Basic Study on Multiplexed Superconducting Quantum Interference Device for Multichannel Biomagnetometer

We have constructed a multiplexed 2-channel, 2-stage superconducting quantum interference device (SQUID) amplifier using double-washer SQUIDs and a 35-SQUID series array. The biasing current of the first-stage SQUIDs is switched by a complimentary metal oxide semiconductor (CMOS) analog multiplexer at liquid helium temperature, and an on-stage SQUID transfers the signal to the second-stage SQUID. Each SQUID has an additional negative feedback circuit to enlarge the linear input region and decrease distortion. Maximum input flux at the first-stage was 0.9Φ opp and the total harmonic distortion of the overall two-stage SQUID was 1% for a sinusoidal input flux of 0.1Φ opp. The voltage swings of the two channels were 1.1 and 1.3 mV.