BentlyNevada P/N 140471-01
此处不是利用MOSFET的转移特性，而是采用电压反馈的方式进行电流控制。在场效应管的漏极与源极间加上24V的电压(由系统的其它模块提供，限于篇幅不作说明)，与外部所接负载构成回路后，漏极电流便成为电流源的输出电流。设输出电流为I，则U8的引脚3引入的采样电压为10I，经过10倍放大后变为100I引入引脚6，由于5与6处的电压值相等，所以Vstand＝100I(Vstand的***大输出为2.5V，而I要求其输出范围为0～20mA,所以100倍的关系比较合适)，由于Vstand的分辨率＝V2‘‘分辨率＝0.002mV，理论上I的分辨率可以达到0.000002mA，完全可以满足预计的0.001mA分辨率要求(Vstand以0.1mV的步进改变即可)，于是高精度电流源得以实现。
测试实验
按照以上的高精度电压与电流的产生方法进行硬件设计，再加上键盘与液晶显示器等模块，利用单片机控制，便可构成一个简易的、可以提供高精度直流电压与电流的仪表(限于篇幅，其它模块与程序设计不作说明)。
系统定标
由于本系统是精密仪表设备。因此，必须采用定标消除系统误差。由于在本设计中MCU采用的是SST公司生产的89E58RC单片机。这种单片机为用户提供了强大的IAP(InApplicationProgram)功能，并有8KflashROM用于存储数据。通过IAP，系统可以提取校准数据，并将数据存储，在仪表输出电压/电流时就可以先提取相应的校准数据进行预处理。这样就保证了该系统的精度。
测试试验
定标后，采用安捷伦公司的3485A进行测试。表1与表2分别为测试的电压输出与电流输出的实验数据。
结语
综上分析可知,本文所提出的宽动态范围、高分辨率的高精度直流电压/电流源的设计方法是切实可行的，同时，此设计方法节约了成本。
NORGREN K41EA00-KS6KV2 *USED*
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