Design And Research On Key Circuits In VCO Based ADC

As the bridge connecting analog signal and digital signal, the development of analog-digital-converter(ADC) towards to higher speed, higher precision, lower power. Advanced CMOS technology offers faster switching speed and higher transistors density, which gives us a new thinking for the way of analog to digital conversion that is signal process in the time domain. Voltage control oscillator can convert the traditional analog signal in voltage domain to the signal in the time domain, and then is transferred to the digital signal, so the conversion from analog to digital has been come true. As the improvement of transistors’ speed, the resolution in the time domain is further improved at the same time. In another word, the structure of ADC built on the quantization of time domain is benefited by the promotion of technology. In this work, in the 180 nm CMOS technology, we study and research the key circuit modules of VCO based sigma-delta ADC and design a 400 MS/s, 10 MHz bandwidth, 11 bit ENOB sigma-delta modulator.According to the sigma-delta principle and loop structure, we use VCO into the quantizer and employ differential dual loop structure for decreasing the harmonic distortion of the system, improving the whole signal-noise-distortion ratio(SNDR) and gaining more effective number of bits(ENOB).Firstly, we mathematical model and theoretical analyze the VCO based quantizer and prove that it holds the property of the first order noise shaping by behavioral simulation. At the same time, the result of the simulation shows that the nonlinearity of quantizer will affect the performance of the whole modulator directly, introducing extra harmonic distortion. For conquering this nonlinearity, we do not control the oscillator by voltage directly. Insteadly, we transfer the voltage signal to the current signal by high linearity voltage-current converter module, and then use this signal to control the oscillation frequency. The simulation proves that this structure can decrease the power of odd harmonic, especially the third harmonic power. The even harmonic can be eliminated by differential quantizers. As a result, the power of harmonic is suppressed in this system.According to the impulse-invariant transformation, we design the continuous time loop filter by transforming the equivalent discrete structure to the continuous structure. Simulating by Cppsim and Matlab, adding noise and mismatch of coefficient, we verify that the continuous filter can operate normally. And then we determine the specific circuit structure and coefficient with the signal stream way. Lastly, we employ the non-returnzero(NRZ) current steer to be the structure of the feedback digital-analog-converter(DAC).VCO based quantizer, continuous time loop filter and feedback DAC, these three key circuit modules compose the continuous time sigma-delta modulator. Simulating the circuit in 180 nm process with Hspice, we achieve the performance of 70 dB SNR, 68.8 dB SNDR and 11 bits ENOB...