Digital Multimeter Calibration: Resolution or Accuracy?

In digital multimeter (DMM) calibration, two concepts are often confused: resolution and accuracy. Resolution is the smallest change the instrument can display; accuracy is how close its measurement is to the true value.

The Digit Concept

On DMMs, the term “6½ digits” means the instrument can display 6 full + 1 half (0–1) digit. So the maximum display is 1,999,999. But this is only resolution — it does not indicate accuracy.

The Accuracy Specification Formula

DMM accuracy is usually given as:

Example: measuring 5 V with a DMM specified at 0.002% of reading + 0.0005% of range on the 10 V range:

• % of reading = 5 V × 0.002/100 = 100 µV
• % of range = 10 V × 0.0005/100 = 50 µV
• Total accuracy = ±150 µV

Choosing Calibration Points

Typical functions and points in DMM calibration:

• DCV: 100 mV, 1 V, 10 V, 100 V, 1000 V; 10%, 50%, 100% of each range
• ACV: the same points, at 50 Hz / 1 kHz / 10 kHz
• DCI/ACI: 100 µA, 1 mA, 10 mA, 100 mA, 1 A, 10 A
• Resistance: 100 Ω, 1 kΩ, 10 kΩ, 100 kΩ, 1 MΩ, 10 MΩ (4-wire measurement)

Reference Source: Multifunction Calibrator

DMM calibration typically uses multifunction calibrators such as the Fluke 5520A, 5730A or Wavetek 9100. The calibrator’s uncertainty must be at least 4 times smaller than that of the DMM being calibrated (the TAR 4:1 rule).

Resolution or Accuracy?

In practice, resolution is independent of accuracy. The last digit of a 6½-digit DMM may not be meaningful because of noise. Calibration reports include the resolution component in the uncertainty budget using the (d/2)/√3 formula.