An Open-Loop VCO-ADC Based on a Linearized Current Control Technique

This brief presents an open-loop current-controlled oscillator (CCO)-based analog-to-digital converter (ADC) intended for ultralow power direct digitizing micro-sensors readout applications. The proposed highly linearized pseudo-differential ${G}_{M}$ -stage mitigates the harmonic distortions induced by the tune circuit to the nonlinear transfer characteristic of the voltage-controlled oscillator (VCO), and it improves the total SNDR considerably. The ADC is implemented in 180-nm CMOS and achieves a peak SFDR of 81.48 dB, THD of −76.8 dB, and SNDR of 70.9 dB (equivalent to 11.48 ENOB) over a 3.6 kHz bandwidth with a 110 mVPP input differential sinewave. A chopper is used for the input transconductance stage to mitigate the input-referred noise and its impact on output phase noise. The ADC consumes $20.3 \mu {\text {W}}$ from a 1-V supply.