Design of Digital Filter for Digital SQUID With Sub-Flux Quantum Feedback Resolution

Digital SQUIDs with the single flux quantum (SFQ) feedback have attracted much attention because of the feasibility of realizing a wide dynamic range and high slew rate. In order to realize higher resolution, we have studied the performance of an advanced, digital SQUID configuration offering sub-flux quantum feedback. Combining the sub-flux quantum feedback to a feedback loop and the direct SFQ feedback to a main loop triggered by carry SFQ pulses from an up/down counter. After optimization of the circuit, we confirmed the maximum magnetic field amplitude √ exceed 1 μT, the magnetic field noise is lower than 100 pT/ Hz with a signal band frequency of 1.22 MHz. Using a first-order sinc filter, we could implement the digital SQUID magnetometer at lowtemperature with reasonable noise reduction performance and the binary data rate reduction in order to transfer the data from low temperature circuit to the room temperature electronics.