Ashwin Rammohan

EECS Department, University of California, Berkeley

Technical Report No. UCB/

May 1, 2024

http://www2.eecs.berkeley.edu/Pubs/TechRpts/Hold/f7d7dd2bad2e54b15636c5bcc3f0b2e9.pdf

Wearable and implantable medical devices are becoming more common now, offering patients minimally invasive health-monitoring capabilities. Neural interfaces, in particular, have the potential to revolutionize our understanding of the nervous system and how to treat and diagnose neurological disorders like epilepsy, Alzheimer's, and many more.

Voltage-controlled oscillator (VCO) based analog-to-digital converters (ADCs) are an attractive architecture for neural frontends in advanced technology nodes, where we can also incorporate complex on-chip digital post-processing. This thesis is part of the AURA project, which employs a VCO-based ADC architecture to measure and digitize neural signals such as electroencephelography (EEG) and electromyography (EMG). This thesis presents a nonlinearity calibration scheme employing a Delta-Sigma DAC to improve the resolution of these ADCs. We begin by explaining the motivation for VCO-based ADCs in modern biosensing applications. We also discuss the challenges that need to be overcome to design high-resolution VCO-based ADCs and details about the VCOs used in the AURA chip specifically. Next, we discuss the theory behind Delta-Sigma modulation and the motivation behind choosing a return-to-zero signaling scheme for the Delta-Sigma DAC. Afterward, we discuss the design methodology of a preliminary version of the Delta-Sigma DAC used in the first tapeout of AURA, and we present its simulated and measured results. We then discuss potential reasons for the DAC's flaws before offering a few potential alternative solutions to the preliminary design. Finally, we discuss the design process and simulated results of the revised DAC which will be implemented in the second tapeout of AURA.

Advisors: Rikky Muller

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BibTeX citation:

@mastersthesis{Rammohan:31285,
    Author= {Rammohan, Ashwin},
    Title= {Digital Calibration of Voltage-Controlled Oscillator (VCO) Nonlinearity using a Delta-Sigma DAC},
    School= {EECS Department, University of California, Berkeley},
    Year= {2024},
    Number= {UCB/},
    Abstract= {Wearable and implantable medical devices are becoming more common now, offering patients minimally invasive health-monitoring capabilities. Neural interfaces, in particular, have the potential to revolutionize our understanding of the nervous system and how to treat and diagnose neurological disorders like epilepsy, Alzheimer's, and many more.

Voltage-controlled oscillator (VCO) based analog-to-digital converters (ADCs) are an attractive architecture for neural frontends in advanced technology nodes, where we can also incorporate complex on-chip digital post-processing. This thesis is part of the AURA project, which employs a VCO-based ADC architecture to measure and digitize neural signals such as electroencephelography (EEG) and electromyography (EMG). This thesis presents a nonlinearity calibration scheme employing a Delta-Sigma DAC to improve the resolution of these ADCs. We begin by explaining the motivation for VCO-based ADCs in modern biosensing applications. We also discuss the challenges that need to be overcome to design high-resolution VCO-based ADCs and details about the VCOs used in the AURA chip specifically. Next, we discuss the theory behind Delta-Sigma modulation and the motivation behind choosing a return-to-zero signaling scheme for the Delta-Sigma DAC. Afterward, we discuss the design methodology of a preliminary version of the Delta-Sigma DAC used in the first tapeout of AURA, and we present its simulated and measured results. We then discuss potential reasons for the DAC's flaws before offering a few potential alternative solutions to the preliminary design. Finally, we discuss the design process and simulated results of the revised DAC which will be implemented in the second tapeout of AURA.},
}

EndNote citation:

%0 Thesis
%A Rammohan, Ashwin 
%T Digital Calibration of Voltage-Controlled Oscillator (VCO) Nonlinearity using a Delta-Sigma DAC
%I EECS Department, University of California, Berkeley
%D 2024
%8 May 1
%@ UCB/
%F Rammohan:31285