Development of Broadband Digital Current Monitors with High Sensitivity

数字电流监测器的主要部件是模数转换器（ADC）。在本研究中，我们提高了基于单通量量子（SFQ）逻辑电路构建的adc的灵敏度和信噪比，并将其应用于实际的传感器系统中。2004年提出的互补ADC具有极高的精度和半导体ADC无法实现的宽带宽度，因为互补ADC利用了量子反馈和高速操作，这是SFQ电路的特殊功能。我们演示了互补ADC，并获得了9位的信噪比。如果我们使用高速二阶抽取滤波器代替这里使用的一阶滤波器，信噪比将提高到14位的目标信噪比。到目前为止，二阶抽取滤波器也得到了证明。我们一直在开发一种用于实际x射线探测器系统的数字电流监测系统。我们使用超导隧道结（STJ）作为x射线探测器，它具有极高的能量分辨率。x射线探测器系统需要增加stj的数量以提高其性能。如果引入我们的数字电流监测系统，由于可以在高速下进行数字复用和数字信号处理，将实现stj数量的显著增加和智能功能的显著增强。ADC的性能要求是高灵敏度和低于几十nA的电流分辨率。通过对输入接口的改进，我们成功地获得了这样的灵敏度和分辨率。该ADC也在实际的STJ x射线探测器系统中进行了测试。虽然信噪比降低了1位，但ADC的正确操作得到了确认。我们相信这些结果是实际应用的第一步。

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.