A Flash A/D Converter Using Complementarily Combined SQUIDs

We propose a new design of a superconductive flash analog-to-digital (A/D) converter circuit with improved linearity. This circuit is composed of the equivalent of two quasi-one-junction SQUIDs (QOSs) that are connected at the comparator. The analog signal is input to the front end of each of the QOSs complementarily, with opposite polarity for each. Though the mechanism of the quantization in each QOS is basically the same as that in conventional ones, the mixed quantized output shows comparatively superior linearity. Moreover, unintended offsets due to
L
/
R
delays are completely canceled in this circuit, suppressing dynamic distortions in the comparator even without a feedback loop for compensation. This makes the circuit free from bandwidth limitation related to the time constant in the loop. We have carried out a computer simulation for A/D converter operation of the circuit and showed a sampling ability of 150 GS/s when the critical current density (
J

_c
) is 40 kA/cm
²
. Functional test results of the circuits are also reported.