UBC Theses and Dissertations
A 43mW single-channel 4GS/s 4-bit flash ADC IN 0.18um CMOS Sheikhaei, Samad
The continued speed improvement of serial links and appearance of new communication technologies, such as ultra wideband (UWB), have introduced increasing demands on the speed and power specifications of high speed low to medium resolution analog to digital converters (ADCs). While multi channel ADCs can achieve high speeds, they often require extensive and costly post fabrication calibration. A single channel 4 bit flash ADC, suitable for abovementioned or similar applications, implemented entirely using current mode logic (CML) blocks, is presented. CML implementation allows for high sampling rates, while typically providing low power consumption at high speeds. To improve the conversion rate, both the analog (comparator array) and the digital (encoder) parts of the ADC are fully pipelined. Furthermore, the logic functions in the encoder are reformulated to reduce wire crossings and delay and to equalize the wires lengths in the layout. To keep the design simple, inductors are avoided. As a result, a compact design with small wire parasitics is achieved. Moreover, some geometric layout techniques, including a common centroid layout for the resistor ladder, are introduced to reduce the effect of mismatches to eliminate the use of digital calibration. The ADC is designed and fabricated in 0.18um CMOS and operates at 4GS/s. It achieves an effective number of bits (ENOB) of 3.71 (3.14, 2.75) for a 10MHz (0.501GHz, 1.491GHz) signal sampled at 4GS/s (3GS/s, 3GS/s). Differential/integral nonlinearity (DNL/INL) errors are between +/-0.35LSB and +/-0.26LSB, respectively. The ADC consumes 43mW from a 1.8V supply and occupies 0.06mm2 active area. Due to the use of CML circuits, the ADC achieves the highest speed reported for a single channel 4 bit ADC in a 0.18um CMOS technology. It also reports the best power performance among the 4-bit ADCs with similar or higher speeds. The active area is also among the smallest reported. In addition, in this thesis, the signal to noise ratio (SNR) of an ADC is formulated in terms of its INL performance. The related formulas in the literature are not accurate for low resolution ADCs, and yet they do not take the input waveform into account. Two standard waveforms, ramp and sinusoid, are considered here. The SNR formulas are derived and confirmed by simulation results.
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