Sequential read-out architecture for multi-channel SQUID systems

We describe a novel multi-channel sequential SQUID read-out technique that requires fewer wires than conventional units and also simplifies the electronics significantly. We designed and experimentally tested the sequential read-out electronics with up to 8 channels using LTS 8/spl times/8 mm/sup 2/ magnetometers with better than 3 fT//spl radic/Hz field resolution. We have investigated noise performance, amplitude-frequency characteristics, and cross-talk of the sequential read-out electronics for 2, 4, and 8 channels. We observed field resolution better than 4 fT//spl radic/Hz, 6 fT//spl radic/Hz, and 9 fT//spl radic/Hz for 2-, 4-, and 8-channel versions, respectively. We observed 10 kHz frequency bandwidth for the 8-channel version using 200 kHz modulation frequency. Cross-talk better than -90 dB was measured for this system. A single-channel simulation was used to estimate the field resolution for systems with up to 128 channels. We found that the expected field resolution can be better than 15 fT//spl radic/Hz, 20 fT//spl radic/Hz, and 30 fT//spl radic/Hz for 32-, 64-, and 128-channel systems, respectively, with the sequential read-out technique.