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This book describes techniques for time-interleaving a number of analog-to-digital data converters to achieve demanding bandwidth requirements. Readers will benefit from the presentation of a low-power solution that can be used in actual products, while alleviating the time-varying signal artifacts that typically arise when implementing such a system architecture.
The increasing data rate of wireline communication systems leads to more inter-symbol interference, due to the dispersive properties of the communication channel. This requires more complex equalization blocks to meet the required bit-error rate. One solution is to use an Analog-to-Digital Converter (ADC) in the front-end, thus enabling a digitally-equalized serial link. To achieve the high-data rates of these communication systems, a time-interleaved ADC is typically used. However, this type of ADC suffers from several time-varying errors, the most prominent of which is timing skew. This thesis introduces a statistics-based background calibration algorithm that compensates for the effect of timing skew. To demonstrate the background calibration algorithm, a proof-of-concept 5 bit 12 GS/s flash ADC has been fabricated in a 65 nm CMOS process. The design of this ADC takes into consideration the tight power bounds imposed on serial links by optimizing both the time-interleaved and the sub-ADC architecture. Power consumption is further reduced by using calibration circuits to correct the offset of the flash ADC's comparators. In the measured results, the timing skew correction improves the dynamic performance of the time-interleaved ADC by 12 dB, and the proof-of-concept ADC has the lowest published power consumption for ADCs with sample rates higher than 10 GS/s.
Abstract: The growth of digital systems underscores the need to convert analog information to the digital domain at high speeds and with great accuracy. Analog-to-Digital Converter (ADC) calibration is often a limiting factor, requiring longer calibration times to achieve higher accuracy. The goal of this dissertation is to perform a fully digital background calibration using an arbitrary input signal for A/D converters. The work presented here adapts the cyclic "Split-ADC" calibration method to the time interleaved (TI) and successive approximation register (SAR) architectures. The TI architecture has three types of linear mismatch errors: offset, gain and aperture time delay. By correcting all three mismatch errors in the digital domain, each converter is capable of operating at the fastest speed allowed by the process technology. The total number of correction parameters required for calibration is dependent on the interleaving ratio, M. To adapt the "Split-ADC" method to a TI system, 2M+1 half-sized converters are required to estimate 3(2M+1) correction parameters. This thesis presents a 4:1 "Split-TI" converter that achieves full convergence in less than 400,000 samples. The SAR architecture employs a binary weight capacitor array to convert analog inputs into digital output codes. Mismatch in the capacitor weights results in non-linear distortion error. By adding redundant bits and dividing the array into individual unit capacitors, the "Split-SAR" method can estimate the mismatch and correct the digital output code. The results from this work show a reduction in the non-linear distortion with the ability to converge in less than 750,000 samples.
This thesis describes a background-calibration technique that overcomes timing errors in time-interleaved analog-to-digital converters (ADCs) in a way that is almost independent of the user-provided ADC input signal. Additive dither is widely used to achieve signal-independent background calibration of many errors in data converters [1]. For example, this technique has been used to calibrate for gain mismatch in time-interleaved ADCs [2]. In most cases, however, binary dither has been used, and binary dither is not able to detect timing errors when the user-provided ADC input is zero or constant because timing errors do not produce amplitude errors when the ADC input is constant. This thesis presents a study of the use of a random ramp-based dither signal to calibrate for timing errors in time-interleaved ADCs. To demonstrate the dither-based timing calibration, a prototype 10-bit 500-MS/s 4-channel ADC was fabricated in 40-nm CMOS. With the proposed timing calibration, the Signal-to-Noise-and-Distortion Ratio (SNDR) is 50.1 dB with a user-provided input at 249 MHz while consuming 6.2 mW, giving a figure of merit (FoM) of 48.4 fJ/step. Disabling the ramp after the timing calibration converges improves the SNDR to 51 dB and reduces the power dissipation to 5.8 mW as well as the FoM to 39.8 fJ/step. [1] H. E. Hilton, "A 10-MHz Analog-to-Digital Converter with 110-dB Linearity," Hewlett-Packard Journal, vol. 44, No. 5, pp. 105-112, Oct. 1993. [2] D. Fu, K. C. Dyer, P. J. Hurst, and S. H. Lewis, "A Digital Background Calibration Technique for Time-Interleaved Analog-to-Digital Converters," IEEE J. of Solid-State Circuits, vol. 33, No. 12, pp.1904-1911, Dec. 1998.
Need to get up to speed quickly on the latest advances in high performance data converters? Want help choosing the best architecture for your application? With everything you need to know about the key new converter architectures, this guide is for you. It presents basic principles, circuit and system design techniques and associated trade-offs, doing away with lengthy mathematical proofs and providing intuitive descriptions upfront. Everything from time-to-digital converters to comparator-based/zero-crossing ADCs is covered and each topic is introduced with a short summary of the essential basics. Practical examples describing actual chips, along with extensive comparison between architectural or circuit options, ease architecture selection and help you cut design time and engineering risk. Trade-offs, advantages and disadvantages of each option are put into perspective with a discussion of future trends, showing where this field is heading, what is driving it and what the most important unanswered questions are.
Analog-to-Digital Converters (ADCs) play an important role in most modern signal processing and wireless communication systems where extensive signal manipulation is necessary to be performed by complicated digital signal processing (DSP) circuitry. This trend also creates the possibility of fabricating all functional blocks of a system in a single chip (System On Chip - SoC), with great reductions in cost, chip area and power consumption. However, this tendency places an increasing challenge, in terms of speed, resolution, power consumption, and noise performance, in the design of the front-end ADC which is usually the bottleneck of the whole system, especially under the unavoidable low supply-voltage imposed by technology scaling, as well as the requirement of battery operated portable devices. Generalized Low-Voltage Circuit Techniques for Very High-Speed Time-Interleaved Analog-to-Digital Converters will present new techniques tailored for low-voltage and high-speed Switched-Capacitor (SC) ADC with various design-specific considerations.