A digitally corrected ADC using a passive pulse frequency modulator

Abstract

Pulse Frequency Modulators have been proposed recently as an alternative to ring oscillators in the implementation of Voltage-Controlled-Oscillators Analog to Digital Converters (VCO-ADC). In this paper, we propose a passive pulse frequency modulator in terms of the integration process that uses digital inverters only and provides a polyphase output similar to a ring oscillator. Oscillation depends on a passive RC circuit acting as oscillator time constant. The voltage-to-frequency characteristic of this oscillator shows a nonlinear relationship respect to the input voltage in a similar way as classical ring oscillators. However, in the proposed circuit the nonlinearity depends mainly in a predictable way from the RC product instead of supply voltage (Vdd), temperature or mismatch. We show in this paper how the nonlinearity can be corrected in the signal post-processing by a simple algorithm depending only on the RC time constant operating on the decimated data and circuit parameters that could be known.