Development of Broadband Digital Current Monitors with High Sensitivity

点击翻译按钮获取中文摘要

Main component of digital current monitor is an analog-to-digital converter (ADC). In this study, we have improved the sensitivity and SNR of the ADCs, which are constructed, based on the single-flux-quantum (SFQ) logic circuits, and have applied them to actual sensor systems. A complementary ADC proposed in 2004 has extremely high precision and a broad band width unachievable in semiconductor ADCs because the complementary ADC utilizes a quantum feedback and high-speed operation that are special features of SFQ circuits. We have demonstrated the complementary ADC and obtained the SNR of 9 bits. If we employ the high-speed second order decimation filter instead of the first order filter used here, the SNR is improved to the targeted SNR of 14-bits. The second order decimation filters have also been demonstrated so far.We have been developing a digital current monitor system used for an actual X-ray detectors system. We use a superconducting tunnel junction (STJ) as an X-ray detector, which has an extremely high energy resolution. The X-ray detectors system needs to increase the number of STJs for improvement in its performance. If our digital current monitor system is introduced, the remarkable enhancement of the number of STJs and intelligent functions will be realized because a digital multiplexing and digital signal processing are available at high-speed. The performances required for the ADC is high sensitivity and current resolution below a few tens of nA. We successfully obtained such sensitivity and resolution after improvement of the input interface. The ADC is also tested in the actual STJ X-ray detector system. The correct operation of the ADC is confirmed, though the SNR is degraded by 1-bit. We believe that these results are the first step to the actual application.