Voltage-controlled Oscillator-based A/D Conversion

There has recently been increasing interest in developing highly digital analog-to-digital converter (ADC) structures for on-chip testing and ease of integration in future CMOS processes. An intriguing circuit to utilize in such cases is a ring oscillator voltage-controlled oscillator (VCO), which outputs a clock waveform whose frequency is a function of an input tuning voltage. By comparing the clock frequency to that of a separate clock reference using digital counters, one can create an all-digital A/D converter that can be readily utilized for on-chip monitoring of supply voltage variations and other on-chip waveforms [1-2]. A shortcoming of the approach in [1] is that the effective conversion rate must be quite low to achieve high resolution, and the shortcoming of the approach in [2] is that the overall A/D implementation ends up being primarily analog in nature.