A Highly Linear OTA-Free VCO-Based 1-1 MASH $\Delta\Sigma$ ADC

In this paper, a new voltage-controlled oscillator (VCO)-based 1-1 MASH delta-sigma ADC structure is presented. The proposed architecture does not require any operational transconductance amplifier-based analog integrators or integrating capacitors. Second-order noise shaping is achieved by using a VCO as an integrator in the feedback loop of the first stage and an open-loop VCO quantizer in the second stage. Simple digital circuitry extracts the phase quantization error of the first stage as a pulse signal that is applied to the second stage. The input of the first VCO is a very small amplitude signal, and the input of the second VCO is a two-level PWM signal. Therefore, the VCO non-linearity does not limit the overall ADC performance, mitigating the need for the power hungry linearization methods. The proposed idea was simulated at the transistor level, and the results verify the analysis.