A 1V 136.6dB-DR 4kHz-BW ΔΣ Current-to-Digital Converter with a Truncation-Noise-Shaped Baseline-Servo-Loop in 0.18μm CMOS

Precise current measurements underpin emerging applications such as photoplethysmography (PPG), electrochemical sensing, and fast-scan cyclic voltammetry (FSCV) [1-6], where the signal is a low-swing current that rides on a large, slow-varying baseline. Therefore, readout systems need a dynamic-range (DR) > 120dB, bandwidth (BW) >1 kHz, noise floor < 1textpArms/surd textHz, and power <1 mW (Fig. 32.3.1 left). To widen DR, prior front-ends employ a prediction DAC [1], threshold-filter-based feedback-loop [2], and a Reset-Then-Open (RTO) DAC [3]. However, they widen the DR by sacrificing BW or power (Fig. 32.3.1 right). For instance, [1] employing a prediction DAC requires a power-hungry digital backend, while [2] with a threshold-filter-based feedback-loop is BW-limited (20Hz). In contrast, [3] achieves wide-DR and BW, but consumes> 1 mW power. This paper presents a continuous-tirne DeltaSigma current-to-digital converter (IDC) that achieves wide-DR and BW at muW power. To this end, it employs: 1) a 2nd-order textCT-DeltaSigma structure employing a highly linear pseudo-differential VCO quantizer, 2) an energy-efficient tri-level resistive DAC, and 3) a digital-intensive truncation-noise-shaped baseline-servo (TNS-BS) loop that extends the DR at low power and area.