Characterization of a superconductive sigma-delta analog to digital converter

Sigma-delta analog to digital converters (ADCs) use a combination of oversampling and feedback to concentrate quantization noise outside the frequency band of interest. Subsequent digital filtering can then be used to suppress the quantization noise and yield a large signal to noise ratio. Sigma-delta ADCs dominate the high performance audio market, where the signal band is limited to frequencies below 50 kHz and 8 octave oversampling requires a sampling rate of only 25.6 MHz. Przybysz et al. have described a superconductive circuit capable of >40 GHz sampling, thereby extending the useful bandwith to tens of MHz. In this paper, we describe the realization of that circuit and present measurements of its performance. Spectral analysis of the modulator performance shows spur-free dynamic range of over 78 dB and third order intermodulation products less than -68 dBc. Quantization noise shaping is also demonstrated.< >