Superconducting sigma-delta analog-to-digital converters

The sigma-delta architecture is the method of choice for designers and manufacturers of analog-to-digital converters (ADCs) for high dynamic range applications. This architecture uses oversampling and precise feedback to generate a shaped spectral distribution of the quantization noise. Subsequent digital filtering suppresses out of band quantization noise, yielding a large signal to in-band noise ratio. This permits the use of quantizers with only a few bits of resolution, most applications use single-bit quantizers. A unique advantage of superconducting electronics is the availability of the flux quantum which can be used to provide quantum mechanically accurate feedback at GHz rates. Josephson digital technology extends the realm of sigma-delta ADCs from MHz sampling rates to GHz sampling rates, from kHz signal bandwidths to MHz signal bandwidths, with comparable or better dynamic range when compared to semiconductor implementations. This paper presents circuits for Josephson sigma-delta ADCs, including single-loop, and double-loop modulators, circuits for quantized feedback, and digital data processing. Experimental results of a double-loop modulator sampling at 1.28 GHz are reported.