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Ultra-Low-Power Analog-to-Digital Converters for Medical Applications
Linköping University, Department of Electrical Engineering, Electronic Devices. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomedical systems are commonly attached to or implanted into human bodies, and powered by harvested energy or small batteries. In these systems, analog-to-digital converters (ADCs) are key components as the interface between the analog world and the digital domain. Conversion of the low frequency bioelectric signals does not require high speed, but ultralow- power operation. This combined with the required conversion accuracy makes the design of such ADCs a major challenge. Among prevalent ADC architectures, the successiveapproximation-register (SAR) ADC exhibits significantly high energy efficiency due to its good trade-offs among power consumption, conversion accuracy, and design complexity. This thesis examines the physical limitations and investigates the design methodologies and circuit techniques for low-speed and ultra-low-power SAR ADCs.

The power consumption of SAR ADC is analyzed and its lower bounds are formulated. At low resolution, power is bounded by minimum feature sizes; while at medium to high resolution, power is bounded by thermal noise and capacitor mismatch. In order to relax the mismatch requirement on the capacitor sizing while still ensuring enough linearity for high resolution, a bottom-up weight calibration technique is further proposed. It utilizes redundancy generated by a non-binary-weighted capacitive network, and measures the actual weights of more significant capacitors using less significant capacitors.

Three SAR ADCs have been implemented. The first ADC, fabricated in a 0.13μm CMOS process, achieves 9.1ENOB with 53-nW power consumption at 1kS/s. The main key to achieve the ultra-low-power operation turns out to be the maximal simplicity in the ADC architecture and low transistor count. In addition, a dual-supply voltage scheme allows the SAR digital logic to operate at 0.4V, reducing the overall power consumption of the ADC by 15% without any loss in performance. Based on the understanding from the first ADC and motivated by the predicted power bounds, the second ADC, a single-supply 9.1-ENOB SAR ADC in 65nm CMOS process has been further fabricated. It achieves a substantial (94%) improvement in power consumption with 3-nW total power at 1kS/s and 0.7V. Following the same concept of imposing maximal simplicity in the ADC architecture and taking advantage of the smaller feature size, the ultra-low-power consumption is achieved by a matched splitarray capacitive DAC, a bottom-plate full-range input-sampling scheme, a latch-based SAR control logic, and a multi-VT design approach. The third ADC fabricated in 65nm CMOS process targets at a higher resolution of 14b and a wider bandwidth of 5KHz. It achieves 12.5ENOB  with 1.98-μW power consumption at 0.8V and 10kS/s. To achieve the high resolution, the ADC implements a uniform-geometry non-binary-weighted capacitive DAC and employs a secondary-bit approach to dynamically shift decision levels for error correction. Moreover, a comparator with bias control utilizes the redundancy to reduce the power consumption.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 114 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1611
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:liu:diva-110387DOI: 10.3384/diss.diva-110387ISBN: 978-91-7519-264-2 (print)OAI: oai:DiVA.org:liu-110387DiVA: diva2:745255
Public defence
2014-10-03, Visionen, hus B, Campus Valla, Linköpings universitet, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2014-10-21Bibliographically approved
List of papers
1. A 53-nW 9.1-ENOB 1-kS/s SAR ADC in 0.13-μm CMOS for Medical Implant Devices
Open this publication in new window or tab >>A 53-nW 9.1-ENOB 1-kS/s SAR ADC in 0.13-μm CMOS for Medical Implant Devices
2012 (English)In: IEEE Journal of Solid-State Circuits, ISSN 0018-9200, E-ISSN 1558-173X, Vol. 47, no 7, 1585-1593 p.Article in journal (Refereed) Published
Abstract [en]

This paper describes an ultra-low power SAR ADC for medical implant devices. To achieve the nano-watt range power consumption, an ultra-low power design strategy has been utilized, imposing maximum simplicity on the ADC architecture, low transistor count and matched capacitive DAC with a switching scheme which results in full-range sampling without switch boot-strapping and extra reset voltage. Furthermore, a dual-supply voltage scheme allows the SAR logic to operate at 0.4 V, reducing the overall power consumption of the ADC by 15% without any loss in performance. The ADC was fabricated in 0.13-mu m CMOS. In dual-supply mode (1.0 V for analog and 0.4 V for digital), the ADC consumes 53 nW at a sampling rate of 1 kS/s and achieves the ENOB of 9.1 bits. The leakage power constitutes 25% of the 53-nW total power.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2012
Keyword
ADC, analog-to-digital conversion, leakage power consumption, low-power electronics, medical implant devices, successive approximation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-80792 (URN)10.1109/JSSC.2012.2191209 (DOI)000306913500008 ()
Available from: 2012-08-30 Created: 2012-08-30 Last updated: 2017-12-07Bibliographically approved
2. Analysis and Calibration of Nonbinary-Weighted Capacitive DAC for High-Resolution SAR ADCs
Open this publication in new window or tab >>Analysis and Calibration of Nonbinary-Weighted Capacitive DAC for High-Resolution SAR ADCs
2014 (English)In: IEEE Transactions on Circuits and Systems - II - Express Briefs, ISSN 1549-7747, E-ISSN 1558-3791, Vol. 61, no 9, 666-670 p.Article in journal (Refereed) Published
Abstract [en]

This brief analyzes the effect of capacitor variation on the design of high-resolution nonbinary-weighted successive-approximation-register analog-to-digital converters in terms of radix, conversion steps, and accuracy. Moreover, the limitation caused by the one-side redundancy of the nonbinary-weighted network is addressed and a corresponding solution with a mathematical derivation is provided. In order to relax the mismatch requirement on the capacitor sizing while still ensuring enough linearity, a bottom-up weight calibration technique accounting for noise and offset errors is proposed, and its effectiveness is demonstrated. This calibration approach can be easily incorporated into a charge-redistribution converter without modifying its main architecture and conversion sequence.

Place, publisher, year, edition, pages
IEEE, 2014
Keyword
Capacitor variation; digital error correction; nonbinary weighted; redundancy; successive approximation; successive approximation register (SAR) analog-to-digital converters (ADCs); weight calibration
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-110385 (URN)10.1109/TCSII.2014.2331111 (DOI)000341985600006 ()
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2017-12-05Bibliographically approved
3. A 3-nW 9.1-ENOB SAR ADC at 0.7 V and 1 kS/s
Open this publication in new window or tab >>A 3-nW 9.1-ENOB SAR ADC at 0.7 V and 1 kS/s
2012 (English)In: ESSCIRC, 2012, Institute of Electrical and Electronics Engineers , 2012, 369-372 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents a 10-bit SAR ADC in 65 nm CMOS for medical implant applications. The ADC consumes 3-nW power and achieves 9.1 ENOB at 1 kS/s. The ultra-low-power consumption is achieved by using an ADC architecture with maximal simplicity, a small split-array capacitive DAC, a bottom-plate sampling approach reducing charge injection error and allowing full-range input sampling without extra voltage sources, and a latch-based SAR control logic resulting in reduced power and low transistor count. Furthermore, a multi-Vt circuit design approach allows the ADC to meet the target performance with a single supply voltage of 0.7 V. The ADC achieves a FOM of 5.5 fJ/conversion-step. The INL and DNL errors are 0.61 LSB and 0.55 LSB, respectively.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers, 2012
Series
IEEEESSCIRC Proceedings, ISSN 1930-8833
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-86346 (URN)10.1109/ESSCIRC.2012.6341331 (DOI)978-1-4673-2211-9 (ISBN)978-1-4673-2212-6 (ISBN)
Conference
IEEE European Solid-State Circuits Conference (ESSCIRC 2012), 17-21 September 2012, Bordeaux, France
Available from: 2012-12-13 Created: 2012-12-13 Last updated: 2014-09-10
4. Power Consumption Bounds for SAR ADCs
Open this publication in new window or tab >>Power Consumption Bounds for SAR ADCs
2011 (English)In: European Conference on Circuit Theory and Design (ECCTD), Linköping, Sweden: IEEE conference proceedings, 2011, 556-559 p.Conference paper, Published paper (Refereed)
Abstract [en]

Power consumption is an important limitation to analog-to-digital converters. The objective of this paper is to estimate a lower bound to the power consumption of successive approximation analog-to-digital converters. This is an extension of our previous work which was limited to pipelined and flash architectures. We find that the power consumption in our case is bounded by capacitor mismatch or thermal noise at high resolution and by digital switching power at low resolution. We also evaluate our methods and the estimated lower bound is compatible with experimental data.

Place, publisher, year, edition, pages
Linköping, Sweden: IEEE conference proceedings, 2011
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-73030 (URN)10.1109/ECCTD.2011.6043594 (DOI)978-1-4577-0617-2 (ISBN)978-1-4577-0616-5 (ISBN)
Conference
20th European Conference on Circuit Theory and Design, Linköping, 29-31 Aug. 2011
Available from: 2011-12-14 Created: 2011-12-14 Last updated: 2014-09-10Bibliographically approved
5. Design of CMOS sampling switch for ultra-low power ADCs in biomedical applications
Open this publication in new window or tab >>Design of CMOS sampling switch for ultra-low power ADCs in biomedical applications
2010 (English)In: NORCHIP 2014, Tampere: IEEE , 2010, 1-4 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper deals with the design of CMOS sampling switch for ultra-low power analog-to-digital converters (ADC) in biomedical applications. General switch design constraints are analyzed, among which the voltage droop due to the subthreshold leakage current constitutes the major error source for low-speed sampling circuits. Based on the analyses, a CMOS sampling switch with leakage-reduction has been designed for a 10-bit 1-kS/s successive approximation (SA) ADC in a standard 130 nm CMOS process. Post-layout simulation shows that the ADC with the proposed switch offers an effective number of bits (ENOB) of 9.5 while consuming only 64 nW.

Place, publisher, year, edition, pages
Tampere: IEEE, 2010
Keyword
CMOS integrated circuits, analogue-digital conversion, biomedical electronics, switches
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-65477 (URN)10.1109/NORCHIP.2010.5669444 (DOI)978-1-4244-8971-8 (ISBN)978-1-4244-8972-5 (ISBN)
Conference
The 32nd Norchip Conference 27-28 October 2014, Tampere, Finland
Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2014-09-10Bibliographically approved

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