High resolution & low noise SAR ADC design for bio-medical signal acquisition

Abstract

In this thesis, we will discuss the low-power design technique for the analog-to-digital converter of successive approximation registers (SAR) type for efficient bio-signal acquisition. The proposed design scheme also proposes an architecture-level approach to realize low power operation in addition to the circuit-level approach. In Chapter 1, we introduce a low-power SAR analog-to-digital converter (ADC) simply and discuss the design issues of the SAR ADC. Also, background and significance of biomedical application is presented. In Chapter 2, the three low-power design techniques for successive approximation registers (SAR) analog-to-digital converter (ADC) for bio-potential signal acquisition are presented: skip-reset, delta ($\delta$) readout with MSB-rounding and tri-level split monotonic switching. The skip-reset scheme reduces not only reference energy but also digital switching energy for the ADC reset. The $\delta-readout$ process with the proposed MSB-rounding technique shifts the location of the resolvable range using the previous digital code to increase the hit-rate. Finally, the tri-level split monotonic switching scheme minimizes the CDAC switching activity in predictive residue generation for the $\delta-readout$ process. A prototype ADC was fabricated in a 0.18um CMOS technology and occupies an active area of $0.17mm^2$. At a 1.5V supply voltage and a 1 kS/s sampling-rate with the ECG signal input, the ADC power consumption could be reduced to 18.5 nW, corresponding to 71 % power saving, owing to the proposed techniques from a conventional SAR ADC consuming 63.5 nW. In Chapter 3, the oversampled SAR ADC architecture with loop-embedded input buffer for low power application is presented. In Chapter 3, we propose a structure of analog-to-digital converter with a built-in input buffer. To implement a low power bio-system, this is a structure study that can replace the large input buffer with low power consumption by the energy-efficient input buffer. This study overcomes the conventional structure using current DAC and succeeded in realizing low power using capacitor DAC. In addition, the proposed low-power source follower has a full-range input range even at low supply voltage conditions, thus enhancing its applicability in bio-systems.