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Proportional-integral-derivative (PID) controllers have been broadly used in many areas. Compared with analog controllers, digital ones by digital signal processor (DSP) or field programmable gate arrays (FPGA) are more stable and easier to adjust parameter. Digital controllers are usually implemented with FPGA for its fast and stable performance. When it is implemented with FPGA, floating-point arithmetic is usually converted to fixed-point arithmetic. However the process of a floating-fixed conversion is complex, the period is long and the precision is proved to be low. Since the above problems, this paper proposes both best-precision fixed point arithmetic and the FPGA implementation details of a digital PID controller. And it illustrates the process of converting analog controllers to digital one. The simulation results show that the best-precision fixed point digital PID controller is simple, and it outperforms by high-speed response, better precision, minor error and high dynamic range.