An Area/Power-Efficient Noise-Shaping SAR ADC for Implantable Biosensor Applications Featuring a Unique Auxiliary Feedback Loop

Abstract

Wide bandwidth and power-area efficient front-ends are required in emerging implantable biosensor applications, e.g., wireless artificial vision systems for the limited vision. This work presents a low-power 0.031 mm $^{2}~3$ -MHz bandwidth noise-shaping (NS) successive approximation register (SAR) analog-to-digital converter (ADC) tailored for implantable biosensor applications. A highly energy- and area-efficient reset-free residue-processing scheme is proposed. This scheme facilitates noise transfer function (NTF) optimization by enabling control over complex poles. It establishes a unique auxiliary feedback path from the infinite impulse response (IIR) back to the finite impulse response (FIR) by eliminating the need for the FIR filter’s reset phase. This auxiliary feedback loop, which could not be achieved in previous FIR-IIR structures, is a key enabler for achieving high energy efficiency and design flexibility. The in-band quantization noise suppression is boosted by the proposed complex conjugate poles optimization technique, while a low $kT/C$ input-referred noise has resulted. The prototype, fabricated in 65-nm CMOS technology with a 7-bit digital-to-analog converter (DAC), consumes $370~\mu $ W from a 1-V supply and occupies an active area of only $0.17\times 0.18$ mm (0.031-mm2). Operating at an 80-MHz oversampling frequency, the proposed ADC achieves a signal-to-noise and distortion ratio (SNDR) of 70.2 dB over a 3 MHz bandwidth, demonstrating its small area, high efficiency, and high performance for implantable biosensor applications that require high data rates.