Two-loop modulator for sigma-delta analog to digital converter

A two-loop modulator has been designed for a superconductive sigma-delta analog to digital converter. In contrast to semiconductor modulators, which use high-gain amplifiers in the signal feed forward path, the superconductive modulator used digital gain in the signal feedback path. The use of superconductive electronics to precisely feed back a single flux quantum into the second integrator loop and multiple flux quanta into the first integrator loop is a key to this circuit. In simulations of a 40 GHz sampling rate, the modulator obtained a 98 dB signal to noise ratio on the dc-60 MHz band. The modulator tolerated thermal noise well, obtaining 98 dB SNR on the dc-4 MHz band, while sampling at a rate of 4 GHz. The modulator tolerated clock timing jitter better than Nyquist-rate A/D converters, obtaining equivalent performance with 3 times as much rms jitter. Compared to single-loop sigma-delta and oversampled lobe-counting A/D converters, the two-loop modulator can achieve equivalent performance at a significantly reduced sampling and digital filter rate.<>