Browsing by Subject "SAR ADC"

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A 10-bit, 10Msps pipelined ADC with first stage conventional SAR ADC and second stage multi-bit per cycle SAR ADC
(2016-05-19) Gulati, Paridhi; Sun, Nan; Orshansky, Michael
A pipelined ADC is generally used for high speeds and high resolutions in applications where latency is not a major concern. This project involves the design of a 10 bit pipelined ADC with a conventional SAR ADC as stage one. The first stage also has an integrated comparator and amplifier. A dynamic automatic gain control scheme is used for the amplification of the first stage residue voltage. Techniques such as redundancy help in achieving higher speed while bidirectional single side switching helps in reducing power consumption. The second stage is a 3 bit per cycle SAR ADC that makes use of a scaled down version of the voltage supply. The ADC designed in this project makes use of 0.13um CMOS technology and is able to achieve a sampling rate of 10MS/s and ENOB of 9.95.
Built-in-self-test and foreground calibration of SAR ADCs
(2017-12) Varier, Vivek; Sun, Nan; Viswanathan, TR
This thesis explores the scope of ‘Built-in-Self-Test’(BIST) schemes to reduce the time cost complexity associated with the production tests for static linearity errors in Successive Approximation (SAR) ADCs. In this regard, an on-chip implementation of the ‘Stimulus Based Error Identification and Removal’ (SEIR) method [1] is sought to be pursued. As an extension, it is proposed that the estimated ADC non-linearities may then be suitably calibrated to achieve higher resolution. A brief review of the testing and calibration algorithm is undertaken. Further, this work elaborates on the design of a prototype front-end test generator and a buffer interface to calibrate a 10MHz 14 bit redundant SAR ADC in the TSMC 180nm process. Simulation results validating the circuit implementation of the integrated front-end system have been presented.
Design and implementation of Radix-3/Radix-2 based novel hybrid SAR ADC in scaled CMOS technologies
(2018-01-24) Rahman, Md. Manzur; Sun, Nan; Viswanathan, T. R.; Swartzlander, Earl; Pan, David; Cao, Changua
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.
Design techniques for low-power SAR ADCs in nano-scale CMOS technologies
(2016-05) Chen, Long; Sun, Nan; Viswanathan, T.R.; Pan, David Z.; Orshansky, Michael; Soenen, Eric
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.
Fully-passive switched-capacitor techniques for high performance SAR ADC design
(2016-02-11) Guo, Wenjuan, Ph. D. in electrical and computer engineering; Sun, Nan; Orshansky, Michael; Tewfik, Ahmed H.; Viswanathan, T. R.
In recent years, SAR ADC becomes more and more popular in various low-power applications such as wireless sensors and low energy radios due to its circuit simplicity, high power efficiency, and scaling compatibility. However, its speed is limited by its successive approximation procedures and its power efficiency greatly reduces with the ADC resolution going beyond 10 bit. To address these issues, this thesis proposes to embed two techniques: 1) compressive sensing (CS) and 2) noise shaping (NS) to a conventional SAR ADC. The realization of both techniques are based on fully-passive switched-capacitor techniques. CS is a recently emerging sampling paradigm, stating that the sparsity of a signal can be exploited to reduce the ADC sampling rate below the Nyquist rate. Different from conventional CS frameworks which require dedicated analog CS encoders, this thesis proposes a fully-passive CS-SAR ADC architecture which only requires minor modification to a conventional SAR ADC. Two chips are fabricated in a 0.13 µm process to prove the concept. One chip is a single-channel CS-SAR ADC which can reduce the ADC conversion rate by 4 times, thus reducing the ADC power by 4 times. In many wireless sensing applications, multiple ADCs are commonly required to sense multi-channel signals such as multi-lead ECG sensing and parallel neural recording. Therefore, the other chip is a multi-channel CS-SAR ADC which can simultaneously convert 4-channel signals with a sampling rate of one channel’s Nyquist rate. At 0.8 V and 1 MS/s, both chips achieve an effective Walden FoM of around 5 fJ/conversion-step. This thesis also proposes a novel NS SAR ADC architecture that is simple, robust and low power for high-resolution applications. Compared to conventional ∆Σ ADCs, it replaces the power-hungry active integrator with a passive integrator which only requires one switch and two capacitors. Compared to previous 1st-order NS SAR ADC works, it achieves the best NS performance and can be easily extended to 2nd-order. A 1st-order 10-bit NS SAR ADC is fabricated in a 0.13 µm process. Through NS, SNDR increases by 6 dB with OSR doubled, achieving a 12- bit ENOB at OSR = 8. An improved version of a 2nd-order 9-bit NS SAR ADC is designed and simulated in a 40 nm process. The SNDR increases by 10 dB with OSR doubled, achieving a 14-bit ENOB at OSR = 16. At a bandwidth of 312.5 kHz, the Schreier FoM is 181 dB and the Walden FoM is 12.5 fJ/conversion-step, proving that the proposed NS SAR ADC architecture can achieve high resolution and high power efficiency simultaneously.
Low-power high-speed ADC design techniques in scaled CMOS process
(2018-01-24) Song, Jeonggoo; Sun, Nan; Viswanathan, TR; Pan, David; Gharpurey, Ranjit; El-Chammas, Manar
The power consumption of a single-channel successive approximation register (SAR) analog-to-digital (ADC) tends to linearly increase with its sampling rate (f[subscript s]), when f[subscript s] is small. However, when f[subscript s] passes a certain point for a given technology node, the ADC power P increases at much higher rate and the normalized power efficiency (P/f[subscript s]) starts to degrade rapidly. To enhance the conversion speed of SAR ADC, while maintaining a good power efficiency, this thesis presents speed-enhancing techniques for SAR ADC in nano-scale CMOS technologies. First chapter presents a 2b/cycle hybrid SAR architecture with only 1 differential capacitor-DAC (CDAC). Unlike prior multi-bit/cycle SAR works that make use of only the DAC differential mode (DM) voltage, the proposed architecture exploits both the DM and the common mode (CM). By using two degrees of freedom, 2b/cycle conversion technique can boost the f[subscript s] of the ADC without any additional DAC arrays. High-speed ADCs can boost the conversion speed not only by increasing the f[subscript s] of a single-channel ADC, but also by time-interleaving multiple ADC sub-channels running at a lower rate. For an N-channel time-interleaved (TI) SAR ADC operating at f[subscript s], each sub-SAR channel only needs to operate at f[subscript s]=N. Therefore, each sub-SAR can operate in the linear power versus speed region, leading to a significant power saving compared to a single-channel ADC running at the same sampling rate. Despite of its power efficiency, TI-ADC suffers from mismatches among sub-ADC channels, including gain, offset, and timing mismatches. Among them, timing skew is one of the most difficult errors to calibrate as it is nontrivial to extract and its induced error depends on both the frequency and the amplitude of the input signal. Second chapter of this thesis presents a TI-SAR with a fast variance-based timing-skew calibration technique. It uses a single-comparator based window detector (WD) to calibrate the timing skew. The WD suppresses variance estimation errors and allow precise variance estimation from a significantly small number of samples. It has low-hardware cost and orders of magnitude faster convergence speed compared to prior variance-based timing-skew calibration technique. The last chapter presents another TI-SAR with mean absolute deviation (MAD) based timing-skew calibration technique. In addition to all the advantages presented with the fast variance-based timing-skew calibration technique, the proposed technique further reduces the digital computation power by 50% by eliminating the squaring operations, which are essential in variance-based calibration technique
A SEIR-based ADC built-in-self-test and its application in ADC self-calibration
(2013-12) Jin, Xiankun; Sun, Nan
The static linearity test is one of the fundamental production tests used to measure DC performance of analog to digital converters (ADCs). It comes with high test equipment cost. An ADC built-in-self-test (BIST) is an attractive solution. However the stringent linearity requirement for an on-chip signal generator has made it prohibitive. The stimulus error identification and removal (SEIR) method has greatly reduced the linearity requirement. However, it requires a highly stable voltage offset, which remains a daunting task. This work exploits the inherit capacitive sample-and-hold circuit used in various ADC architectures to inject offset with very good constancy. A 16-bit successive approximate register (SAR) ADC with the proposed BIST scheme is modeled and simulated in Matlab to prove its validity. The results show that the estimation error on the maximum INL is less than 0.07 LSB. This BIST solution is then naturally extended to the calibration of an ADC. It is shown missing codes of such ADC can be effectively estimated and calibrated out.
A study of capacitor array calibration for a successive approximation analog-to-digital converter
(2013-05) Ma, Ji, active 2013; Sun, Nan
Analog-to-digital converters (ADCs) are driven by rapid development of mobile communication systems to have higher speed, higher resolution and lower power consumption. Among multiple ADC architectures, successive approximation (SAR) ADCs attract great attention in mixed-signal design community recently. It is due to the fact that they do not contain amplification components and the digital logics are scaling friendly. Therefore, it is easier to design a SAR ADC with smaller component size in advanced technology than other ADC architectures, which decreases the power consumption and increases the speed of the circuit. However, capacitor mismatch limits the minimum size of unit capacitors which could be used for a SAR ADC with more than 10 bit resolution. Large capacitor both limits conversion speed and increases switching power. In this design project, a novel switching scheme and a novel calibration method are adopted to overcome the capacitor mismatch constraint. The switching scheme uses monotonic switching in a SAR ADC to gain one extra bit, and switches a dummy capacitor between the common mode voltage level (Vcm) and the ground (gnd) to obtain another extra bit. To keep the resolution constant, the capacitor number is reduced by two. The calibration method extracts missing code width to estimate the actual value of capacitors. The missing code extraction is accomplished by detecting metastable state of a comparator, forcing the current bit value and using less significant bits to measure the actual capacitor value. Dither method is adopted to improve calibration accuracy. Behavior model simulation is provided to verify the effectiveness of the calibration method. A circuit design of a 12 bit ADC and the simulation for schematic design is presented in this report.
A study of SAR ADC and implementation of 10-bit asynchronous design
(2013-08) Kardonik, Olga; Sun, Nan
Successive Approximation Register (SAR) Analog-to-Digital Converters (ADCs) achieve low power consumption due to its simple architecture based on dominant digital content. SAR ADCs do not require an op-amp, so they are advantageous in CMOS technology scaling. The architecture is often the best choice for battery-powered or mobile applications which need medium resolution (8-12 bits), medium speed (10 - 100 MS/s) and require low-power consumption and small form factor. This work studies the architecture in depth, highlighting its main constraints and tradeoffs involving into SAR ADC design. The work researches asynchronous operation of SAR logic and investigates the latest trends for ADC’s analog components – comparator and DAC. 10-bit asynchronous SAR ADC is implemented in CMOS 0.18 µm. Design’s noise and power are presented as a breakdown among components.
Utilizing digital design techniques and circuits to improve energy and design efficiency of analog and mixed-signal circuits
(2021-05-01) Gandara, Miguel Francisco; Pan, David Z.; Sun, Nan; Soenen, Eric; Gharpurey, Ranjit; Orshansky, Michael E
Technology scaling has long driven large growth in the electronics market. With each successive technology generation, digital circuits become more power and area efficient. The large performance increases realized for digital circuits due to digital scaling have not translated to similar performance improvements for analog circuits. First, noise-limited analog circuits are not capable of leveraging the reduced parasitics of advanced processes, since capacitor sizes are generally set by noise requirements. Second, analog circuit performance is closely tied to the achievable device intrinsic gain, which degrades as process sizes shrink. Reduced supply voltages further exacerbate this issue, as the achievable gain per stage is limited by the number of devices that can be stacked while maintaining all devices in saturation. Finally, process variation increases with decreased feature sizes, so analog circuits have deal with increased mismatch and wider variations in threshold voltages, increasing the time required to design a circuit that is robust across process, voltage, and temperature (PVT) variation. This work seeks to address the limitations of analog circuits in advanced technologies by leveraging digital techniques and digital-like circuits that offer improved scalability. The first half of this dissertation investigates replacing the traditional closed-loop residue amplifier in a pipeline analog-to-digital converter (ADC) with an open loop dynamic amplifier. Previous works incorporating dynamic amplifiers have struggled to achieve large gains and have suffered from offset mismatch between the comparator and amplifier, which will only get worse in more advanced technologies. We propose the usage of a residue amplifier that combines an integration stage, to ensure low noise operation, with a positive feedback stage, to ensure high gain and high speed operation. By utilizing this topology, the proposed amplifier was the first dynamic amplifier to achieve a high gain of 32. Additionally, the proposed amplifier can reuse existing comparator hardware in the ADC, removing all offset mismatch between comparator and amplifier. Digital calibration techniques were applied to ensure a constant gain across PVT. The next part of this dissertation tries to overcome the scaling challenges for noise-limited ADCs with band-limited input signals. By leveraging digital filtering techniques to generate a prediction of the band-limited signal, the conversion can be limited to a range that is a fraction of the total ADC input range, allowing for significant decreases in reference and comparator power consumption. This work extends previous works by enabling accurate predictions for any band-limited signal characteristic. Previous works only focused on accurate predictions for low-activity signals. Finally, the large compute power enabled by modern technology scaling is leveraged to improve the design efficiency of analog circuits. A new automated circuit sizing tool is proposed that can achieve better performance than manual designs done by experts in a much shorter amount of time. All of these techniques help to alleviate the power and design efficiency limitations caused by technology scaling.