High-speed ADCs using time-domain signal processing = 시간영역 신호처리를 이용한 고속 아날로그 디지털 변환기 연구

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As networks have been developing in recent years, the data rate between computing devices (via wireline or wireless) has been increasing. For these communications, the demand for high-speed and moderate resolution analog-to-digital converter (ADC) has also been increasing. However, the modern CMOS process is not friendly to the analog and mixed signal circuits due to the drastic scale-down. Because of the low intrinsic gain of the modern CMOS process's transistor, it is hard to achieve a high-precision amplifier. On the other hand, the modern CMOS process has fast edge transitions due to low parasitic capacitance, leading to high-performance digital circuits. In order to cope with the barriers presented to analog circuits, the author claims that the time-domain signal processing is suitable analog design methodology for the modern CMOS process. The time-domain signal has digital-like waveform which takes advantage of the modern CMOS process. The author improves two types of ADCs using the time-domain signal processing: (1) an ADC using time-domain quantization and (2) an ADC using zero-crossing time information. By adopting time-domain signal processing, the ADCs can take advantage of the fast edge transition property of the modern technologies. At high speeds, the ADC using the time-domain quantization has two challenges: (1) nonlinearity and large power consumption of the voltage-to-time converter and (2) small time-resolution, $T_LSB$, required for proper linearity. First, since the input signal is in the voltage-domain, there should be a domain converter prior to the time-domain quantizer. However, in order to create a linear conversion transfer function, the voltage-to-time converter (V2T) should use a power hungry ramp generator in conventional single-slope (SS) ADCs. Another problem is that $T_LSB$ becomes too small to resolve it as the sampling rate increases. This is because the fullscale of the time-domain signal, $T_FS$, is limited by the sampling period. For example, a single-slope architecture is one of the time-domain ADCs which have a ramp-generator as the V2T and a counter as the TDC. Unfortunately, its operating speed is substantially low due to the thermometric counting method. In order to achieve a 250MS/s, 9-bit ADC, the required quantization clock should be 128GHz to achieve $T_LSB$ of 7.8125psec. In order to increase the speed without sacrificing the linearity and without high frequency clock, the author proposes a multiphase-counting single-slope ADC, which quantizes the input signal in a two-step subranging manner without high frequency quantization clocks or multiple ramps. To increase the operating speed, the proposed ADC employs a 16-way time-interleaving technique, which enlarges T_FS sixteen times. Its power consumption is minimized by sharing a multiphase clock generator and a counter with sub-channels. The author also proposes a method to remove offset errors between the counter sequence and multiphases as well as a method to correct false decision due to the metastability of latches. As a result, the prototype SS-ADC achieves 250MS/s, 9-bit with only 500MHz of the quantization clock. The individualized and simplified V2T reduces power consumption further while ensuring an enough linearity. A prototype 9-bit ADC implemented in 90nm CMOS achieves 245fJ/c.s. with a DNL/INL of 0.25/0.36LSB and SFDR of 55.3dB at 250MS/s while consuming 7.12mW. In order to achieve a much higher sampling rate (>1GS/s), the author improves the ADC using zero-crossing time by applying the time-domain signal processing. Traditionally, a zero-crossing based circuit (ZCBC) is a promising technique for low-power, low-speed pipeline ADCs. Unfortunately, operating ZCBC ADC at speed near 1GS/s is quite challenging due to the delay of the zero-crossing detector (ZCD), which introduces nonlinear gain and offset errors. To alleviate the issue of nonlinearity, the author proposes a ZCBC pipeline ADC that employs a passive resistor as a current source. As its characteristic is inherently linear, the resistor-based ZCBC eliminates the input dependency of the inter-stage gain and offset errors, allowing simple calibration. Furthermore, a background offset calibration scheme is proposed to cope with a large offset that results from high-speed operation. A prototype ADC implemented in 65nm CMOS achieves an SNDR/SFDR of 47.26dB/62.64dB at 1GS/s while consuming 46.52mW from 1V supply. From the results, it can be seen that the time-domain signal processing is well-suited analog design methodology for the modern scaled CMOS technology. Note that the representative characteristic of the modern CMOS process is hard to make high DC gain opamp. The time-domain ADC eliminates the opamps and calculates the signal in the time-domain, leading to performance improvements. Therefore, the time-domain analog signal processing should be investigated further in order to exploit the modern CMOS technology, to reduce the power consumption, and to increase the speeds.
Cho, SeongHwanresearcher조성환researcher
한국과학기술원 :전기및전자공학부,
Issue Date

학위논문(박사) - 한국과학기술원 : 전기및전자공학부, 2016.2 ,[ix, 92 p. :]


analog integrated circuit; mixed mode signal; analog to digital converter; ADC; single slope; two step; zero-crossing based circuit; ZCBC; pipelined ADC; high speed; 아날로그 IC; 데이터 컨버터; 아날로그 디지털 변환기; 고속; 파이프라인

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