Design of a Time Based Analog to Digital Converter

Abstract

Analog to digital converter (ADC) plays a very important role in any mixed analog/digital system. Because digital CMOS technology can take advantage of technology scaling, system designers try to increase the percentage of the digital part of the system. This means moving the ADC more and more towards the input of the system which results in making the role of the ADC more and more critical. With technology scaling, the switching characteristics of MOS transistors offer superb timing accuracy at high frequencies. This makes the time based analog to digital converter (TADC) a good alternative to the conventional ADCs in sub-micron region. In this thesis, an all digital TADC structure is proposed. This TADC is based on an analog to time converter (ATC), followed by a time to digital converter (TDC). The TDC is based on sigma-delta modulation. A non-linear multi-bit internal quantizer in sigma-delta modulator is used to counteract the nonlinearity introduced when the VCO is used as the ATC. The novel TADC also uses an implicit sample and hold (S/H) circuit to reduce area. Dynamic element matching (DEM) is used to improve the robustness of the system against random mismatch in the multi-bit quantizer. Both first and second order sigma-delta modulator TADC are proposed. Simulations and measurements on the proposed TADC are provided. Measurements, from a prototype chip fabricated using 0.13um CMOS technology, show that the first order TADC has achieved a dynamic range of 11 bits for a bandwidth of 2MHz. While simulation results show a dynamic range of 12 bit. Simulations show that the second order TADC has achieved a dynamic range of 12bit for a bandwidth of 20MHz.