A 1.8-GHz Near-Field Dielectric Plethysmography Heart-Rate Sensor With Time-Based Edge Sampling

This article presents a near-field dielectric plethysmography (DPG) heart-rate sensor for low-power operation. By interrogating the contracting and expanding blood vessels with gigahertz-frequency fringing electric fields from the on-board coplanar waveguide (CPW) transmission line, pulsing heart-rate signals are detected using a phase-sensitive interface circuit through changing permittivity at the fingertip. Signal analysis, frequency response measurements, and variability studies are provided to demonstrate the robustness of DPG for heart-rate detection. The readout circuit employs a highly digital time-based edge sampling with the rail-to-rail signaling and double-integration chopper stabilization technique to maximize the electronics sensitivity and to minimize the flicker noise. Implemented in the 0.18- $\mu \text{m}$ CMOS technology, the phase-sensitive readout circuit exhibits a limit of detection of 0.0033° and consumes 20.1 $\mu \text{W}$ under a sampling and duty-cycling rate of 100 Hz and 0.0256%, respectively. With the integration of CPW sensor in a low-cost FR-4 printed-circuit board (PCB), the 1.8-GHz DPG heart-rate sensor achieves an rms inaccuracy of 1.64 bpm.