United States Nuclear Regulatory Commission - Protecting People and the Environment

Selection of Sample Rate and Computer Wordlength in Digital Instrumentation and Control Systems (NUREG-1709)

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Publication Information

Manuscript Completed: June 2000
Date Published: July 2000

Prepared by:
T. W. Jackson

Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555

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Abstract

Digital sampling of analog signals adds two types of errors, aliasing and finite wordlength error, to the sampled version of the signal. Aliasing is characterized by high frequency components misrepresented as low frequency components in the sampled signal. It is greatly influenced by the sample rate, and may lead to degraded performance in monitoring, alarm, control, and protection systems. Since computer wordlengths are finite in length, digital systems are limited in their capability to represent real number values. Finite wordlength errors related to round-off, truncation, and data conversion have the potential to adversely impact the performance of digital instrumentation and control (I&C) systems. The Office of Nuclear Regulatory Research is investigating good engineering practices regarding aliasing and finite wordlength errors in nuclear facilities. Hazards associated with these errors are minimized through proper design and selection of sample rates and computer wordlengths. This document provides the regulatory background, theoretical information, practical issues, best engineering practices, and examples associated with sample rate and computer wordlength selection.

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