A Current-to-Digital Converter Based on Hybrid CT/DT Sigma-Delta Modulator for Current-Output Biosensors

This article presents an area- and power-efficient current-to-digital converter (CDC) system for bioelectric current sensor applications. To address the requirements of high resolution, compact size, and low power in such systems, a hybrid continuous-time/discrete-time (CT/DT) incremental sigma-delta modulator (SDM) architecture is proposed. Compared to conventional CT-based CDCs, the hybrid approach offers advantages in area and efficiency, which are systematically analyzed and validated through chip implementation. The proposed system employs a resistor-free continuous-time (CT) integrator as its first stage and a switched-capacitor (SC) integrator as the second stage. The integration of a recycling folded-cascode (RFC) operational amplifier further optimizes power consumption. In addition, the implemented D flip-flop (DFF)-assisted feedback loop and enhanced charge injection cancellation switch improve the system’s harmonic and noise characteristics. The chip is implemented using a standard 0.18- $\mu $ m process, the feedback resistor and integration capacitor can be adjusted through on-chip digital registers to accommodate different sensor current output ranges. The proposed CDC achieves 94.9-dB signal-to-noise and distortion ratio (SNDR) with 55- $\mu $ A current consumption and core area of 0.17 mm2. The figure of merit (FoM) for the system is 164.9 dB, achieving a good balance between resolution and power consumption.