Linearizing techniques for voltage controlled oscillator based analog to digital converters

Voltage controlled oscillator (VCO) based ADC is an important class of time-domain ADC that has gained widespread acceptance due to their several desirable properties. VCO-based ADCs behave like an open-loop continuous time ΔΣ modulator and achieve excellent resolution by first order noise shaping the quantization error. However, the SNDR of an open-loop VCO-based ADC is severely distortion limited by the voltage-to-frequency tuning characteristics of the VCO. This work examines various techniques that have already been proposed to overcome the VCO tuning non-linearity problem. Two new VCO-based ADC architectures, that overcome the limitations of the conventional approaches, are proposed. In the first approach, the ADC is linearized by forcing the VCO to operate at only two operating points using a front-end two level modulator. With this technique, the linearity is improved without using either a multi-bit feedback DAC or calibration. Fabricated in a 90 nm CMOS process, the prototype ADC achieves better than 71 dB SFDR and 59.1 dB SNDR in 8 MHz signal bandwidth while consuming 4.3 mW power. The ADC achieves a figure of merit of 366 fJ/conv-step, which is comparable with other state of the art time based ADCs. In the second approach, the need for a front-end two level modulator is obviated using linearizers, which introduce an inverse of VCO’s voltage to frequency characteristics in the signal path. A deterministic digital calibration unit runs continuously in the background and builds the inverse voltage to frequency transfer function. Implemented in a 90nm CMOS process, this on-chip calibration improves SFDR of the prototype ADC from 46 dB to more than 83 dB. The ADC consumes 4.1 mW power and achieves 73.9 dB SNDR in 5 MHz signal bandwidth resulting in an excellent figure of merit of 101 fJ/conv-step