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Design of a Gain-stage for Pipelined SAR ADC Using Capacitive Charge Pump
Linköping University, Department of Electrical Engineering. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Electrical Engineering, Integrated Circuits and Systems. Linköping University, Faculty of Science & Engineering.
Abstract [en]

This paper presents the design of a multi-stage capacitive charge pump (CCP) as a gain-stage which is used in the two-stage pipelined successive approximation analog-to-digital converter (SAR ADC). The topology of multi-stage CCP and the design considerations are provided. Thereafter, the power comparison between switch capacitor (SC) integrator and multi-stage CCP is analyzed with the parameters from 0.35-mu m CMOS process. The comparison results show that the proposed gain-stage is more power efficient than SC integrator. To verify the analysis, two types of gain-stage, SC integrator and multi-stage CCP, were simulated in 0.35-mu m CMOS process. Simulation results show that the three-stage CCP achieves a gain of 7.9 while only consuming 1.1 mu W with the gain bandwidth of 178.7 kHz. But the SC integrator consumes 1.58 times more power than CCPs to reach the similar gain and gain bandwidth.

Place, publisher, year, edition, pages
IEEE , 2016. 187-190 p.
Keyword [en]
Capacitive charge pump; successive approximation analog-to-digital converter; power consumption; pipeline; two-stage
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:liu:diva-132104DOI: 10.1109/MIXDES.2016.7529729ISI: 000383221700035ISBN: 978-8-3635-7808-4 (print)OAI: diva2:1038346
23rd International Conference on Mixed Design of Integrated Circuits and Systems (MIXDES)
Available from: 2016-10-18 Created: 2016-10-17 Last updated: 2016-12-15
In thesis
1. Energy-Efficient Data Converters for Low-Power Sensors
Open this publication in new window or tab >>Energy-Efficient Data Converters for Low-Power Sensors
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wireless sensor networks (WSNs) are employed in many applications, such as for monitoring bio-potential signals and environmental information. These applications require high-resolution (> 12-bit) analog-to-digital converters (ADCs) at low-sampling rates (several kS/s). Such sensor nodes are usually powered by batteries or energy-harvesting sources hence low power consumption is primary for such ADCs. Normally, tens or hundreds of autonomously powered sensor nodes are utilized to capture and transmit data to the central processor. Hence it is profitable to fabricate the relevant electronics, such as the ADCs, in a low-cost standard complementary metal-oxide-semiconductor (CMOS) process. The two-stage pipelined successive approximation register (SAR) ADC has shown to be an energy-efficient architecture for high resolution. This thesis further studies and explores the design limitations of the pipelined SAR ADC for high-resolution and low-speed applications.

The first work is a 15-bit, 1 kS/s two-stage pipelined SAR ADC that has been implemented in 0.35-μm CMOS process. The use of aggressive gain reduction in the residue amplifier combined with a suitable capacitive array digital-to-analog converter (DAC) topology in the second-stage simplifies the design of the operational transconductance amplifier (OTA) while eliminating excessive capacitive load and consequent power consumption. A comprehensive power consumption analysis of the entire ADC is performed to determine the number of bits in each stage of the pipeline. Choice of a segmented capacitive array DAC and attenuation capacitorbased DAC for the first and second stages respectively enable significant reduction in power consumption and area. Fabricated in a low-cost 0.35-μm CMOS process, the prototype ADC achieves a peak signal-to-noise-and-distortion ratio (SNDR) of 78.9 dB corresponding to an effective number of bits (ENOB) of 12.8-bit at a sampling frequency of 1 kS/s and provides a Schreier figure-of-merit (FoM) of 157.6 dB. Without any form of calibration, the ADC maintains an ENOB > 12.1-bit up to the Nyquist bandwidth of 500 Hz while consuming 6.7 μW. Core area of the ADC is 0.679 mm2.

The second work is a 14-bit, tunable bandwidth two-stage pipelined SAR ADC which is suitable for low-power, cost-effective sensor readout circuits. To overcome the high open-loop DC gain requirement of the OTA in the gain-stage, a 3-stage capacitive charge pump (CCP) is utilized to achieve the gain-stage instead of using the switch capacitor (SC) amplifier. Unity-gain OTAs have been used as the analog buffers to prevent the charge sharing between the CCP stages. The detailed design considerations are given in this work. The prototype ADC, designed and fabricated in a low-cost 0.35-μm CMOS process, achieves a peak SNDR of 75.6 dB at a sampling rate of 20 kS/s and 76.1 dB at 200 kS/s while consuming 7.68 μW and 96 μW, respectively. The corresponding Schreier FoM are 166.7 dB and 166.3 dB. Since the bandwidth of CCP is tunable, the ADC maintains a SNDR > 75 dB up

to 260 kHz. The core area occupied by the ADC is 0.589 mm2.

As the low-power sensors might be active only for very short time triggered by an external pulse to acquire the data, the third work is a 14-bit asynchronous two-stage pipelined SAR ADC which has been designed and simulated in 0.18-μm CMOS process. A self-synchronous loop based on an edge detector is utilized to generate an internal clock with variable phase. A tunable delay element enables to allocate the available time for the switch capacitor DACs and the gain-stage. Three separate asynchronous clock generators are implemented to create the control signals for two sub-ADCs and the gain-stage between. Aiming to reduce the power consumption of the gain-stage, simple source followers as the analog buffers are implemented in the 3-stage CCP gain-stage. Post-layout simulation results show that the ADC achieves a SNDR of 83.5 dB while consuming 2.39 μW with a sampling rate of 10 kS/s. The corresponding Schreier FoM is 176.7 dB.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 93 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1816
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Communication Systems Telecommunications Signal Processing Computer Engineering
urn:nbn:se:liu:diva-133231 (URN)9789176856178 (ISBN)
Public defence
2017-01-12, John von Neumann, Hus B, Campus Valla, Linköping, 13:15 (English)
Available from: 2016-12-15 Created: 2016-12-15 Last updated: 2016-12-29Bibliographically approved

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