An AC-Coupled Instrumentation Amplifier Achieving 110-dB CMRR at 50 Hz With Chopped Pseudoresistors and Successive-Approximation-Based Capacitor Trimming

High common-mode rejection ratio (CMRR) with concurrent electrode offset rejection is essential for physiological signal acquisitions. This article presents a CMRR enhancement technique for ac-coupled instrumentation amplifiers (ACIAs), where the mismatch of passive components limits the CMRR performance primarily. A modified chopping structure is proposed to mitigate the mismatch effect of the pseudoresistors, and a successive-approximation based capacitor trimming loop is exploited. Fabricated in a 0.18-μm CMOS technology, the ACIA draws 2.3 μA from a 1.2-V supply and exhibits 3.2-μV_rms input-referred noise over 0.5-400 Hz. The measured prototypes achieve >110-dB CMRR at 50/60 Hz without any off-chip tuning.

I. Introduction

High-performance bio-potential signal acquisition requires sufficient rejection of common-mode interference and electrode dc offset (EDO) [1]–[3]. Taking the electroencephalography (EEG) as an example, the 5–300- signal comes often with large in-band common-mode interference, e.g., hundreds mV from 50-/60-Hz power lines, resulting in a 60–90-dB common-mode rejection ratio (CMRR) requirement. Clinical EEG has even higher CMRR requirement, e.g., 110 dB, as regulated by the International Federation of Clinical Neurophysiology (IFCN) [4]. Moreover, the electrochemical reaction in the electrode–tissue interface introduces EDO which can be as large as 300 mV, resulting in a 600-mV differential EDO in the worst case. The front-end instrumentation amplifiers (IAs) have to accommodate the design challenges of high CMRR and EDO rejection concurrently.