This work presents an ultralow-voltage and ultralow-power current-mode delta-sigma modulator designed for biomedical applications. To implement both the integrator and quantizer of the modulator, a bulk-driven current conveyor circuit is introduced. By satisfying the Barkhausen criterion, the proposed modulator can intrinsically oscillate, producing a pulse-density modulated (PDM) output. This clock-less operation avoids conventional quantization noise associated with discrete-time sampling. The current-mode design not only provides improved matching properties and a wider bandwidth but also simplifies the implementation of passive components, resulting in a smaller silicon area compared to voltage-mode modulators—though this area efficiency is partly achieved using on-chip components. The circuit has been fabricated using standard TSMC 0.18-nm CMOS technology, occupying a silicon area of 226um*264 um. Despite consuming only 34-nW of power, experimental results demonstrate a signal-to-noise and distortion ratio of 57.1 dB, corresponding to an active resolution of 9.2 bits with a bandwidth of 81 Hz. Additionally, the use of the bulk-driven method achieves an input dynamic range of 24 nA under a supply voltage of 0.3 V.
