Development of Broadband Digital Current Monitors with High Sensitivity

数字电流监测仪的主要部件是模数转换器。在本研究中，我们改进了基于单磁通量子(SFQ)逻辑电路构建的模数转换器的灵敏度和信噪比，并将其应用于实际的传感器系统。2004年提出的互补模数转换器具有极高的精度和半导体模数转换器无法实现的宽带，因为互补模数转换器利用了量子反馈和高速运算，这是SFQ电路的特点。我们已经演示了互补ADC，并获得了9位的信噪比。如果采用高速二阶抽取滤波器，而不是这里使用的一阶滤波器，则信噪比可提高到14位的目标信噪比。到目前为止，我们已经演示了二阶抽取滤波器。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，由于可以实现高速的数字复接和数字信号处理，将实现STJ数量和智能化功能的显著提高。ADC的性能要求是高灵敏度和几十nA以下的电流分辨率。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还在实际的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，但确认了ADC的正确工作。我们相信，这些结果是迈向实际应用的第一步。

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.