7.8 History of Superconductor Analog­to­digital Converters

An Analog-to-Digital Converter (ADC) is a mixed-signal electronic circuit that converts an electrical signal from the analog domain to the digital domain, typically providing N binary bits at the sampling frequency f s. The higher speed of superconducting circuits comparable to conventional circuits provided an initial interest in superconductor technology for ADC applications. One can find technical details of superconductor ADCs in earlier reviews, e. g., [1], [2]. In contrast here, we focus on a historical perspective of the ADC development identifying important trends and milestones as well as acknowledging main actors. Similar to superconductor digital electronics, early superconductor mixed-signal circuits were based on cryotrons, pre-Josephson switching elements. A patent for the first superconductor analog-to-digital converter (ADC) was filed in 1960 [3], just in a few years after the initial cryotron-based digital circuit implementation. The projected speeds of the order of microseconds per ADC switching operation was rather attractive goal at that time. Several years later, the cryotrons were replaced by faster Josephson junctions in superconductor circuits including ADCs. The early Josephson-junction based ADC was a simple thermometer-code " totalizer " circuit patented in 1969 and presented an A-to-D conversion idea rather than a complete ADC circuit [4]. The first superconductor ADC circuit of a practical significance was a successive-approximation ADC invented in 1974 by M. Klein of IBM [5]. It consisted of a sample-and-hold circuit and four comparators made of serially connected Josephson junctions being switched to a voltage state at a predetermined control signal levels. This design was fabricated and tested in 1977 to show a 25 MHz bandwidth. In general, this ADC design was following semiconductor implementations using Josephson junctions. All early ADCs were focused on exploiting only the high switching speed of superconducting devices starting from cryotrons and then Josephson junctions. As in digital circuits, these first ADCs were viewed as a faster version of their semiconductor counterparts. The realization that superconductivity offers much more than the higher speed led to innovations in ADC circuits. These new truly superconductive ADCs were exploiting fundamental features of superconductivity unavailable in other technologies: magnetic flux quantization, extremely high sensitivity, quantum accuracy, and low noise. For instance, the magnetic flux quantization provides a natural ruler which may be used to provide a large number of quantum-mechanically accurate thresholds for the ADCs. Newer superconductor ADCs generally fall into two categories: Nyquist-sampling parallel ADCs and oversampling ADCs. An ideal Nyquist ADC samples …