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IEEE Transactions on Applied Superconductivity

Design of SFQ-counting analog-to-digital converter

J.C. Lin, Vasili K. Semenov, Konstantin K. Likharev

Published Jun 1, 1995• 51 citations
Publisher

Abstract

We describe the design of an oversampling single-flux-counting analog-to-digital converter with the estimated bandwidth of a few tens of MHz, based on Rapid Single-Flux-Quantum (RSFQ) devices. It consists of an input analog circuit, high-speed quantizer with an original analog/digital negative feedback loop, decimation (comb) filter, and supporting subsystems including clock distribution circuits, quantization signal conditioner, and output drivers. We have applied a new approach, the 'symmetrizing' negative feedback, to free the quantizer from the intrinsic hysteresis without dynamic range sacrifice. The 16-bit version of the system comprises about 1,500 Josephson junctions and consumes 1.2 mW of DC power, when implemented using Nb-trilayer technology with 1 kA/cm/sup 2/, 3.5 /spl mu/m Josephson junctions. All subsystems of the converter have been successfully tested and testing of the system as a whole is in progress.>

DOI
10.1109/77.403034

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IEEE Transactions on Applied Superconductivity•Mar 1, 1998•17 citations
We describe the design of an oversampling single-flux-counting analog-to-digital converter with the estimated bandwidth of a few tens of MHz, based on Rapid Single-Flux-Quantum (RSFQ) devices. It consists of an input analog circuit, high-speed quantizer with an original analog/digital negative feedback loop, decimation (comb) filter, and supporting subsystems including clock distribution circuits, quantization signal conditioner, and output drivers. We have applied a new approach, the 'symmetrizing' negative feedback, to free the quantizer from the intrinsic hysteresis without dynamic range sacrifice. The 16-bit version of the system comprises about 1,500 Josephson junctions and consumes 1.2 mW of DC power, when implemented using Nb-trilayer technology with 1 kA/cm/sup 2/, 3.5 /spl mu/m Josephson junctions. All subsystems of the converter have been successfully tested and testing of the system as a whole is in progress.>
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.
A low-skew frequency divider and clock controller have been designed for high-frequency timing of superconductor rapid single-flux-quantum (RSFQ) digital systems. The circuits have only about 10-ps skew between input and output signals and are applicable for multirate digital systems (e,g., oversampling analog-to-digital converter and bit-serial digital systems). Several circuits have been fabricated in conventional Nb-trilayer technology with a critical current density of 1 kA/cm/sup 2/. The most complex clock controller generates trains of 2/sup 24/ single-flux-quantum pulses with a period of less than 70 ps. The long-term relative stability of these intervals has been measured to be better than 6/spl times/10/sup -5/. The basic component of the controller, a frequency divider, operates at input frequencies above 85 GHz.>
We are developing an analog-to-digital converter (ADC) based on rapid single-flux quantum technology for radio-astronomic spectroscopy applications where very wide bandwidth is required and relatively small resolution is sufficient. The ADC consists of periodic-flash comparators, a demultiplexer and output drivers; and is sufficiently simple to be implemented on a single chip using a conventional 2–3 μm Nb trilayer technology. A 2-bit ADC has been designed for operation at 16 GHz of the sampling frequency. All the subsystems of the ADC have been fabricated using standard 1.6 kA cm −2 Nb/AlO x /Nb technology and have been successfully tested.
Several novel circuits of the rapid single-flux-quantum (RSFQ) family of Josephson-junction digital devices have been designed, fabricated using a 2.5- mu m 1000-A/cm/sup 2/ Nb trilayer technology, and tested at low frequencies. Numerical simulation and measurements have shown that these circuits have considerably wider parameter margins, due to application of several novel design methods. The authors have also carried out an experiment to measure the rate of errors in a simple RSFQ circuit including an inverter, confluence buffer, and Josephson transmission line. Near the middle of the parameter window at 4.2 K, the error probability was definitely lower than 3*10/sup -15/ per logic operation, despite experimentation with rudimentary shielding and filtering.>
We present the design of a rapid single-flux-quantum all-digital 1-bit autocorrelator for submillimeter spectrometry applications, featuring 4-GHz input signal bandwidth, double oversampling quantizer, and 16-Mb/s-per-channel output rate. The correlator consists of a digital delay line with multiplication and an array of accumulators with parallel readout. We report experimental results for a three-stage delay line (294 Josephson junctions) and a 4/spl times/4 array of accumulators (288 Josephson junctions) fabricated in HYPRES' standard 3.5-/spl mu/m 1000-A/cm/sup 2/ Nb-trilayer process.