VCO-based ADCs for Low Power Precision Sensor Interfaces

VCO-based ADCs has long existed as an alternative way of digitization of analog signal. Thanks to its time-domain operation, VCO-based structures using phase domain signal processing have become very promising in highly scaled CMOS processes. The general idea is that since voltage-domain quantization is increasingly difficult to do well in scaled CMOS processes with low supply voltages, it is potentially a better idea to exploit what scaled CMOS processes are very good at: having lots of small transistors that switch fast. Thus, translating input voltage variations to a corresponding phase/frequency variation puts information into the time domain, which can be easily quantized via simple digital circuitry. On the other hand, one well known issue of VCOs is the non-linear voltage-to-frequency transfer characteristic, particularly when input amplitude is large. The distorted frequency output ultimately translates to a distorted digital output, which limits the maximal achievable spurious free dynamic range of the ADC. This dissertation presents a new architecture for VCO-based ADCs called differential pulse code modulation (DPCM) that virtually eliminates the VCO V-to-F nonlinearity by substantially reducing the signal amplitude that the VCO sees so that the VCO operates in the small signal linear region. By using this technique along with other calibration and circuit schemes, three prototype ICs (in which two are for bio-signal and one for audio signal) were fabricated and measured. They all achieved significantly better linearity not only amongst VCO-based ADCs, but also free of any measurable distortions in the output spectra, thus enabling a virtually distortion-less VCO-based ADCs suitable for high dynamic range precision sensing applications