What Does Digits of Precision Mean for DMMs? Hardware: Modular Instruments>>Digital Multimeters (DMM)
Problem: I'm looking at the specifications for the DMM products and they are described as having a certain number of digits of precision. What does this mean and why are some DMMs described as having ½ of a digit? Solution: For a traditional benchtop DMM, the number of digits indicates how many decimal places the DMM can display on its digital readout. The number of digits is often specified as the number of full digits, digits that can take any value from 0 to 9, and a single overrange digit, referred to as the ½ digit. The overrange digit is limited to only specific values, generally 0 or 1. For example a 6½ digit DMM would have seven decimal places on its readout. The most significant digit on the display could take on a value of 0 or 1; the rest could range from 0 to 9. An example of a digital readout display with 6½ digits can be seen in the image below (DigitsOfPrecision.jpg). Even though there are 7 total digits the first digit is only counted as ½ digit since it can only be 0 or 1.
Computer-based DMMs, on the other hand, do not have digital readouts in the traditional sense. The number of digits is used to specify the number of meaningful counts, or unique digitized values, the DMM is capable of producing. A computer-based 6½ digit DMM can produce 1,999,999 unique positive values and 1,999,999 unique negative values, or +/- 1,999,999 counts. A typical way to determine the meaningful number of digits is to calculate the effective number of digits(ENOD). ENOD is a statistical quantity that gives an indication of the stability of measurements. If a DMM has 6 digits of effective resolution then 6 digits will be very stable from one measurement to the next given the voltage source does not vary. The ENOD is based on the amount of RMS noise present on the measurements. Specifically:
It is important to note that the number of digits is not the same as resolution in the normal sense but is related to effective resolution. The resolution is defined by the number of bits available in the Analog to Digital Converter (ADC) which may be more or less than the effective resolution of a particular measurement. Related Links: Attachments:   - enod.JPG
Report Date: 12/13/2006 Last Updated: 07/27/2007 Document ID: 44CFNSE5 |
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