Low-drift and compact readout electronics for practical SQUID magnetocardiography working in unshielded environment

Abstract

To promote the adaptability of the practical Magnetocardiography (MCG) system working in unshielded environment, the SQUID readout electronics is required to improve its stability and the Electromagnetic Compatibility (EMC). It should be low-drift to the variation of cryogenic liquid level in long-time running, avoid Electromagnetic Interference (EMI) internally from the control circuit, be electrical isolated from host computer, and finally be compact to be embedded in the cryostat to implement external EMI shielding and filtering. Accordingly, we presented a low-drift and compact readout circuit design. It applies the differential amplifier in the Flux-Locked Loop (FLL) to reject the voltage drift, and avoids the internal EMI by using control circuit without Micro-Controller Unit (MCU). It simplifies the integration of multi-channel FLLs and the isolated accessing from host computer by using the Inter-Integrated Circuit (IIC) bus with fiber optic interface. In the experimental results, our MCU-free FLLs successfully suppressed the voltage drift, and exhibited the high consistency in reading of 9-channel gradiometers with noise level of about 6.5μΦ0 /√Hz. Finally, we set up a 9-channel MCG system using our multi-channel SQUID readout electronics, and demonstrated the high quality 9-channel measurements of adult MCG signals in the unshielded environment.