A low power and low distortion VCO based ADC using a pulse frequency modulator

This paper shows a novel VCO based ADC which uses an inverter delay line as a monostable to implement a pulse frequency modulator. The inverter delay line doubles as a Time to Digital converter, providing a multibit digital output similar to a ring oscillator. The pulse frequency modulator is based on a gm-C integrator, operating as a closed loop transconductor for the low bandwidth input signal path and as a charge pump for the monostable feedback. This way a high linearity is achieved while the total power consumption is kept on the order of a calibrated VCO-ADC. The paper shows the basic system level operation and behavioral simulations, and also a transistor level design example in 0.25μm CMOS technology.