Evaluation of superconducting quantum interference devices interfaced with digital signal processing electronics for biomagnetic applications

摘要

The performance of a dc-SQUID magnetometer driven by both analog electronics and digital signal processors are investigated and compared for biomagnetic applications. Low-noise ( < 5 (mu) (Phi) (sub 0)/(radical)Hz at 1 Hz) dc-SQUIDs were fabricated by Conductus, Inc. using the all-refractory Nb/Al/Al(sub 2)O(sub 3)/Nb process on silicon substrates with on-chip modulation coils and integral washer damping resistors. A second-order gradiometer was magnetically coupled to the input coil of the SQUID to maximize the detected signal strength. The readout of this SQUID gradiometer was achieved using a conventional flux-locked loop (FLL) circuit to provide a linearized voltage output that was proportional to the flux applied to the SQUID. A shielded cylinder was constructed to house the magnetometer to reduce ambient field noise. To realize the digital feedback loop, the analog FLL is replaced except for the preamplifier by a digital signal processing board with dual 16-bit A/D and D/A converters. This approach shows several advantages over the analog scheme including operational flexibility, cost reduction, and possibly, the enhancement of dynamic ranges and slew rates.