Linearization of synthesizable VCO-based ADCs using delta modulation

Abstract

VCO-based ADC is an attractive candidate for the synthesis of all-digital ADCs using standard cells. However, the non-linearity of a synthesizable VCO requires digital postprocessing to obtain good performance. We propose another solution where the input analog signal is pre-coded into a delta-modulated pulse stream which is used to drive a VCO-based converter. This causes the oscillator to operate at two distinct frequencies thereby eliminating the VCO non-linearity from the converter transfer function. A circuit is proposed that consists of a synthesized digital block realizing all the active parts of the circuit and a passive RC net used as an integrator. Spectre simulation of the netlist synthesized using a 65 nm standard cell library shows a performance of 8.2 bit ENOB over a 3 MHz bandwidth without using any digital post-processing.