Ultrafast switching speed, low power, natural quantization of magnetic flux, quantum accuracy, and low noise of cryogenic superconductor circuits enable fast and accurate data conversion between the analog and digital domains. Based on rapid single-flux quantum (RSFQ) logic, these integrated circuits are capable of achieving performance levels unattainable by any other technology. Two major classes of superconductor analog-to-digital converters (ADCs) are being developed - Nyquist sampling and oversampling converters. Complete systems with digital sampling at rates of /spl sim/20 GHz and above have been demonstrated using low-temperature superconductor device technology. Some ADC components have also been implemented using high-temperature superconductors. Superconductor ADCs have unique applications in true digital-RF communications, broadband instrumentation, and digital sensor readout. Their designs, test results, and future development trends are reviewed.

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Superconductor analog-to-digital converters

Superconductor digital electronics using Josephson junctions as ultrafast switches and magnetic-flux encoding of information was proposed over 30 years ago as a sub-terahertz clock frequency alternative to semiconductor electronics based on complementary metal-oxide-semiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to high-performance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin s...

Superconductor digital electronics: Scalability and energy efficiency issues (Review Article)

Superconductor digital electronics using Josephson junctions as ultrafast switches and magnetic-flux encoding of information was proposed over 30 years ago as a sub-terahertz clock frequency alternative to semiconductor electronics based on complementary metal-oxide-semiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to high-performance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin superconducting layer for making compact high-kinetic-inductance bias inductors. All circuit layers are fully planarized using chemical mechanical planarization (CMP) of SiO2 interlayer dielectric. The physical limitations imposed on the circuit density by Josephson junctions, circuit inductors, shunt and bias resistors, etc., are discussed. Energy dissipation in superconducting circuits is also reviewed in order to estimate whether this technology, which requires cryogenic refrigeration, can be energy efficient. Fabrication process development required for increasing the density of superconductor digital circuits by a factor of ten and achieving densities above 10^7 Josephson junctions per cm^2 is described.

Superconductor Digital Electronics: Scalability and Energy Efficiency Issues

HYPRES is developing a class of digital receivers featuring direct digitization at radio frequency (RF). Such a receiver consists of a wideband analog-to-digital converter (ADC) modulator and multiple digital channelizer units to extract different frequency bands-of-interest within the broad digitized spectrum. The single-bit oversampled data, from either a lowpass delta or bandpass delta-sigma modulator, are applied to one or more channelizers, each comprising digital in-phase and quadrature mixers and a pair of digital decimation filters. We perform channelization in two steps, the first at full ADC sampling clock frequency with rapid single flux quantum (RSFQ) digital circuits and the second at reduced (decimated) clock frequency with commercial field programmable gate array (FPGA) chips at room temperature. We have demonstrated lowpass and bandpass digital receivers by integrating an ADC modulator and a channelizer unit on the same chip at clock frequencies up to 20 GHz. These 1-cm2 single-chip digital-RF receivers contain over 10,000 Josephson junctions. The channelizing receiver approach can be extended to include multiple ADC modulators and multiple channelizer units on a multi-chip module.

Digital Channelizing Radio Frequency Receiver

We have developed a flux-quantizing A/D converter (ADC) based on RSFQ elements, employing a novel front end capable of generating high-linearity multibit differential code within a wide dynamic range (up to 16 bits). The front end operates as a phase modulator/demodulator and uses fractional-flux-quantum least significant bit (LSB). It runs at multi-GHz speed, enabling ADCs with large oversampling ratio and effective resolution in excess of 20 bits (after decimation filtering). We have designed, fabricated and tested several versions of a complete ADC using this new architecture and demonstrated its operation with dynamic range of 14 bits. We have also confirmed continuous phase modulation of the flux quantizer with a carrier frequency of 10 GHz. >

Superconducting high-resolution A/D converter based on phase modulation and multichannel timing arbitration

We have developed a new architecture for a superconducting high-resolution analog-to-digital converter (ADC) which is highly beneficial for IR array readout applications due to its high sensitivity, wide dynamic range and sub-mW power consumption. This ADC is based on the principles of magnetic flux quantization, direct differential coding and decimation filtering. The digital part of the ADC employs elements of the Rapid Single Flux Quantum logic family which are capable of clock frequencies in excess of 100 GHz. We discuss the limits of ADC performance and present an analysis of the power consumption in these ADCs. We also report experimental verification of several key components of this architecture in Niobium technology (operating at 4.2 K), particularly the implementation of a synchronizer unit (necessary for the generation of the differential digital code) as well as a dual-counter synchronous ADC.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Low-power high-resolution superconducting flux-quantizing A/D converter for IR array readout

Superconducting Analog-to-Digital Converters (ADCs) are attractive for use on cryogenic focal plane arrays because of their ultra-low power consumption and their ability to operate at cryogenic temperatures. We have developed a 12 bit ADC based on Nb thin film superconducting integrated circuit technology which dissipates less than 0.44 mW while in operation at 4.2 K. Extensions of this deign to lower junction critical currents would allow the production of an ADC which dissipates less than 0.1 mW when fully biased. The ADC had at least 9.75 effective bits of resolution for 20 kHz input signals, limited by the harmonic distortions of the signal source. We estimate that the ultimate resolution of this ADC can be greater than 20 bits at 10 MHz bandwidth with our current 2.5 micron fabrication process. Potential applications for this device include focal plane array read out electronics for low temperature (4.2 K and below) imaging arrays, such as those being used on the SIRTF mission being planned by NASA. Other applications include high precision instrumentation for metrology uses.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

R. P. Robertazzi

Papers

Superconducting high-resolution A/D converter based on phase modulation and multichannel timing arbitration

Low-power high-resolution superconducting flux-quantizing A/D converter for IR array readout

Low-power 12-bit superconducting analog-to-digital converter for cryogenic focal plane array readouts