A highly linear OTA-free VCO-based 1-1 MASH ΔΣ ADC.

In this paper a new VCO-based MASH delta-sigma ADC structure is presented. The proposed architecture does not require any OTA-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. Based on the architecture, the input of the first VCO is a very small 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 power hungry linearization methods. Behavioral-model simulations show a 70 dB SNDR improvement from a standalone open loop VCO-based ADC in the presence of nonlinearity.