Research On VCO-based Time-Domain Digital Analog-to-Digital Converter

With the rapid development of CMOS technology,the speed of transistors is increasing,the density of unit integration is getting higher and higher,the logic delay continues to decrease,and the time resolution is continuously improved.Therefore,when designing analog-to-digital converters,traditional voltage-domain signals can be converted to time-domain signals,which can be sampled and quantized to obtain the digital signal.The voltage-controlled oscillator can complete the conversion of the signal from voltage-domain to time-domain due to its inherent linear relationship between the control voltage and the output frequency and the integral relationship between the voltage and the phase.Therefore,VCO-based analog-to-digital converters have become a research hotspot.Traditional quantizer in Sigma-Delta modulator operates in voltage-domain and uses comparators to obtain multi-level quantizatizer.However,as the number of quantization bits and speed of the system increases,the probability of metastable state at the output of comparators increases.Therefore,there is a higher requirement for the performance of comparator.VCO-based quantizer has a simple structure and first-order noise shaping feature.The continuous Sigma-Delta modulator based on VCO obviates the need for complex anti-aliasing filters and sample-and-hold circuits due to the inherent anti-aliasing characteristics,which reduces the complexity of system and saves the area and power consumption.This thesis mainly designs two kinds of VCO-based Sigma-Delta ADC.Between them,the VCO-based frequency-domain ADC mainly uses the linear relationship between the input control voltage and the output frequency of VCO.The designed architecture employs VCO and a Gray-counter as a quantizer to count the VCO output cycles within a certain sampling period to obtain the quantized digital result,which mitigates the problem of partial sampling of digital data in multi-bit VCO-based quantizers.To increase the resolution of the quantizer,a method of simultaneously counting the rising and falling edges of the VCO output waveform is used.This design arranges two sub-ADCs in a differential manner,which cancels out even-order distortions and reduces the influence of the VCO nonlinearity.This synthesizable digital circuit occupies 1.05mm~2 in SMIC 180nm process and delivers an ENOB of 7.67-bit over a signal bandwidth of 10MHz while sampling at 250MHz.The VCO-based phase-domain Sigma-Delta ADC mainly utilizes the phase difference between two differentially operated VCOs to quantify the analog input.And the quantizer is placed in a first-order Sigma-Delta loop,which ensures the linearity of the VCO operation.Due to the working principle of the phase VCO in the current-domain,the circuit has an intrinsic mismatch shaping capability that automatically addresses the DAC mismatches.In addition,because the linear range of the XOR-based phase detector is0~?,this thesis proposes an fully-digital phase-extended quantizer that expands the resolution of the quantizer by detecting the phase lead-lag information of the two differential VCOs and extends its linear range to-?~?.Based on this quantizer,a current-steering DAC with both inflow and outflow currents is designed.The designed scaling-friendly VCO-based closed-loop Sigma-Delta ADC occupies 1.88mm~2 in 180nm process and achieves an ENOB of 8.82-bit over 1MHz BW while sampling at 300MHz.