Asynchronous sigma-delta modulator and fast demodulator

Asynchronous sigma-delta modulator (ASDM) is closed-loop nonlinear system that transform the information in the amplitude of a bounded input signal into time information in the output signal, without suffering from quantization noise such as in synchronous sigma-delta modulators. This is an important advantage with many interesting applications. This paper investigates the commonly used asynchronous sigma-delta modulator, which consists of a Schmitt trigger and a continuous-time loop filter. A detailed analysis is presented to accurately describe the properties of such modulator. The circuit for fast demodulation of modulated signal is also described. The theoretical derivations were compared with simulation and measuring results, and they illustrate a good match. The presented circuits were analyzed, simulated, constructed and measured.