An Analog Front-End for a Multifunction Sensor Employing a Weak-Inversion Biasing Technique With 26 nVrms, 25 aCrms, and 19 fArms Input-Referred Noise

Abstract

An analog front-end (AFE) for a multifunction environmental sensor is proposed. The AFE is designed to be able to sense $1~\mu \text {V}$ , 1 fC, and 1 pA level signals in a low-frequency band around DC, and consists of an instrumentation amplifier (IA), a charge amplifier (CA), and a continuous time delta-sigma (CT- $\Delta \Sigma $ ) ADC. To reduce the low-frequency noise of operational amplifier (OPAMP) which is used for signal path, this paper proposes a weak-inversion biasing technique. The designed IA achieved 26 nVrms input-referred noise (IRN) in the band of 0.1–10 Hz with 0.22 mm2 die area. The designed CA achieved 25 aCrms IRN in charge sensing mode and 19 fArms IRN in current sensing mode in the band of 0.01–1 Hz, respectively, and occupies 0.12 mm2 die area. The AFE has been fabricated in a $0.18~\mu \text {m}$ CMOS process with 4.5 mm2 die area. The current consumption at 2.56 kHz output data rate (ODR) is 2.5 mA, reducing to $2~\mu \text {A}$ at 0.01 Hz ODR.