In the context of power supplies and measurement instruments, the term "digit" is used to define both display resolution and measurement accuracy.
Display Resolution
When specifying a display, "digits" indicates the number of full digits (0 to 9) combined with the range of the Most Significant Digit (MSD).
- ½ Digit: Indicates the MSD can only display 0 or 1. For example, a 4 ½ digit multimeter can display values from 00000 to 19999 (20,000 counts).
- ¾ Digit: Indicates the MSD can typically display values up to 3. A 3 ¾ digit display generally reads from 0000 to 3999 (4,000 counts).
Accuracy Specification
The term "digit" is also used to specify instrument accuracy (e.g., "±0.05% of reading + 3 digits"). In this context, "digits" refers to the count of the Least Significant Digit (LSD), accounting for internal noise and quantization errors.
Example:
If a 4 ½ digit meter set to the 20 V range reads 10.000 V, the LSD represents 0.001 V. An error specification of "3 digits" corresponds to a voltage uncertainty of ±0.003 V.