Difference coding in analog-digital converters based on Josephson junctions

The natural periodic dependence of superconductive current I_csin φ through a Josephson junction upon phase difference φ permits creation of simple analog-digital converters (ADC's). At present two techniques have been developed for such conversions: serial and parallel. The authors propose an alternative approach to construction of an ADC with Josephson junctions. It is shown that by combining a high-speed narrow ADC which measures a few (2-4) of the least significant bits of the signal with a special digital device which reconstructs the remaining (more significant) bits, one can obtain a multibit high speed ADC without precision (approximately 2^-N) components. The basic ideas of this new approach can be applied not only to Josephson junction converters, but also other ADC's which make use of natural periodicity, for example, electrooptical devices. The authors first present a general description of the method, then turn to several methods for its concrete realization in Josephson technology. One of these methods, in amplification of the input signal by use of the Josephson junction's negative differential inductance. The basic properties of an amplifier of this type are of independent interest and are considered separately in the appendix.