Current-mode quadrature ring oscillator for bioimpedance applications

Abstract

A tunable frequency current-mode quadrature ring oscillator for bioimpedance spectroscopy applications is presented. It consists of four cross-coupled differential delay cells implemented using log domain circuits. The oscillation frequency can be fine-tuned using the bias current of the delay cells and coarse tuned by modifying the capacitors of the delay cells. The ring oscillator output current can be used for bioimpedance measurements as a current stimulus via a current buffer. Two quadrature output currents are converted using current-input comparators to in-phase and quadrature square voltages needed for demodulation on the bioimpedance readout side. The ring oscillator was implemented in a standard 180nm CMOS technology using MOS transistors operating in weak inversion. The oscillation frequency can be modified from 7.27kHz to 15.13MHz, while consuming from 1.05mA to 1.23mA from a 2V supply voltage.