Often the terms resolution, precision, and accuracy are used interchangeably, but they actually indicate very different entities, as we discussed in the previous section. Although common sense indicates that a 6 1/2 digit multimeter must be accurate to the 6 1/2 digit level, this may not be the case. The number of digits simply relates to the number of figures that the meter can display and not to the minimum distinguishable change in the input (refer to Digits Displayed and Overranging definition above). Therefore, when using or evaluating an instrument, remember that the number of digits may refer to the display and not directly to the resolution of the instrument.
You need to verify that the instrument sensitivity and effective resolution are enough to guarantee that the instrument will give you the measurement resolution you need. For example, a 6 1/2 digit multimeter can represent a given range with 1,999,999 counts or units. But if the instrument has a noise value of 20 counts peak to peak, then the minimum distinguishable change must be at least 0.52 ´ 20 counts. Referring to equation (4) above, the effective number of digits is:
Often, this error is added in the DMM specifications under Percent Range, where the source of the error -- nonlinearity, noise, or offset -- is not identified.
This aspect of the technical specifications relates to the first digital multimeters, which had a limited number of digits displayed to keep the cost of the instrument as low as possible. With the advent of more sophisticated digital instruments and, ultimately, of virtual instruments, the cost of the instrument display is no longer an issue. Therefore, care must be taken in specifying the number of digits of a measurement device (whether computer-based, PXI/CompactPCI, VXI, or GPIB controlled). The resolution, accuracy, nonlinearity, and noise of the measurement device must be considered when determining the number of digits to display to the user. For example, consider an instrument that uses a 24-bit analog-to-digital converter (ADC) and can display seven digits of data (seven 9s). However, if the six least-significant bits are noisy and thus not carrying any valuable information, the resolution of this ADC is reduced to 18 bits (five digits) and the instrument vendor should display no more than five digits.
The NI 4050 and NI 4060 digital multimeters and the NI 4350/4351 temperature and voltage instruments are based on a 24-bit ADC and provide 24-bit data (7 digits). However, following these guidelines, the information returned to the user has been voluntarily limited to 5 1/2 digits (18.6 bits) to maintain the correct relationship between the number of digits displayed and the effective resolution of the instrument. Because National Instruments has adopted the definition of resolution listed at the beginning of this document, the number of digits displayed matches the instrument resolution.
Sometimes it is difficult to make a clear distinction between precision of the instrument and its accuracy. Precision,which relates to the repeatability of the measurement, is determined by noise and short-term drift of the instrument. (Long-term drift of an instrument affects precision only if it is considered over extended period of time.) The precision of an instrument is often not provided directly, but it must be inferred from other specifications such as the 24 hour ±1 °C specification, the transfer ratio specification, noise, and temperature drift. Precision is meaningful primarily when relative measurements (relative to a previous reading of the same value) need to be taken -- a typical example is device calibration.
Accuracy of an instrument is absolute and must include all the errors resulting from the calibration process. It is interesting to note that sometimes the accuracy specifications are relative to the calibration standard used. In such a case, it is important to include in your error budget the additional errors due to this calibration standard.
In the following section we will go through an error budget calculation to determine the instrument total accuracy.