Spectral analysis of multibit VCO-ADCs and PFM-ADCs with sinusoidal inputs

The analytical formulation of the output DFT of an oversampled converter based on a VCO has been recently disclosed for the single-bit case. However, most practical VCO-ADCs are constructed with a multibit configuration using a multiphase ring oscillator architecture. This paper shows that the spectral properties of a single-bit VCO-ADC are retained in the multibit case if an equivalent model is employed. This means that similar equations used to study the performance of single-bit ring oscillator can also be applied to the multibit case. Also, a multibit PFM-ADC has been proposed as an alternative to a conventional VCO-ADC, showing differences in the output spectrum respect to the VCO-ADCs based on ring oscillators. This paper describes these differences and how to apply the previous equations to study the PFM-ADC architecture.