V.     

Comparator-based binary-search ADC architectures for UWB receivers

Recently, modern wireless communication applications have pushed ADCs power consumption into the range of fJ/conversion step by introducing circuit and architectural level enhancements. In this paper we propose improvements to binary-search topologies and demonstrate them on two ADC designs. The first one, a 4-bit ADC, uses 2 ^N  ? 1 comparators arranged in N stages, and a set of N time-interleaved track-and-holds is introduced along with a pipelined operation of the comparators, leading to an increase of the ADC throughput rate. The second is a 5-bit ADC in which the number of comparators is reduced to N. The reduction is possible because we employ reconfigurable comparator with multiple thresholds, thus splitting the comparison range. As the implementation of accurate threshold voltages has a critical impact on ADC performance, an effective design methodology based on optimization through genetic algorithms was used for the comparators. Monte Carlo simulations performed on the first ADC show that, sampling at 1.5 GSps, the ADC consumes 4.2 mW, providing 3.67 effective bits, leading to a figure of merit (FOM) of 219 fJ/conversion step. With the reduction in the number of comparators, the second ADC consumes 5 mW providing 4.6 effective bits and a FOM of 138 fJ/conversion step at the same sampling rate.     

A 1.0-V 6-b 40 MS/s time-domain flash ADC in 0.18 μm CMOS

This paper presents a 6-bit low power low supply voltage time-domain comparator. The conventional voltage comparison is moved to time-domain so as to remove pre-amplifier and latch, which enables its feasibility to low supply voltage. The voltage-to-time converter is realized by the proposed linear pulse-width-modulation. The set-up time of the D flip-flop determines the sampling rate of the converter. The resistive averaging relaxes the matching requirement of the parallel comparison cells. The total input capacitance is decreased to less than 40fF in this architecture. The above digital-intensive setting makes the analog-to-digital converter (ADC) benefit from technology scaling in both power consumption and sampling rate. The prototype ADC is fabricated in SMIC 0.18 μm CMOS process. At 40 MS/s and 1.0-V supply, it consumes 540 μW and achieves an effective-number-of-bit of 5.43, resulting in a figure-of-merit of 0.31 pJ/conversion-step and active area of 0.1 mm².     

A superconducting 6-bit analog-to-digital converter with operation to 2 × 109samples/second

The design and performance of a 6-bit superconducting A/D converter are described. The converter is based on double junction interferometers used as current comparators. The unique periodic response of these comparators makes possible a fully parallel N-bit converter requiring only N comparators. Conversion rates up to 2 × 10⁹samples per second have been demonstrated.     

An 8/10 bit 200/100MS/s configurable asynchronous SAR ADC

This paper presents an asynchronous 8/10 bit configurable successive approximation register analog-to-digital converter (ADC). The proposed ADC has two resolution modes and can work at a maximal sampling rate of 200 and 100MS/s for 8 bit mode and 10 bit mode respectively. The ADC uses a custom-designed 1 fF unit capacitor to reduce the power consumption and settling time of capacitive DAC, a dynamic comparator with tail current to minimize kickback noise and improve linearity. Moreover, asynchronous control technique is utilized to implement the ADC in a flexible and energy-efficient way. The proposed ADC is designed in 90 nm CMOS technology. At 100MS/s and 1.0 V supply, the ADC consumes 1.06 mW and offers an ENOB of 9.56 bit for 10 bit mode. When the ADC operates at 8 bit mode, the sampling rate is 200MS/s with 1.56 mW power consumption from 1.0 supply. The resulted ENOB is 7.84 bit. The FOMs for 10 bit mode at 100MS/s and 8 bit mode at 200MS/s are 14 and 34 fJ/conversion-step respectively.     

A 1 Gsps and 840 µW fully dynamic asynchronous pipelined binary search ADC

In this paper a very low power asynchronous 5-bit ADC in CMOS 45 nm process technology is described which combines the pipeline and binary search architectures. Due to utilization of dynamic non-linear amplifier, power consumption of the converter is very low. The ADC circuit uses digital calibration technique to update the reference voltages of the comparators. The power consumption of ADC is 840 µW, and the ENOB is 4.05 at 1 Gsps with input signal at the Nyquist rate. At sampling rate of 10 0Msps, the power consumption is reduced to 89 µW and the ENOB is equal to 4.6 again at the Nyquist rate.     

A reconfigurable two-stage cyclic ADC for low-power applications in 3.3 V 0.35 µm CMOS

This article presents a reconfigurable pipeline analog-to-digital converter (ADC) using a two-stage cyclic configuration. The ADC consists of two stages with 1.5 effective bit resolution, two reference circuits for voltage and current biasing, and a clock generator and timing circuit. Throughout the development of this ADC, several techniques were combined for reducing the power consumption as well as for preserving the converter linearity. To reduce the power consumption, the circuit has a single operational trans-conductance amplifier shared by both pipeline stages. To keep conversion linearity, circuits such as the bootstrapped complementary metal-oxide semiconductor (CMOS) transmission gates and a robust comparator topology were implemented. The circuit can be configured to perform conversion between 7 and 15 bit resolutions, and it works with the master clock frequency in the range of 1 kHz to 40 MHz. The circuit has been prototyped in a 3.3 V 0.35 µm CMOS process and consumes 14.1 mW at 40 MHz and 8 MSample/s sampling rate. With this resolution and sampling rate, it achieves 60.1 dB SNR, 56.57 dB SINAD and 9.1 bit ENOB at 0.666 MHz input frequency and 53.6 dB SNR, 52.4 dB SINAD and 8.6 bit ENOB at 3.85 MHz input frequency. The technological FOM obtained was 13.2 A s/m².     

A 7-bit 40 MS/s single-ended asynchronous SAR ADC in 65 nm CMOS

This paper presents a 7-bit 40 MS/s single-ended asynchronous SAR ADC intended for in-probe use in medical applications, which requires small area and good power efficiency. A single-ended architecture is proposed for a moderate resolution for its simplicity. Together with a double reference technique, the architecture reduces the area of the technology-limited large capacitors. The speed is optimized by an asymmetric delay line embedded in the asynchronous digital logic, enabling a sampling frequency of 40 MS/s. The prototype is fabricated in a 65 nm CMOS technology. Measurement shows that at 1 V supply and 40 MS/s, the ADC achieves an SNDR of 39.73 dB and an ENOB of 6.3 bit, while consuming 298.6 µW, resulting in an energy efficiency of 94.74 fJ/conversion-step. The core circuit layout only occupies 0.017 mm².     

A Method to Reduce Sampling Rate of the ADC in Feedback Channel for Wideband Digital Predistortion

This paper presents a new digital predistortion (DPD) solution for wideband signals with low feedback sampling rate. To reduce the minimum sampling rate of the analog-to-digital converter (ADC) for wideband digital predistortion, the proposed method uses a bandpass filter to form a narrowband signal before the ADC. Then, a deconvolution operation is performed to recover the original wideband signal from the ADC samples. The proposed method is evaluated with an international mobile telecommunication-advanced signal with 100 MHz bandwidth. The simulation results show that the recovered signal of the proposed method closely approximates to the original signal in the passband of the filter, and the mean square error of the deconvolution decreases as the signal-to-noise ratio increases. The proposed algorithm can reduce the sampling rate of the ADC from 1105.92 million samples per second (MSPS) to 368.64 MSPS, and improve the adjacent channel power ratio more than 20 dB, which is merely 5.6 dB less than the conventional DPD with 1105.92 MSPS sampling rate.     

A 1.1-mW 10-bit 50-MSample/s hybrid two-step ADC in 0.13-µm CMOS technology

This paper presents a hybrid two-step analog-to-digital converter (ADC) that employs a successive approximation register (SAR) ADC and a time-to-digital converter (TDC)-based ADC as coarse and fine converters, respectively. By exploiting the respective advantages of the SAR and TDC architectures, the two-step ADC is realized without a high-gain amplifier for high linearity of a multiplying digital-to-analog converter. Thus, the proposed architecture can implement a low-power ADC without compromising operational speed. In addition, two digital error corrections are used to compensate for TDC error and the final ADC output, respectively. A 10-bit 50 MS/s ADC is fabricated in a 0.13-μm complementary metal–oxide–semiconductor process and occupies a 0.12-mm² die area. Furthermore, it consumes only 1.1 mW and achieves a signal-to-noise distortion ratio and spurious-free dynamic range of 53.67 and 60 dB, respectively, resulting in a 53.7 fJ/conversion-step at a 25-MHz full-scale input.     

Architecture optimization for energy-efficient resolution-scalable 8–12-bit SAR ADCs

Low power analog-to-digital converters (ADCs) in energy constrained devices, such as wireless sensor readout modules, often target dynamic resolution scalability with application context to reduce the average power consumption. This work implements such an 8–12-bit resolution scalable ADC, using an oversampling and noise-shaping successive approximating register (SAR) architecture. This architecture is selected for its high power efficiency after a detailed comparison of various resolution enhancing techniques within the SAR framework. Specifically, in this paper, three resolution enhancing techniques are reviewed and compared on their energy usage namely: the majority voting, the oversampling, and the oversampling with noise shaping SAR ADC. Furthermore, the proposed resolution scalable ADC simplifies the design of the noise shaping filter by enabling the use of a first order switched-capacitor low-pass filter for shaping the comparator noise and the in-band quantization noise. The ADC design also alleviates the matching concerns by using only an 8-bit capacitive digital-to-analog converter (DAC) for a maximum 12-bit resolution, or 11-bit effective number of bits (ENOB). The architecture can be configured to allow an operation from 8-bit traditional SAR ADC up to an 11-bit ADC by enabling the oversampling and noise shaping loops within the SAR architecture. This ADC is designed to operate with up to 320 kS/s and achieves a power scaling from 80 nW to 1.5 \(\upmu\)W, resulting in an steeper energy-ENOB scaling trend compared to state-of-the art resolution scalable ADCs.     

An energy-efficient reconfigurable analog-to-digital converter for orthopedic implants

This paper presents a pipelined analog to digital converter (ADC) with reconfigurable resolution and sampling rate for biomedical applications. Significant power saving is achieved by turning off the sample-and-hold stage and the first two pipeline stages of the ADC instead of turning off the last two stages. The reconfiguration scheme allows having three modes of operation with variable resolutions and sampling rates. Reconfigurable operational transconductance amplifiers and an interference elimination technique have been employed to optimize power-speed-accuracy performance in biomedical instrumentation. The proposed ADC exhibits a 56.9 dB SNDR with 35.4 mW power consumption in 10-bit, 40 MS/s mode and 49.2 dB SNDR with only 7.9 mW power consumption in 8-bit, 2.5 MS/s mode. The area of the core layout is 1.9 mm² in a 0.35 μm bulk-CMOS process.     

A 0.6 V 100 KS/s 8–10 b resolution configurable SAR ADC in 0.18 μm CMOS

A resolution configurable ultra-low power SAR ADC in 0.18 μm CMOS process is presented. The proposed ADC has maximum sampling rate of 100 KS/s with configurable resolution from 8 to 10 b and operates at a supply of 0.6 V. Two-stage bootstrapped switch and voltage boosting techniques are introduced to improve the performance of the ADC at low voltage. To reduce the power consumption of the analog components of the ADC, monotonic capacitor switching procedure and fully dynamic comparator are utilized. The implementation of dynamic logic further reduces the power of the digital circuits. Post-layout simulation results show that the proposed SAR ADC consumes 521 nW and achieves an SNDR of 60.54 dB at 10 b mode, resulting in an ultra-low figure-of-merit of 6.0 fJ/conversion-step. The ADC core occupies an active area of only 350 × 280 μm².     

Integrated mixed signal control IC for 500-kHz switching frequency buck regulator

The main purpose for this work is to study the challenges of designing a digital buck regulator using pipelined analog to digital converter (ADC). Although pipelined ADC can achieve high sampling speed, it will introduce additional phase lag to the buck circuit. Along with the latency brought by processing time of additional digital circuits, as well as the time delay associated with the switching frequency, the closed loop will be unstable; moreover, raw ADC outputs have low signal-to-noise ratio, which usually need back-end calibration. In order to compensate these phase lag and make control loop unconditional stable, as well as boost up signal-to-noise ratio of the ADC block with cost-efficient design, a finite impulse response filter followed by digital proportional-integral-derivative blocks were designed. All these digital function blocks were optimised with processing speed. In the system simulation, it can be found that this controller achieved output regulation within 10% of nominal 5 V output voltage under 1 A/µs load transient condition; moreover, with the soft-start method, there is no turn-on overshooting. The die size of this controller is controlled within 3 mm² by using 180 nm CMOS technology.     

A DLL clock generator for a high speed A/D-converter with 1 ps jitter and skew calibrator with 1 ps precision in 0.35 μm CMOS

This paper presents a clock generator circuit for a high-speed analog-to-digital converter (ADC). A time-interleaved ADC requires accurate clocking for the converter fingers. The target ADC has 12 interleaved fingers each running at a speed of 166 MS/s, which corresponds to an equivalent sampling frequency of 2 GS/s. A delay-locked loop (DLL) based clock generator has been proposed to provide multiple clock signals for the converter. The DLL clock generator has been implemented with a 0.35 μm SiGe BiCMOS process (only MOS-transistor were used in DLL) by Austria Micro Systems and it occupies a 0.6 mm² silicon area. The measured jitter of the DLL is around 1 ps and the delay between phases can be adjusted using 1 ps precision.     

A 25-GS/s 6-bit time-interleaved SAR ADC with design-for-test memory in 40-nm low-leakage CMOS

This paper presents a 25-GS/s 6-bit time-interleaved (TI) SAR ADC in a 40-nm CMOS low-leakage (LL) process. The prototype utilizes 4 × 12 hierarchical sampling architecture to reduce the complexity of track-and-hold circuits and the timing skew calibration. The single-channel SAR ADC adopts asynchronous processing with two alternate comparators. A partially active reference voltage buffer is designed to reduce the power consumption. The method based on sinusoidal signal approximation is employed to calibrate timing skew errors. To characterize the ultra-high-speed ADC, an on-chip design-for-test memory is designed. At 25 GS/s, the ADC achieves the SNDR of 32.18 dB for low input frequency and 27.28 dB for Nyquist frequency. The chip consumes 800 mW and occupies 1.3 × 2.6 mm², including the TI ADC core and memory.