Multimeter Digits Explained: 5½ vs 6½ Digit DMMs
Filed under Meters
This guide explains the concept of display digits in digital multimeters (DMMs), focusing on the differences between 5½-digit and 6½-digit instruments. For engineers, technicians, and technical buyers, choosing the correct resolution is critical to balancing measurement precision with application requirements. While handheld meters often feature 3½ or 4½ digits, precision laboratory and benchtop environments frequently require 5½ or 6½ digits to resolve minute signal variations. This page covers how fractional digits relate to counts, the distinction between display resolution and actual measurement accuracy, and how to interpret manufacturer specifications using industry-standard examples.
- 6½-Digit Count Limit
- 1,999,999 or 2,000,000 counts typical
- 34401A Maximum Resolution
- 100 nV (on 100 mV range)
- 34461A Best DCV Accuracy
- 0.0035% of reading + 0.0005% of range
- Operating Temperature Limits
- 0 to 55 °C
What the Fractional "Half" Digit Means
The resolution of a digital multimeter is often specified in "digits," such as 5½ or 6½. A full digit can display any integer from 0 through 9. A fractional or "half" (½) digit refers to the most significant (leftmost) digit on the display, which has a restricted range. In most instruments, a half-digit can only display a 0 or a 1.
Because of this limitation, a 5½-digit multimeter can display values up to 199,999 (commonly referred to as 200,000 counts). Similarly, a 6½-digit multimeter, such as the Keysight Technologies (Agilent HP) 34401A or the Keysight Technologies (Agilent HP) 34461A, has six full digits plus one leading half-digit, allowing it to display readings up to 1,999,999 (or 2,000,000 counts).
Resolution vs. Accuracy
It is critical to distinguish between resolution and accuracy. Resolution is the smallest change in a signal that an instrument can detect and display. For example, on its 100 mV range, the Keysight Technologies (Agilent HP) 34401A provides a maximum resolution of 100 nV.
Accuracy, however, is the limit of error for a measurement under specified operating conditions. A DMM can have high resolution (showing many digits) without a corresponding level of absolute accuracy if it has drifted, is out of calibration, or is operating outside its specified temperature range. Absolute accuracy depends on internal reference stability, noise floor, and calibration quality rather than the number of display digits alone.
How to Read an Accuracy Specification
DMM accuracy is typically presented as a combination of two variables: a percentage of the actual reading plus a percentage of the selected range. This is expressed as ±(% of reading + % of range) and is valid for specific timeframes and temperature bands.
Consider the 1-year DC voltage accuracy specification for the Keysight Technologies (Agilent HP) 34401A on the 10 V range: ±(0.0035% of reading + 0.0005% of range). If measuring a 5 V signal on this 10 V range, the total uncertainty is calculated as follows:
- Error from reading: 5 V × 0.000035 = 0.000175 V (175 µV)
- Error from range: 10 V × 0.000005 = 0.000050 V (50 µV)
- Total maximum error limit: 175 µV + 50 µV = ±225 µV
When evaluating AC parameters, the calculations often include frequency-dependent terms. For example, the Keysight Technologies (Agilent HP) 34461A has an AC voltage 1-year accuracy of 0.06% of reading + 0.03% of range specifically for signals within the 10 Hz to 20 kHz frequency band.
When 6½ Digits are Necessary
While 5½-digit or high-count handheld meters are suitable for general troubleshooting, component testing, and basic circuit design, a 6½-digit benchtop DMM is required for more demanding metrology tasks.
Typical applications for 6½-digit instruments include precision calibration of other equipment, measuring very small signal drifts on large DC voltage offsets (such as monitoring noise on a 5 V or 10 V power rail), or tracking subtle sensor responses over time. Additionally, modern 6½-digit DMMs like the Keysight Technologies (Agilent HP) 34461A include graphical interfaces to display trend charts and histograms directly on a 4.3-inch color TFT-LCD, which simplifies the visualization of long-term drift without needing an external PC.
Example instruments
Frequently asked questions
- What is the relationship between the Keysight 34401A and the 34461A?
- The Keysight Technologies (Agilent HP) 34461A is designed as the direct replacement for the legacy Keysight Technologies (Agilent HP) 34401A. Both are 6½-digit multimeters with equivalent 1-year DC voltage accuracy of 0.0035% of reading + 0.0005% of range on the 10 V range, but the 34461A adds modern interfaces (USB and LAN) and a color TFT-LCD graphical display.
- Does a 6½-digit DMM require more frequent calibration than a 5½-digit DMM?
- The calibration interval is determined by the manufacturer's specification and your tolerance requirements, not strictly by the digit count. However, because 6½-digit meters are used for higher-precision work, they are typically calibrated on a strict annual cycle to maintain their certified accuracy specifications.