A Continuous Time Analog-to-Digital Converter With 90μW and 1.8μV/LSB Based on Differential Ring Oscillator Structures

A low-power, high-resolution analog-to-digital converter (ADC) for sensor applications is presented in this paper. Its principle emanates from the popular slope techniques by replacing the voltage controlled current source with a voltage controlled oscillator (VCO). The analog input voltage modulates the oscillator frequency, which is integrated continuously and quantised using delay elements of a ring oscillator. The resulting phase information represents the moving average (MA) of the input voltage during the integration interval. The use of a continuously operating and integrating VCO eliminates the need for a separate sample and hold stage. Due to the averaging process, pre-filtering in order to prevent anti-aliasing and to perform noise reduction is often unnecessary. To the author´s knowledge (Watanabe et al., 2003), (Watanabe et al., 2003) represent the only published realisations of the principle. Both papers use inverter-based ring oscillators which are inherently nonlinear and need a synchronisation circuitry to ensure oscillation. The linearity of the converter is determined by the linearity of the VCO transfer characteristic. Differential structures recommended in this paper greatly improve the overall linearity and lead to a low-power converter design. Thorough insights of the performance-limiting phenomena are obtained by utilisation of sophisticated modelling techniques from related fields of research. The provided tools enable designers to meet stringent specifications with respect to power consumption, resolution and linearity.